C11 collimation woes
John Moore
After stripping my C11 to fit various accessories and rebuilding it I have been
attempting to recollimate it, without much success. Please see the image at:
taken with a webcam and 2X Barlow. My question is are the 2 offcentre blobs
merely caused by tube currents or is there some major problem with the optics?
The tube had been outside for at least 2 hours before this picture was taken.
Thanks for any help.
John Moore
Fleet, Hants, England
Roger Hamlett
"John Moore" <john...@iee.org> wrote in message
news:yZD9h.31404$Xh3....@newsfe6-win.ntli.net...
What were the 'accessories'?.
It is really impossible to tell from a single picture. However, assuming
the camera is orientated as you would look at the back of the scope, the
'suspicious' thing is that the top plume is where you would rather expect
tube currents to show, but the second plume, aligns along the line of the
focusser arm. Could any of the accessories be causing a strain in this
area?.
Best Wishes
John Moore
> What were the 'accessories'?.
> It is really impossible to tell from a single picture. However, assuming the
> camera is orientated as you would look at the back of the scope, the
> 'suspicious' thing is that the top plume is where you would rather expect tube
> currents to show, but the second plume, aligns along the line of the focusser
> arm. Could any of the accessories be causing a strain in this area?.
>
I should have mentioned that the image was taken out of focus, although I hope
that was obvious.
I don't remember the camera orientation unfortunately. The 'accessories' were:
1. Black flocking of most of the internal surfaces.
2. An extractor fan (which I forgot to run, partly because I thought it would
have cooled down after a couple of hours).
3. Three point mirror locking screws, which were tightened at the time, but the
problem was present even with the screws completely loose. I also completely
loosened the corrector plate screws in case that was under strain.
4. A second lifting handle.
There was a dew heater strip round the outside of the front of the tube, but it
was not even noticeably warm to touch.
I had certainly removed and refitted the focusser assembly, but it was running
freely as far as I can tell.
Is it possible that there were two sets of tube currents from different causes?
Regards
John Moore
Allan Mayer
Interesting... I can guess that it is
tube currents. (I can be wrong too...)
BTW, do you have anything on, or against
the OTA at those two points ?
I've seen some interesting images while
my C 11's cooling down, just don't recall
any quite like that.
Now the the three mirror lock screws....
--
AM
http://sctuser.home.comcast.net
OS X 10.4.8
Roger Hamlett
only has one mounting arm (running out to the focusser), and the centre
tube assembly itself, I cannot see anyway that a three point mirror lock
can be attached to the scope, without major changes round the mirror!...
It could be just an unusual thermal plume. On some nights, a scope will
never cooldown properly, with the temperature changing so fast in the
night. However If it is visible on another night, or with a daytime test,
I'd be worrying...
Best Wishes
John Moore
> only has one mounting arm (running out to the focusser), and the centre tube
> assembly itself, I cannot see anyway that a three point mirror lock can be
> attached to the scope, without major changes round the mirror!...
> It could be just an unusual thermal plume. On some nights, a scope will never
> cooldown properly, with the temperature changing so fast in the night. However
> If it is visible on another night, or with a daytime test, I'd be worrying...
>
The modifications including the locking bolts were based on ideas in:
http://www.rothritter.com/C11_Project/
The bolts seat on three small stainless steel plates that I glued to the back of
the mirror.
I think I'll have to repeat the test and see if the pattern is still the same,
also run the fan to see if that helps. I have seen tube currents before in other
scopes and this just doesn't look like that to me.
Regards
John Moore
John Moore
> the OTA at those two points ?
> I've seen some interesting images while
> my C 11's cooling down, just don't recall
> any quite like that.
>
No, nothing against the tube, apart from the usual Losmandy G11 dovetail plate,
and that only touches the tube at a few points.
John Moore
Allan Mayer
Ok.
When I set my C 11, G 11 up at one of our local
club sites, it is on a small knoll. Sometimes
it seems that the temp is either falling, or raising
but never staying at one point. Nights like these
I never get a clear view as the primary is always
changing, along with the corrector.
My C 8 always seems to equalize temp in around an hour
or so max. I've had nights with the C 11 where it
just never cools down after several hours.
Thanksgiving, at dawn started out near freezing.
After the sun had set, the temp was around 50F
Go figure..........
Big mirrors suffer when the temp never stabilizes
at night.
Roger Hamlett
When attaching mirror cells to Newtonian mirrors, it is vital to use a
'soft' adhesive, like Silicon RTV (and the non acid version of this),
rather than anything rigid like epoxy. I must admit, if anything more
rigid has been used, I would be inclined to think that the shapes are the
result of thermal expansion between the metal parts and the mirror,
resulting in distorting the primary.
Best Wishes
John Moore
> When attaching mirror cells to Newtonian mirrors, it is vital to use a 'soft'
> adhesive, like Silicon RTV (and the non acid version of this), rather than
> anything rigid like epoxy. I must admit, if anything more rigid has been used,
> I would be inclined to think that the shapes are the result of thermal
> expansion between the metal parts and the mirror, resulting in distorting the
> primary.
>
Thanks for the suggestion. I hope you aren't right! I used Loctite plastic
padding car body filler (junior size Art No 27). It is supposed to be slightly
elastic to accomodate movement in the car body as it goes along the road.
Regards
John Moore
RMOLLISE
>
> Thanks for the suggestion. I hope you aren't right! I used Loctite plastic
> padding car body filler (junior size Art No 27). It is supposed to be slightly
> elastic to accomodate movement in the car body as it goes along the road.
>
> Regards
> John Moore
Hi:
The mirror was originally attached to the primary cell with RTV, that's
for sure, and that's what you should have reattached it with. That
said, your problem is more likely related to cooldown issues.
RMOLLISE
;-)
CeeBee wrote:
> Maybe of some interest, not so much the actual problem, but the collimation
> troubles and links mentioned.
>
> http://astroshed.com/observatory/c11saga/c11saga.htm
>
Roger Hamlett
Seriously, this sort of adhesive, _heats_ as it sets. What will have
happened, is that the plate will have expanded sigificantly with this
heat, and then when it cools, will have applied tension to the mirror. the
forces needed to make a visible distortion on a mirror are _tiny_. The
adhesive needs to be an order of magnitude softer than the polyester
adhesive you have used.
People have had problems with visible distortions on Newtonians, using
these adhesives, on mirrors that are at least twice as thick as the SCT
primary.
I'm afraid I think this is the problem.
Best Wishes
Uncle Bob
> a dew heater strip round the outside of the front of the tube, but it
> was not even noticeably warm to touch.
It need not be warm to the touch to cause issues. It only has to be
a different temperature than the equipment.
You can let your scope cool down with the cover on or the scope pointed at
the ground and the dew heater removed and try the test again. Try making a
dew shield out of household material if you don't have a factory dew
shield.
I have had some very bizarre effects from tube currents. Not as bizarre as
yours, though. What we can't tell is if the dark smudges are static or
dynamic. Do they move around? Shrink and grow?
If you want to see if your optics are hosed, you can do a ronchi test on a
star. That will show you if something's tweaked.
Looks like thermal issues to me, though, fwiw.
You any relation to Sir Patrick? ;-)
Regards,
Uncle Bob
Fairfax, CA
Reply to Super MORON
(somehow) and saw the kind of adhesive that was there in the first place. I wont say
more. My blood 0pressure is rising -
Opera Bannana
John Moore wrote:
> Hello,
>
> After stripping my C11 to fit various accessories and rebuilding it I have been
> attempting to recollimate it, without much success. Please see the image at:
>
> http://tinyurl.com/yy2j3e
>
> taken with a webcam and 2X Barlow. My question is are the 2 offcentre blobs
> merely caused by tube currents
No. These are not tube curent patterns. These are optical-mechanical distortion
patterns with what look like associated thermal distortion
patterns. The question because what did you do "mechanically" to
cause the optics to perform in this manner.
Release all stresses on optical components including mirror locks etc, rough
collimate not on a star but by merely looking down the eye tube
and collimate the secondary to center all components, then put the scope on a star
andsee what diffraction pattern you get if any!. I
assume your mirror is free to travel and squared to the optical axis.
Two of the blobs are consistent with some mechanical pattern - in
something you did with this scope? Mirror locks (3) in a triangle or something? I
assume the missing third blob is where your focuser
attaches to the tanger arm and if that is the case then obviously your
focuser/tanmgent arm are not inducing stress on the primary whereas
the other two equidistant points are, somehow?
I have a feeling Im close to the truth here whatever you have done.
Next time, frankly, pick a project that is commensurate with your
skill level, but... that's why they call this Amateur astronomy, which
is akin to gambling . sci.astro.gambl.ers.
hee hee
John Moore
feel I should explain why I made these modifications. I already have an Intes
Micro MN78 which has taken some decent planetary pcitures (see
http://tinyurl.com/ckxk5 ), but I am striving for better. Having seen some
spectacular planetary images taken with a C11, I picked one up secondhand. I
immediatley compared it with the MN78 on Jupiter and its performance was much
worse, despite the much bigger aperture. Looking at the very glossy inside of
the C11 I surmised that at least some of the problem was caused by internal
reflections, so I found the previously quoted webpage and made the flocking
modifications described there. I had also already noticed severe tracking
problems when autoguiding on DSOs, and ascribed this to the noticeable mirror
flop, hence the locking bolts as well. As far as I was concerned the scope was
unusable without the modifications.
The bolts are at 120 degreee intervals and not aligned with the focusser. To
clarify, I did not remove the focusser arm from the mirror; this was not
necessary. Certainly the 2 blobs on the image I posted could correspond with the
position of 2 of the seating plates I installed, but it's strange that the third
has not caused any problem in that case.
You've all given me some food for thought, and I need to make some more tests.
Regards
John Moore
Roger Hamlett
when setting up a mirror cell on a Newt. I just don't feel that the
polyester, will give enough compliance. This stuff is designed to bend
when a significant force is applied, and if such force is present it'll
show. The scope actually makes an incredibly sensitive 'strain gauge'.
This is why silicon RTV, is the favourite choice!. A significant amount
will also depend on just how thick the glue is at each point, and how
close to 'set' it was when applied.
As a completely 'off the cuff' suggestion, why not test it?.
If you remove the secondary and corrector, the primary on an SCT, is a
spherical mirror. A basic Foucault tester, can be made for a few pence
(you don't need the abilities that cost money, of being able to measure
the position of the tester accurately...). A simple tester, would give you
data in a very few seconds, on whether there is any detectable distortion
in the primary. If so, some careful removal is going to be needed. If not,
you can go looking elsewhere.
Best Wishes
John Moore
> when setting up a mirror cell on a Newt. I just don't feel that the polyester,
> will give enough compliance. This stuff is designed to bend when a significant
> force is applied, and if such force is present it'll show. The scope actually
> makes an incredibly sensitive 'strain gauge'. This is why silicon RTV, is the
> favourite choice!. A significant amount will also depend on just how thick the
> glue is at each point, and how close to 'set' it was when applied.
> As a completely 'off the cuff' suggestion, why not test it?.
> If you remove the secondary and corrector, the primary on an SCT, is a
> spherical mirror. A basic Foucault tester, can be made for a few pence (you
> don't need the abilities that cost money, of being able to measure the
> position of the tester accurately...). A simple tester, would give you data in
> a very few seconds, on whether there is any detectable distortion in the
> primary. If so, some careful removal is going to be needed. If not, you can go
> looking elsewhere.
>
That sounds a very good idea. I'll see if I can do it. Many thanks
John Moore
RMOLLISE
I've done a lot of planetary imaging with SCTs from 5 - 11 inches in
diameter. They've performed spendidly when precisedly collimated.
What matters for imaging? More than anything? Seeing. If you're not
getting the high resolution images you want, it's SEEING (and maybe
collimation...you should be collimating on the _in focus_ first
diffraction ring of a stars...don't have the seeing to do
that...?...well you'll be limited either way, then). Not some quest to
darken the field with flocking.
The MN78 might present a darker background, but that ain't gonna have
much effect on your images. Put your C11 back together, get a copy of
the new Registax, and wait for a night of good seeing.
Oh...and remember this one little thing: "The Only Enemy of Good Enough
is More Better."
;-)
Peace,
Rod Mollise
Author of:
Choosing and Using a Schmidt Cassegrain Telescope
and
The Urban Astronomer's Guide
<http://skywatch.brainiac.com/astroland>
RMOLLISE
CeeBee wrote:
> "RMOLLISE" <rmol...@hotmail.com> wrote in sci.astro.amateur:
>
> > Which link can be titled "How I Ruined a Perfectly Good OTA"....
> >
> > ;-)
>
>
> OTOH, the documentation for maintenance of an SCT - at least the Celestron I
> own - is sadly lacking. Any problem you encounter and should be prevented in
> the first place is met in the user's manual "do not....!".
>
> I would say : "we should have built a flawless performing unit, but as we
> didn't, at least we should give you the opporunity to correct it yourself or
> find out if you have to return it to us incompetent idiots in the first
> place...."
>
What? You think they should have instructions for yanking the primary?
This thread alone ought to show you different. This is not intended to
be an operation nor an option for the average user.
There probably should be a section about pulling the corrector, but I'm
also sure Celestron (and Meade) have visions of Joe/Jane Blow yanking
their correctors to remove a speck of dust, breaking the things, and
calling customer service demanding a 'nother one.
Anyhoo, the result of _fiddling with what don't need to be fiddled
with_ is all too often this: a once-lovely C11 with a mirror now FIRMLY
CEMENTED to its mirror holder. ;-)
RMOLLISE
Roger Hamlett wrote:
> People have had problems with visible distortions on Newtonians, using
> these adhesives, on mirrors that are at least twice as thick as the SCT
> primary.
> I'm afraid I think this is the problem.
>
Hi:
I'd say so...at any rate, it is NEVER, EVER a good idea to affix a
primay mirror in any type of scope as he's done. Just won't work.
RMOLLISE
Roger Hamlett wrote:
> People have had problems with visible distortions on Newtonians, using
> these adhesives, on mirrors that are at least twice as thick as the SCT
> primary.
> I'm afraid I think this is the problem.
>
Hi:
I'd say so...at any rate, it is NEVER, EVER a good idea to affix a
primray mirror in any type of scope as he's done. Just won't work.
RMOLLISE
Roger Hamlett wrote:
> People have had problems with visible distortions on Newtonians, using
> these adhesives, on mirrors that are at least twice as thick as the SCT
> primary.
> I'm afraid I think this is the problem.
>
Hi:
I'd say so...at any rate, it is NEVER, EVER a good idea to affix a
primary mirror in any type of scope as he's done. Just won't work.
RMOLLISE
After 40 years in this biz, believe me: some are, some _aren't_. ;-)
The main problem I run into? People who've really done a number on
their OTAs because they can't stand that one little speck of dust the
"flashlight test" reveals. Folks: RESIST the temptation. ;-)
Peace,
Rod Mollise
Author of:
Choosing and Using a Schmidt Cassegrain Telescope
and
The Urban Astronomer's Guide
<http://skywatch.brainiac.com/astroland>
CeeBee wrote:
>
> BTW, Most people using such scopes aren't average users - on the contrary.
> To keep such a precision instrument in top shape you have to know how the
> thing works, is assembled, and should be maintained. Maybe it has to do
> with another culture in the US for handling liability. Not everyone however
> wants their unit missing for months due to maintenance they could be
> perfectly capable of themselves given correct instructions (and warnings).
>
>
> --
> CeeBee
>
> *** Democracy is not a spectator sport ***
lal_truckee
>
> Anyhoo, the result of _fiddling with what don't need to be fiddled
> with_ is all too often this: a once-lovely C11 with a mirror now FIRMLY
> CEMENTED to its mirror holder. ;-)
I don't think that's what he's described. If I'm reading correctly, he
glue three small "tabs" to the back of the mirror for three thrust rods
to bear against in an attempt to correct mirror flop, as done
successfully by several web site publishers. Using an unfavorable glue,
unfortunately.
All is not lost.
I don't recall if this has been suggested, but the OP may have "locked"
the mirror with so much pressure as to distort it directly - if, so he
should test with the thrust rods backed out.
If it's determined that the glue itself is at fault, the OP needs to
remove the glue - I'd cut through each glue patch with a thin abrasive
thread (dental floss suggests,) removing as much as possible that way,
and retesting. Possible follow on dremel surgery will be required (with
a very delicate touch.)
John Moore
"lal_truckee" <lal_t...@yahoo.com> wrote in message
news:Fztah.4308$wc5....@newssvr25.news.prodigy.net...
I did test the collimation with the locking screws completely undone, and there
was no improvement in the pattern shown.
If the glue turns out to have been a mistake then I will indeed be following the
lines you suggest.
Regards
John Moore
Dennis Woos
> Hello,
>
> After stripping my C11 to fit various accessories and rebuilding it I have
> been attempting to recollimate it, without much success. Please see the
> image at:
>
> http://tinyurl.com/yy2j3e
>
> taken with a webcam and 2X Barlow. My question is are the 2 offcentre
> picture was taken. Thanks for any help.
>
> John Moore
> Fleet, Hants, England
I know some SCT owners who are reluctant to collimate their scopes - and
believe me they need it - so I give you a lot of credit for taking a scope
that you found unsatisfactory and trying to make it perform better. Given
that the corrector and secondary are oriented the way they were originally,
I don't see how anything you did can be at fault except the glued pads, and
I would check if the blobs' orientation coincide with the pads. If they are
at fault, it shouldn't be too big a deal removing them and reattaching with
silicone. I use aquarium silicone for all of my mirrors.
Dennis
John Moore
> I know some SCT owners who are reluctant to collimate their scopes - and
> believe me they need it - so I give you a lot of credit for taking a scope
> that you found unsatisfactory and trying to make it perform better. Given
> that the corrector and secondary are oriented the way they were originally, I
> don't see how anything you did can be at fault except the glued pads, and I
> would check if the blobs' orientation coincide with the pads. If they are at
> fault, it shouldn't be too big a deal removing them and reattaching with
> silicone. I use aquarium silicone for all of my mirrors.
>
Thank you for the remarks. If we are reluctant to experiment we make no
progress, and yes the scope was unsatisfactory as far as I'm concerned.
As clear skies are quite rare here at the moment, I'm about to colect a Foucault
tester from a friend in the next hour and follow Roger's suggestion of seeing if
the mirror is still spherical. It will mean stripping the scope yet again. but
I'm persuaded that's the best path.
Regards
John Moore
AstroApp
wrote:
>Thanks to all for their suggestions/comments so far (some rather critical). I
>feel I should explain why I made these modifications. I already have an Intes
>Micro MN78 which has taken some decent planetary pcitures (see
>http://tinyurl.com/ckxk5 ), but I am striving for better. Having seen some
>spectacular planetary images taken with a C11, I picked one up secondhand. I
>immediatley compared it with the MN78 on Jupiter and its performance was much
>worse, despite the much bigger aperture. Looking at the very glossy inside of
>the C11 I surmised that at least some of the problem was caused by internal
>reflections...
In my own experience, a telescope design such as an SCT with large
obstruction is not optimal for planetary observing. I have owned
three SCTs, at least six refractors, and probably eight Newts, I
believe; and the SCTs, despite other excellent virtues, were not
telescopes that ever gave me enjoyable planetary images.
My C-11 is collimated well so that in-and-out of focus diffraction
patterns look symmetrical and nearly identical. Yet, Jupiter is
"soft" and regularly disappoints me. I a direct "shoot off" between
the C-11 and a very well collimated, sharp figured 10" Newt with
smaller obstruction, the latter looked crisper but the diffraction
spikes from the spider were irritating...I preferred looking at
Jupiter in a lower power in my 120mm refractor. But, the best views
of Jupiter I've ever had, year in, year out, were always in
refractors, even achromats but preferably something like a 7" AP
StarFire, which--in perfect seeing--has such detail that one almost
imagines that it's a satellite image! Side by side SCTs -- Meades,
Celestrons, smaller Maks -- never look as good, bigger or smaller
aperture.
Now, I suppose it is possible for somebody to make a nice planetary
classical Cassegrain scope. But I do not think that commercial SCTs
are just the ticket for that type of observing.
Furthermore, with 11" aperture you will not get superbly sharp
planetary images except on rare nights with near-steady air. On those
occasions Jupiter might still *seem* almost perfectly steady and
detailed in a 4-6 inch refractor compared to your C-11 but not, of
course, if you run both scopes at "full bore", 50x per inch of
aperture (which, BTW, an 11" SCT can never do on a planet and seem to
retain sharp clarity.)
Furthermore, I find that Jupiter's detail tends to start breaking down
somewhere above 25x per inch of aperture though this will vary quite a
bit depending on the aperture diameter and any central obstruction in
the system.
So, I suspect that the problem here is that your original
determination that the C-11 was faulty or substandard is the thing you
might want to re-examine.
I notice that from night to night (observing at 3400 feet altitude in
the mountains, not far from Lick Observatory) that even though we have
a Meditteranean climate here in the SF bay area, and laminar airflow,
at least at high altitude sites, there is a HUGE variation in how
sharp things look in my C-11. I have to resist the impulse to "play"
with the scope or to feel let down. It's the air; and the large
aperture; and of course the slight extra image degradation of the
large central obstruction. When you improve each of these situations
the image gets better and better...but in the case of the C-11, you
can't change the optical design and size of the obstruction, so at
some point there is a limit to the way it will render the finest
detail at the highest magnification.
Yet I find that often I use mine at powers of 373x to 466x, to try to
see central stars of planetary nebulae, details in galaxies, etc. And
when doing so, on SOME nights the stars have nice, delicate, defined,
unbroken first diffraction rings; and on MOST nights they are fuzzy
blobs. So, I have determined that there is nothing wrong with my
telescope, nor its general collimation. Since the performance is good
but only in the confluence of weather and wind events that even here
rarely occur, I need not worry about endlessly modifying or tweaking
my scope: it's fine but simply will not show Jupiter the way a
friend's 7" StarFire will show it.
As to the internal reflections: well, for decades I struggled to try
to see IC 59/63 near brilliant Gamma Cass. I even once tried the 36"
refractor at Lick Observatory. Failed EVERY time; on Newts,
refractors, etc. At last I was able to see the faint nebulae using an
ancient 4" f/10 with thirty year old Edmund mirrors that had pitted
coatings: because the inside of the scope tube was nicely blackened,
and the light from Gamma Cass was relatively dim with this small
aperture, and did not affect my vision. I could see the nebulae,
finally. Then, I spent ten more years trying to see them with ANY
OTHER scope, and failed.
But, I *did* see them the first time I tried with my C-11. When Gamma
Cass was out of the field, by carefully choosing my eyepieces and
filters, I got an incredibly good view of both nebulae, much better
than before, and could actually recognize their shape. So, my
conclusion was that for visual use, my C-11's internal baffling is
quite satisfactory: again, it beats all the other scopes that I have
used to see particularly challenging, faint, pale objects that are
RIGHT near a blinding star. I have yet to try for Leo 1 but am
planning on it later this season. I have no doubt that it will be
accomplished: for the same reason. So I consider the baffling to be
more than adequate: better than my home made reflectors that I sweated
over for years, putting in flocking, using special paint, messing with
spider types, changing mirror coatings, etc.
In my opinion, for the kinds of observing I intend the C-11 to enable
me to do -- extremely faint extended objects of small angular diameter
-- it is almost perfectly optimized. It is also NOT perfectly
optimized for planetary viewing.
However, I have yet to find a telescope that does everything precisely
correctly. The solution I am favoring now is not to attempt to
redesign anything as complex as a commercial SCT, but merely to get a
couple of different telescopes to suit different needs.
I read the webpage you cite about modifying the C-11, shortly after I
acquired mine 18 mos. ago, and frankly was appalled by it.
Years ago I toured the Celestron factory and went into their sort of
low key clean room -- not exactly "NASA grade", but effective -- where
they laser-collimate scopes. I watched them adjust a small Mak using
900x, and saw that the Airy disk pattern was textbook perfect. I
realized then and there that no matter how much effort I'd put into
star testing at night, under any old sky conditions, I could NEVER
achieve that kind of accuracy and repeatability. It made me respect
the product, knowing that they were putting their scopes thru that
kind of test procedure after assembling the components.
If I were ever to undertake a program to modify my C-11, I would first
acquire optical bench testing components -- at least home made ones --
and a laser collimator. I would have a setup where I could
repeatably, carefully, analyze a perfect point source light so that I
could verify the condition of the scope BEFORE as well as during the
changes, and finally AFTERWARD when it was all finished and ready for
use.
But, I see no need. When my C-11 does produce those exquisite
textbook star images -- OCCASIONALLY -- then I can sigh with relief
and remember that it's an excellent optical system, and is working
correctly.
Now, this is not to say that any advanced observer can look through
one and not find details that fall short of perfection. But,
perfection is expensive. And, as you have seen, it is not achievable
by merely following some instructions given by any old web page,
without taking the precautions of having some kind of optical
reference test standards.
Your defocused image made me squirm with discomfort. At first, I was
thinking it might be due to axis alignment problems with your imager
but it is certainly far more likely that you have -- as several people
indicate -- pinched the optics.
The C-11's corrector plate is so sensitive to warping and temperature
change that I have noticed a very small aberration in star images when
I attach the heavy counterweight system that I purchased, and slide
the weight too close to the corrector. Furthermore, it is possible
that it can take MORE than just two hours to get the scope to total
thermal equilibrium. Often I find that stars just do not look near
perfect until my scope has been outside for five or six hours -- but
of course this could also be because my air seems to get steadier and
steadier as the night rolls on. Again: without a laser beam to test
the diffraction pattern, you cannot really tell by looking at stars,
because you CANNOT CONTROL THE AIR!
Your tone of despair was useful to me, at least. Reading of your
woes, and seeing your image of the terrible result, I could never be
tempted to take my C-11 apart and do anything to it...so, even though
it can't be of any consolation to you, at least I'd like to thank you
for telling us about the results.
I don't think "all is lost" but of course Celestron would be appalled.
Some further work will make this better and perhaps you can even end
up where you want to be (allowing of course for the fact that you
cannot reduce the size of the central obstruction.)
AstroApp
Trailer Trash ReUnion
lal_truckee wrote:
> RMOLLISE wrote:
> >
> > Anyhoo, the result of _fiddling with what don't need to be fiddled
> > with_ is all too often this: a once-lovely C11 with a mirror now FIRMLY
> > CEMENTED to its mirror holder. ;-)
>
> I don't think that's what he's described. If I'm reading correctly, he
> glue three small "tabs" to the back of the mirror for three thrust rods
> to bear against in an attempt to correct mirror flop, as done
> successfully by several web site publishers. Using an unfavorable glue,
> unfortunately.
>
> All is not lost.
>
> I don't recall if this has been suggested, but the OP may have "locked"
> the mirror with so much pressure as to distort it directly - if, so he
> should test with the thrust rods backed out.
>
I think that is exactly what he's done. Two distortions at exactly
the positions of the two installed lock rods and none at the third which
is thetangent arm itself so free of stress. Pretty simple really.
Roger Hamlett
"Trailer Trash ReUnion" <hog...@ai5.net> wrote in message
news:456D36DD...@ai5.net...
>
>
> lal_truckee wrote:
>
>> RMOLLISE wrote:
>> >
>> > Anyhoo, the result of _fiddling with what don't need to be fiddled
>> > with_ is all too often this: a once-lovely C11 with a mirror now
>> > FIRMLY
>> > CEMENTED to its mirror holder. ;-)
>>
>> I don't think that's what he's described. If I'm reading correctly, he
>> glue three small "tabs" to the back of the mirror for three thrust rods
>> to bear against in an attempt to correct mirror flop, as done
>> successfully by several web site publishers. Using an unfavorable glue,
>> unfortunately.
>>
>> All is not lost.
>>
>> I don't recall if this has been suggested, but the OP may have "locked"
>> the mirror with so much pressure as to distort it directly - if, so he
>> should test with the thrust rods backed out.
>>
>
> I think that is exactly what he's done. Two distortions at exactly
> the positions of the two installed lock rods and none at the third which
> is thetangent arm itself so free of stress. Pretty simple really.
locking screws slackened...
>> If it's determined that the glue itself is at fault, the OP needs to
>> remove the glue - I'd cut through each glue patch with a thin abrasive
>> thread (dental floss suggests,) removing as much as possible that way,
>> and retesting. Possible follow on dremel surgery will be required (with
>> a very delicate touch.)
Best Wishes
Baby Rasheed
Roger Hamlett wrote:
In this case "he said" and what he did are probably different things.
The patterns are self evident.
John Moore
"AstroApp" <Blo...@BLOCKED.net> wrote in message
news:dp2pm2dduihtfl3e2...@4ax.com...<snipped>
I have read your post with much interest. In everything you say I would agree
with you, but what really triggered me to try the C11 for planetary imaging was
the many first class images of Saturn and Jupiter taken by Damian Peach, many
from his home location here in UK. All were made with C9.25, C11 and C14 scopes,
and I think are probably among the best planetary images in the world. I don't
know how he does it, but he seems to be able to overcome the inherent
limitations of the large central obstruction!
There are still several other posters who clearly haven't read my posts, so I
won't waste time replying to them. I'm well down the road building a Foucault
tester at the moment, and hope soon to prove or disprove whether I have indeed
distorted the mirror, and whether it's recoverable.
John Moore
Roger Hamlett
any normal timescale, at low stress levels), does not 'flow', so if the
source of the stress is removed, it will recover. The flow that does
occur, is tiny (the old idea that windows in cathedrals etc., had visibly
'flowed' was the result of the builders setting the slightly non-flat
crown glass with the thick edge down, not actual flow).
Best Wishes
John Moore
> normal timescale, at low stress levels), does not 'flow', so if the source of
> the stress is removed, it will recover. The flow that does occur, is tiny (the
> old idea that windows in cathedrals etc., had visibly 'flowed' was the result
> of the builders setting the slightly non-flat crown glass with the thick edge
> down, not actual flow).
>
John Moore
Bryan
scopes doing astro-photography have large central obstructions. For visual
work, the large central obstruction _is_ a problem. My C11 gives nice views
of planets visually, but the slightly smaller Mak-Newt gives even better.
All this is due to the increased contrast of the Mak-Newt. Of course around
here, the seeing conditions are the larger factor in successful visual or
webcam images.
Bryan
"John Moore" <john...@iee.org> wrote in message
news:3NBbh.411$r95...@newsfe6-win.ntli.net...
AstroApp
wrote:
>
>"AstroApp" <Blo...@BLOCKED.net> wrote in message
>news:dp2pm2dduihtfl3e2...@4ax.com...
>> On Sun, 26 Nov 2006 10:17:24 GMT, "John Moore" <john...@iee.org>
>> wrote:
>>
><snipped>
>
>I have read your post with much interest. In everything you say I would agree
>with you, but what really triggered me to try the C11 for planetary imaging was
>the many first class images of Saturn and Jupiter taken by Damian Peach, many
>from his home location here in UK. All were made with C9.25, C11 and C14 scopes,
>and I think are probably among the best planetary images in the world. I don't
>know how he does it, but he seems to be able to overcome the inherent
>limitations of the large central obstruction!
Yes, I have followed Damian's pictures for a long time, and agree that
they are spectacular. In fact, every one of them surpasses anything
that I have seen BY EYE with my own C-11.
Why?
Well: I have given this a great deal of thought, augmented by my own
testing. It's largely due to (a) seeing; (b) his processing; (c) the
number of images he takes, over time.
I do not consider the C-11 that I own to have any optical compromises
that would PREVENT me from doing an excellent planetary image, if I
cared to work at it long enough to optimize every variable.
The way I have collimated my C-11, which I have done exactly two times
in the last 16 months, is by using an artificial star, which consists
of a small perfectly round transparent plastic globe, mounted on a
small base, which I take down the street to a friendly neighbor's
house and place on the top rail of his fence. It ends up being about
150 meters from my garage. I set up the C-11 in the shade, and
allowed it to acquire temperature equilibrium there for at least 3
hours. Then I collimated the scope using a 3.7 mm eyepiece at a
ridiculously high power: 757x. (I have actually used this
magnification to do such things as view the faint central stars of dim
planetary nebulae, so it's not as ludicrous as it sounds.) The VERY,
very slight adjustment that I made from the original factory settings
after the scope was shipped from S. Calif. to Watsonville, to
Cupertino, to my home in San Jose, were extremely slight: maybe 1-2
degree turns of one screw, and a 1 degree turn of another screw.
Just to satisfy my curiosity, a year later I checked again and found
that the collimation was NEARLY the same, requiring only approximately
a 1 degree turn of one of the three screws on the back of the
secondary.
Why people think that they need so-and-so's knobs to do this, so that
it constantly can be tweaked, is a mystery to me. In a years' time
the scope was not far enough off to be obvious to the eye in normal
use...
At any rate, the Airy disk pattern at very high magnification -- say,
466x, which I use all the time for faint small galaxies and
stellar-like small planetaries -- is excellent, either side of focus;
normally there is no pinched appearance and the surface looks very
smooth (allowing of course for various currents that are evident when
you look during the day. I might add that I chose a sort of cool day
to do the adjustment, when the outside temperature was not much higher
than 60F.)
So, when I look at Jupiter, and it fails to look to the eye like one
of Damian's photos, what is the reason?
Seeing.
Furthermore, Jupiter is very hard to magnify a good deal. You can
sometimes put 600x on Saturn; on Jupiter you get mush.
I would say that there are also the usual reasons that Damian's images
are so good: surely he must use a Hartmann mask for focusing; maybe he
does an even better collimation than I do, using a laser; he probably
works night after night after night, tirelessly, and throws away 100
shots for every one that is "great". Maybe the air in Selsey where he
works is even steadier than the laminar airflow near Mt. Hamilton hear
in N. California. I am sure he has worked out every conceivable issue
related to the digital sampling of his images, achieving state of the
art results; and his software processing must boggle the mind,
compared to the things that I have tried to do with low end graphics
editing programs or freeware.
So, even though the instrument that I have looks good, I can't even
BEGIN to duplicate the rest of Damian's fantastically optimized
procedures.
For instance: I tried exactly TWO TIMES to image Jupiter. Period.
The better night, I got the results discussed in the link below at the
end of the page, with a cheap webcam that cost me $30, and the free
graphics editing program that came bundled with my $29 Canon scanner,
plus some fiddling with Registax if I recall.
See:
http://home.earthlink.net/~steve_waldee/digital/webcam.htm
Look how AWFUL my Jupiter is, compared to Damian's. (I have to admit
that I have indeed seen *worse* -- and when I used to work at a
telescope dealer in the 1980s, customers frequently brought their best
tries to the store, using film photography: and my very novice digital
image is actually better than most of those were...)
Now, I am completely satisfied with the C-11 for the purposes that I
use it for. I don't do planetary imaging with it, aside from the
experiments shown on this page link above, but use it visually for
hunting down galaxies and other small diameter faint objects. I have
confidence, though, that if I took up deep sky imaging, it would work
fine. But, since I never see BY EYE anything that is even remotely as
sharp as the planetary images that Damian regularly publishes, I
wouldn't expect that my beginner's talents, and low grade peripheral
devices and software, would yield pictures that are remotely
comparable to his.
I rarely see Jupiter more than one night per year when it looks as
good as one of Damian's images.
Now, after being a bit let-down by the view in the C-11, perhaps I
take out my 120 mm refractor and look at Jupitert, and it almost
always seems at first to look better than it does in the C-11. Why?
Well: the image scale is much smaller. A very careful comparison at
exactly the same image scale by choosing appropriate oculars, and
keeping the magnification down to nothing higher than, say, 165x or
so, will definitely show that the C-11 looks BETTER: the image is
brighter; the details of loops, belts, festoons, ovals are clearer.
But, one is always tempted to take the Schmidt-Cassegrain scopes
higher than that: and the Jupiter image gets soft.
On the occasions I have fiddled with the achromat refractor to try to
capture a planetary image by webcam, the result was appalling: too
short a focal length, too dim an image; a useless picture. My Jupiter
pictures with the refractor weren't even worth trying to process and
improve; I haven't done much with them and have not bothered to post
them on that page. The C-11 Jupiter images were vastly superior....
Now, the night that I took the ones that I posted, the seeing was
perhaps 4 on a scale of 1-10, 10 being perfect. Why did I do an image
at all on that night? Simple...it was an experiment, to see how to
set up the whole thing. I found the result not so ghastly that I
couldn't share it with other novices. But it's not something that I'd
ever post to a Yahoo planet imaging forum such as the one where Damian
hangs out. Why bother?
The point of my article about this was to show ABSOLUTE BEGINNERS that
you can actually get something recognizable, using a webcam, the very
first time you try to image the Moon or an appropriate planet.
Since I rarely even look at planets -- they ruin my dark adaptation!
-- I concentrate instead on visual observations of extremely faint
objects. I save planetary observing for the end of a long deep sky
session, just before I put the scope away, if a planet is in the sky
at a reasonable altitude above the horizon. The last time I looked at
Jupiter on my mountain top sight, I did it twice, once using my "bad"
eye (my left, which I never use for looking at faint d-s objects in a
telescope eyepiece, so as not to ruin my dark adaptation) when Jupiter
was near the zenith, and then again when Jupiter was way down, up only
about 30 degrees, as I was ending my 'run'. The difference between
those two views, on the same night, was staggering. The planet looked
very fine, sharp, clear, and detailed at the zenith; and virtually
unrecognizable as Jupiter later on! I could only see the faintest
trace of one belt. I could not get an apparent sharp focus on the
limb; it was a blob that faded gradually into the sky background:
hideous, ugly, and discouraging. If I had used this to try to judge
my scope's optics, I'd have thought that the thing was utterly broken.
So, in my view, you still aren't really looking at the problem from
the right perspective.
BEFORE I would have taken the scope apart to modify it, I would have
spent about a year studying Jupiter visually. I would want to know
how often it looked really crisp, sharp, and detailed; would want also
to have a baseline for my local seeing conditions, and if I *ever* got
a crisp, sharp view. I'd also compare the scope performance with
other peoples' similar telescopes. I would then know if I really
could ever get a near-perfect view of the planet: the kind of result
that you have to get on the nights when you try to make a "Damian
Peach Rated" image.
You will notice, also, that searching for great Jupiter shots will
turn up a lot of them that are done in the Meditteranean region, or in
Florida, or in other climates where the air is exceptional. How is
YOUR seeing compared to this?
Now, once I had a baseline for knowing (a) how my scope performs
generally; and (b) how often I get "great nights" with near-steady
seeing and spectacularly detailed planetary views BY EYE, I would then
approach the job of analyzing any perceived shortcomings in the
telescope.
But, I'd do it with as much instrumentation as possible.
As far as the problem of baffling is concerned: you can do worst-case
tests with most reflectors and find issues, especially with Jupiter
and the Moon. There are ways to mollify internal reflections when you
take pictures of planets and the Moon. For one thing: you can make
improvements by processing the images, stretching the contrast range
and altering the sky background color and luminance around the planet.
I'd do that before I would dare to take apart my C-11 without proper
optical test equipment...
I think that what you possibly should have considered is to go through
a steady series of marginal improvements, small step by small step,
rather than to tear the scope down and make major changes all at once.
Maybe I'm being presumptuous, and you already did that.
Since planetary images benefit from the smallest possible exposure
time, I fail to understand the need to clamp the primary. Any attempt
to do so at the rear and periphery of the mirror, by ANY means, would
perforce pinch the optics. That would be a cure worse than the
disease. I think the idea of stabilizing the mirror for, say, a 2
hour film exposure makes sense. But, for 1/500th second snapshots of
Jupiter? Or for the short exposures of individual video frames? I
would not think this ever to be necessary.
And, did you make or acquire a Hartmann mask for focusing? I haven't
bothered to do that yet myself: and I tell you, it is a PAIN to fine
focus my webcam without it!
People like Damian Peach know so much about the fine details that it
is almost impossible for them even to TELL you how they do it! They'd
have to write a book -- and some have done just that (thinking of,
say, Don Parker.)
I have two very close friends who are superb deep sky photographers,
one of them being world-class, recognized by anybody who reads this
ng. I see what he has to do to produce his images: I have observed
with him. I know how he develops film, or works with digital
processing. It's simply beyond me, and I recognize that I WILL NEVER
GET THERE...period. It's not in me.
Yet, I could explain pretty much what he does; it isn't magic. But,
is one willing to set up experiments with micrometers and test the
adjustments of lenses to rotational increments of sub-thousandths of
an inch? Take months to do it? I won't, and can't: so even if I
bought the same gear, my results would be VERY casual and mediocre,
compared to his.
Now: as to the issue of the central obstruction.
Yes; it will degrade the ultimate contrast and resolution -- SLIGHTLY.
But, you will not be able to afford an 11 inch long focal length full
corrected triplet in a gigantic, lengthy tube that requires a mount
like one that would be used on a ten-tonne 19th century observatory
telescope! So, one has to put up with this SLIGHT degradation to get
the light-gathering, and resolution, of an 11 inch aperture.
I wouldn't worry about it: the very fact that Damian gets the results
he does, shows that the losses can at least partially be made up by
the processing.
You'd have to set up comparable telescopes in exactly equivalent
conditions to SEE and quantify the difference that the obstruction
made, anyway.
Best,
AstroApp/srw
AstroApp
wrote:
>
>The way I have collimated my C-11, which I have done exactly two times
>in the last 16 months, is by using an artificial star, which consists
>of a small perfectly round transparent plastic globe, mounted on a
>small base,
I should have clarified that this globe reflects a tiny image of the
Sun, so small that at the distance where I have the scope, it's barely
larger than the diameter of a very bright star seen at night. I am
not looking at the GLOBE: I am looking at the tiny reflected solar
image, a pinpoint of light. And, with the extremely high
magnification and small exit pupil, this light is no brighter to my
eye than, say, Sirius would be.
AstroApp
Bryan
then, very small adjustments are needed.
Bryan
AstroApp
>> I know some SCT owners who are reluctant to collimate their scopes - and
>> believe me they need it - so I give you a lot of credit for taking a scope
>> that you found unsatisfactory and trying to make it perform better.
HOW was the scope 'found to be unsatisfactory' -- that's the issue.
A C-11 out of the box is not likely to give a distorted pattern of the
primary mirror reflection like the one shown in John's link to his
image, which takes one's breath away. Mine shows a perfect, even,
smooth donut.
So, whatever small dissatisfaction that the scope SEEMED to give in
its planetary performance, as received, is nothing compared to the way
it's working now.
Was an artificial star used at first to check collimation? A laser
collimator? Were the optics checked on a bench? Those things would
have been useful BEFORE deciding to tear the scope apart and modify
it.
Now, presuming that all these techniques were used to examine the
optics, I can say from experience at least with mine that if the C-11
was (a) correctly collimated; and (b) had not had its optics altered
by, say, rotating the corrector plate or doing something else to the
interior, that any remaining "defects" would be very small ones that
would require quantification and measurement, step by step, to
eliminate.
This is assuming that the reason John was not getting images like
Damian Peach does is INDEED due to the C-11, and not something else
(which is a pretty bad assumption on its face.)
I still do not see why the primary mirror must be CLAMPED HARD into
position at three points when doing short exposures for planetary
imaging. The method used -- gluing things to the back of the mirror
and then screwing them down to something -- is a virtual guarantee of
stressing it. One could not clamp it this way and have it thermally
stable over time. Even the slight pressure exerted by screws hanging
off the back, not touching anything, would distort the surface of the
primary.
My scope's corrector plate is, in fact, SO sensitive to flexure that
if I slide my heavy counterweight up near it, along the bar mounted
underneath the scope, that I can see a slight deformation in the
pattern. I have to make sure the counterweight is at least 2 to 3
inches back.
This counterweight is not an official Celestron product so I have no
idea if the company that makes it is aware of that potential flaw in
the setup. I discovered the problem and made the correction; even so,
it does so little 'damage' to the focused image that it might be hard
to see the aberration except in moments of perfect seeing at extremely
high magnification.
This gives you an idea of how sensitive the optical components are to
stresses. Furthermore, if you defocus the image of a bright
artificial light source just enough so that you get a big round
"donut" that shows the surface of the primary, you can see stresses
and if there are any gross deformations (that's what John's image
shows.) In my case, if I do this when the scope is not thermally
stabilized, I can see ALL KINDS of defects, which gradually go away
and smooth out when all the components of the scope finally become
thermally stable. Now, that is the condition the optics must be in
when fine adjustments are made to the collimation or axial alignment
of the components, and that is the way they are set up at the factory.
Do we know how John determined his belief that the optics were
unsatisfactory, and if he considered any of these issues? I just
re-read the entire thread, and I'm not clear at all on this point. We
know that he got the scope second-hand; tested it "immediately"
against his other small aperture scope, and that the C-ll was found,
in his words to be "unusable without the modifications" and that he
"surmised" that it needed improvements in the internal baffling.
Well, it would be nice to know what EXACTLY was done to determine this
"finding" and to come up with this "surmise".
John's web page shows magnificent images of planets that he's done
with his previous scope, so one can assume that his judgment of what
looks good, and what doesn't, is sound. But, when you are modifying,
altering, redesigning, and realigning a Schmidt-Cassegrain - including
going so far as to GLUE STUFF onto the back of the primary mirror -
this is not a trivial matter, compared to merely collimating a scope!
The point that needs to be made strongly, for other SCT owners who are
dissatisfied with their planetary images when compared to what experts
who have struggled for YEARS to reach a pinnacle, is that when you are
tracking down and eliminating all the weaknesses in your setups, you
need to use some sort of scientific method, and to do repeatable
testing employing some kind of standards. That is, in fact, how the
scopes are designed, constructed, and aligned in the first place; they
aren't as easy to make work properly as home brew Newtonians slapped
together from mirrors ground at an astronomy club workshop!
For instance, with respect to John's "surmising" that the scope needed
improved baffling, what were his criteria? How did he come up with
that judgment? What experiments did he do BEFORE altering any of the
scope's design to determine this? Did he contact Damian Peach and ask
advice? What other information was gathered, aside from looking at
the web page that suggests a radical modification of the scope? Was
there any testing done with artificial light sources to determine the
significance of light scatter? Was critical visual observation done
of stars and bright planets -- with a variety of eyepieces to
eliminate THEM from being part of the problem -- to check on-axis and
off-axis internal reflections? Was there any comparison made this way
between HIS used C-11, and other owners' C-11s? We have no answers to
these questions I'm posing. These are some of the things that I think
would have been prudent to do before stripping the telescope apart,
and changing the design, and gluing clamps to the primary...
>As clear skies are quite rare here at the moment, I'm about to colect a Foucault
>tester from a friend in the next hour and follow Roger's suggestion of seeing if
>the mirror is still spherical.
John, you must get ahold of somebody else's C-11, and look at a bright
star slightly defocused and examine the smoothness of the image as the
scope cools down, PRESUMING that the seeing is good. If not, then use
an artificial light source. You can tell a lot about the smoothness
of the primary and corrector plate this way.
Secondarily, if you remove the corrector you'll get spherical
aberration which will cause a halo around the star images; but you can
probably still see if the primary is stressed.
AstroApp
John Moore
The Foucault test was not conclusive, maybe not sensitive enough, but I could
see no distortion in the mirror. Nevertheless I have now stripped the scope,
removed the plastic padding and metal plates (quite easy as it turned out) and
refitted them using Dow Corning RTV silicone rubber. I think this may be the
material that Celestron use, but am not sure. Last night was the first chance I
had to try another star test. I could see no signs of the previous distortion,
and was able to perform an approximate collimation using various eyepieces, but
seeing was quite bad. I think the problem's solved, but need another night to be
sure.
Just for the record, I may not have made clear my intentions in earlier posts,
but I want to use the scope both for planets and DSOs becasue I think it will be
good for both. For the former, I hadn't intended to publish my comparison with
the MN78 scope, but now do so - see:
http://tinyurl.com/yd4y9s
These images were taken as close together in time as possible, but the lower
altitude of Jupiter actually should have favoured the C11, as should its bigger
aperture. Yes, I do know that seeing is important, but this was a direct
comparison, and the C11 should have been at least as good as the MN78 but
wasn't. A comparison visually through the eyepiece gave the same result -
noticeably more contrast with the belts much more clearly visible in the MN78.
Looking down the C11 tube in daylight the next day the obvious reflectivity of
the tube seemed (and still does) like an obvious possible cause of the
difference. I hadn't checked the collimation of the C11 at this stage, but
believe that it wasn't far out: see for example:
http://tinyurl.com/yck5ku
taken some days later. In fact this image would have been quite a bit better if
it hadn't been marred by tracking errors, ascribed to mirror flop, and that was
the other problem with the scope. Judicious deconvolution has removed much of
the evidence from the presented image, but the original was much worse! For DSO
work I want to use autoguiding, as I have with other scopes. This is the reason
for the mirror locks, and is nothing to do with planetary imaging.
I can't do any testing with an artifical star (yes it would be a good idea) as I
live on a housing estate and there's no convenient line of sight around.
John Moore
AstroApp
John, et al.:
This reply of mine is all out of order because my ISP, Earthlink, for
some reason has completely stopped updating s.a.a. so I cannot get the
last posts to put my reply to them, in proper time relationship. I
had to take John's followup off the Google archive, which IS up to
date; but because I don't have a Google usenet account, am putting my
reply to it here.
John Moore wrote:
>
>The Foucault test was not conclusive, maybe not sensitive enough, but I could
>see no distortion in the mirror.
Glad to know that you have eliminated the flexure.
>Just for the record, I may not have made clear my intentions in earlier posts,
>but I want to use the scope both for planets and DSOs becasue I think it will be
>good for both. For the former, I hadn't intended to publish my comparison with
>the MN78 scope, but now do so - see:
>http://tinyurl.com/yd4y9s
>These images were taken as close together in time as possible, but the lower
>altitude of Jupiter actually should have favoured the C11, as should its bigger
>aperture. Yes, I do know that seeing is important, but this was a direct
>comparison, and the C11 should have been at least as good as the MN78 but
>wasn't. A comparison visually through the eyepiece gave the same result -
These pictures confirm EXACTLY what I thought. I am an observer of
Jupiter, visually, for about forty years, and can say with complete
conviction that you are troubled, here, by seeing problems, the planet
being low in altitude above the horizon (you are in fact stipulating
that it is only about 20 degrees up, I believe.)
Seeing turbulence becomes apparently and significantly WORSE with
larger apertures. Furthermore, adding a central obstruction -- in
conditions of bad seeing -- will wreak havoc with planetary image
detail, especially that of Jupiter. Furthermore, at low elevation
then you are adding the final unsolvable problem to the combination of
difficulties: the denseness of the air and the diffusion that it
causes.
If you recall one of my recent comments, I compared a view of Jupiter
when it was nearest the meridian, with one when it was somewhere
between 30 and 20 degrees above the horizon, at the site that I use
that is 3,400 ft. altitude not very far from the famed Mt. Hamilton
locale of Lick Observatory (well known for the clarity of its air and
for high quality early observations of the planets when the 36" and
12" refractors were first installed.)
Jupiter was just about like your picture, when it was low in the sky.
But a few hours earlier that same night, when it was higher up, it was
spectacularly clear and sharp. Same place, same night -- different
altitude.
Furthermore, the larger aperture scope, with probably a larger
percentage of obstruction in its secondary than your Mak, is more
sensitive to the air currents and turbulence of the bad seeing than a
6" scope. This increase in sensitivity is not, in my experience,
"linear" with respect to an increase of aperture size; it suddenly
gets very much worse as you go from (say) 6 or 7' aperture, to 10" and
larger.
So the image fuzziness is more than 40% worse in a 10 inch aperture
scope, than it is in a 6 inch aperture scope. It's hard to quantify
this as it's a sort of approximation of your perception; but I'd guess
that the larger 10" aperture will seem *several times fuzzier* in bad
seeing than the 6" will.
All things being optimal -- collimation; optical refinement, etc. --
in perfect seeing of course the 10 inch scope will have much higher
resolution. But, nothing up only 20% above the horizon will be a
useful object for determining that. Anything down that low will be
HORRIBLY affected by atmospherics, and significantly extinguished in
contrast and brightness. At my location, I can barely see galaxies
that are up only that high; bring them up 20 degrees higher and they
show up in profusion. And, I look directly out at the Pacific ocean
after a precipitous drop off from 3,400 feet, so am not looking at air
that is over a city.
I don't know where you are, but in my experience it is inconceivable
that planets will ever be sharp and clear enough at 20 degrees
elevation either to study them visually, or to image them.
Large aperture scopes -- and I'd call the C-11 "large" -- demand
perfect seeing to achieve their diffraction limit. When I owned a
17.5 inch Dob, I made an apodizing screen for looking at double stars
or planets, because when used full-aperture, the scope was very fuzzy
and indistinct on small angular objects, especially bright ones, 98%
of the time. But alongside my large Dob, a friend's 7 inch aperture
StarFire apo refractor would look amazingly sharp if not flawless,
full aperture. That's the difference between an unobstructed small
aperture scope, and a very large obstructed aperture scope, in
imperfect seeing.
On very rare occasions of flawless seeing, however, I could discern
the central star in M-57 in the big Dob, but it wasn't detectable in
the 7" StarFire.
Therefore there wasn't anything particularly wrong with the optics of
the big scope; it merely gathered light over too large an area to
achieve its diffraction limit under poor seeing conditions.
In examining your C-11 and Mak images, I also perceive a very obvious
difference in the signal to noise ratio, and perhaps also in the image
scaling you have had to do to make the two about the same size: the
Mak picture has what looks like unsharp masking artefacts, or perhaps
at least some 'pixellization' or sampling problem. It LOOKS seemingly
sharper; but is this perhaps due to artificial edge enhancement that
has occurred when the picture was resized to look the same as the
C-11's image?
From my not very considerable experience doing imaging, I would say
that if these are single exposure shots, then if you were to take
videos, and use Registax to throw away inferior frames, and then do
the steps of Registax wavelet processing on composited frames, you'd
come up with a lot better picture than the one on the left. In fact,
in the imaging experiments I've done with Jupiter, raw frames looked
individually slightly WORSE than this single C-11 frame of Jupiter
that you post; but the registered, processed, composite image was
vastly better in detail, S/N, and contrast.
>Looking down the C11 tube in daylight the next day the obvious reflectivity of
>the tube seemed (and still does) like an obvious possible cause of the
>difference.
Well, it may seem so to *you*. But it does not seem so to me when I
do the same experiment with mine. I can assure you that Jupiter DOES
look like your example, above, in bad seeing and low altitude
placement; but in excellent seeing when the planet is high, it does
NOT look like that at all in MY C-11. And I have made no modification
to mine.
> I hadn't checked the collimation of the C11 at this stage, but
>believe that it wasn't far out: see for example:
>http://tinyurl.com/yck5ku
A Schmidt-Cassegrain scope -- and I've owned several of them dating
back to the 1980s -- is in fact, in my experience, more sensitive to
collimation errors than a Newtonian. The first thing you will notice
on a very slightly mis-collimated SCT, viewing Jupiter, is that the
limb will be unevenly illuminated and not sharply distinct; there may
be a slight halo of light on one side. And, YES: the contrast of the
disk's details will go all to hell. You will not be able to see ovals
and festoons, but smears.
Under nearly perfect California skies -- which are rare here too -- I
have seen loops, festoons, ovals, and small details that are
definitely the kinds of details that Damian's pictures record, though
they are far paler and more subtle in a live view thru my C-11 than
they are after contrast-stretching, unsharp masking, and all the other
processing steps that I'm sure he uses. But, the "raw data" are
present to the eye. However, in fact I have found that I have seen
such detail much BETTER in a 7" StarFire apo at the same site. When
Shoemaker-Levy slammed into Jupiter, the view the next night was an
awesome experience, not greatly inferior to the monochrome images from
the Hubble that soon were published. My wife Regina, my friend Rich
(owner of the scope) and I were struck almost dumb by the sight. This
was of course many years before the C-11 was introduced so I could not
make a direct comparison; but I can say that at the time, my excellent
10" f/5.6 Dob, with fairly large central obstruction, could not hold a
CANDLE to the view in the unobstructed 7" refractor.
That being said, however, whatever loss of ultimate contrast and
sharpness of fine planetary detail is caused by the secondary
obstruction of the C-11 is entirely swamped and rendered
inconsequential by looking at the planet at LOW ELEVATIONS.
> In fact this image would have been quite a bit better if
>it hadn't been marred by tracking errors, ascribed to mirror flop, and that was
>the other problem with the scope.
But...it is just this accumulation of errors that will reduce your
ultimate planetary resolution. There is a good deal of tracking error
in your M57, more even than in the very first attempt I ever made to
image M57 with my C-11, with a friend's loaned Meade DSI imager. I
can't quite tell also if there is a focus problem or if the star
images are blurred due to a combination of overexposure and tracking
error; but the stars in your picture of M57 are sufficiently blurred
and smeared that I wonder if this is also an evidence that something
is not quite right in the way you have focused your Jupiter image?
Are you using a Hartmann mask?
>the evidence from the presented image, but the original was much worse! For DSO
>work I want to use autoguiding, as I have with other scopes. This is the reason
>for the mirror locks, and is nothing to do with planetary imaging.
Before I would want to "hang" screws on the back of the primary, via
blobs of cement, in order to lock the mirror for long exposures, I
would want to do a careful examination of the Airy disk concentricity
with, and without, the locking mechanism, to make sure that the cure
is not worse than the disease.
If you ever do get your Foucault testing procedure worked out you will
instantly be able to see that the slightest pressure, even blowing
your breath strongly on the back of the mirror, will show up as a
significant alteration of the smoothness of the surface test. A tenth
wave mirror will instantly be altered in performance, robbing it of
resolution. I really suspect that you will not, by the means of
clamping your mirror DIRECTLY on its physical substrate (the back of
the glass) be able to then use the scope for ultimate high res
planetary imaging. You may indeed need and want TWO scopes for this
purpose: one that has been optimized for long exposures by eliminating
all potential causes of mirror shift due to enclosure stresses and
gravity, and one that has a relatively free floating mirror just for
short exposures.
That is, if you want planetary pictures as good as Damian's!
>I can't do any testing with an artifical star (yes it would be a good idea) as I
>live on a housing estate and there's no convenient line of sight around.
OH: I see that you are using the telescope "on a housing estate".
That means, inevitably, you are looking up at the sky in a region of
air that is prone to local air turbulence due to chimneys, roofs,
asphalt or concrete road heat radiation, etc. I have rarely been able
to get my sharpest planetary images when using my telescope at home.
Take it to a large grassy field, far away from houses -- especially in
winter time. If your eyes could ONLY SEE the wavy heat radiation from
all the roofs and chimneys around you, you'd never even consider
trying to look through that at a planet!
At the Mars opposition in 2003, my wife and I went to a semi-rural
school high in the wooded hills of the San Francisco peninsula for a
Mars observing party. Many fine telescopes were set up, including a
superb large aperture refractor (8", large that is for a refractor!),
various Newtonians, and a Meade 12" GOTO SCT. Unfortunately all were
located on an asphalt parking lot and many of them were pointed right
over the roof of the school. The WORST views of Mars were thru the
large refractor, the Meade SCT, and the Newts. The best view of Mars
was with a 4" refractor that a very intelligent user had set up far
away from all the other people, so that it was (a) off the asphalt;
and (b) not pointing over the roof. I had one of the very finest,
clearest views of Mars I've ever achieved, and went home and made a
drawing from memory that turned out to be pretty reasonable, compared
to a computer simulator image. I had exactly the same kind of
comparative experiences that I am trying to describe here: the larger
aperture scopes showed a sometimes very unstable, swimmy, shimmery
image with barely any distinction of the dark markings against the
blindingly bright orb. The small refractor, with no filters of any
kind, had a sharp disk with nicely etched limb, and very clearly
discernible albedo features; it did not constantly swim around and
waver the way almost all the other telescopes showed the planet.
I sense that what I am trying to say seems unconvincing to you; so be
it; I think I have put forth all that I know from practical
experience, which certainly can't hold a candle to what the expert
planetary imagers have learned. I can say that one of my friends, who
is a world-renowned deep sky photographer and digital imager -- with
an impressive website and international recognition and publication of
many of his pictures -- tells me that he does NOT do planetary
imaging, considering it "another thing altogether" and requiring an
entirely different approach, and different equipment, than taking d-s
shots. The two tasks are almost orthogonally related, and are
distinct specializations in which optimal techniques are often
entirely different in approach and execution.
AstroApp
John Moore
Thank you for your lengthy and helpful reply. I'll try to respond:
The gist of your comments is that the C11 is actually expected to give a worse
image than the MN78 on a planet under such poor observing conditions. I will
accept your advice on this. I hope soon to start on Saturn (when I can get round
to getting up at 4am on a cold morning!), and it will be interesting to see. An
interesting experiment next time might be for me to stop down the C11 while
making such comparisons.
> In examining your C-11 and Mak images, I also perceive a very obvious
> difference in the signal to noise ratio, and perhaps also in the image
> scaling you have had to do to make the two about the same size: the
> Mak picture has what looks like unsharp masking artefacts, or perhaps
> at least some 'pixellization' or sampling problem. It LOOKS seemingly
> sharper; but is this perhaps due to artificial edge enhancement that
> has occurred when the picture was resized to look the same as the
> C-11's image?
>
I think in turn you have to accept that I do have some experience in planetary
imaging. Both images were constructed from quite long avis (I forget now exactly
how long, but certainly a few minutes, and obviously both the same length) by
aligning and stacking the same fixed percentage in K3CCD Tools. This is my
normal initial procedure. I am also very familiar with Registax, but on this
occasion did not use it because both images have had no further processing or
enhancement of any sort. That would have been silly. Also both images were very
carefully focussed. I have used a Hartmann mask in the past and not found it
helpful on planetary images. I focus very carefully manually on a blown up image
on the screen. Again I think if you look at my website you will see that I know
what I'm doing in this area at least.
The MN78 image is indeed noisier. I think this is because the much smaller
telescope aperture forced the use of more gain to achieve the same level from
the webcam. I read no more into it than that.
>> In fact this image would have been quite a bit better if
>>it hadn't been marred by tracking errors, ascribed to mirror flop, and that
>>was
>>the other problem with the scope.
>
> But...it is just this accumulation of errors that will reduce your
> ultimate planetary resolution.
>
Here I think you are wrong. Exposure times on planets are in the region of 1/50
second for ecah frame and are unaffected by slight but smooth movement. In fact
slight drift over a few minutes helps to smooth out any problems caused by dust
in the camera.
> Before I would want to "hang" screws on the back of the primary, via
> blobs of cement, in order to lock the mirror for long exposures, I
> would want to do a careful examination of the Airy disk concentricity
> with, and without, the locking mechanism, to make sure that the cure
> is not worse than the disease.
>
The screws aren't on the mirror, just small plates of stainless steel. But yes I
agree, I will be making a comparison of the star pattern with and without the
locks done up.
> OH: I see that you are using the telescope "on a housing estate".
> That means, inevitably, you are looking up at the sky in a region of
> air that is prone to local air turbulence due to chimneys, roofs,
> asphalt or concrete road heat radiation, etc.
>
Yes, I know. For that reason most of my planetary imaging is done late at night
after most local heat sources have died down. Even then it's far from ideal.
> I sense that what I am trying to say seems unconvincing to you;
>
On reflection, I accept your main point. I just hope that whenever I next get
good conditions the C11 performs as well as it should!
> many of his pictures -- tells me that he does NOT do planetary
> imaging, considering it "another thing altogether" and requiring an
> entirely different approach, and different equipment, than taking d-s
> shots. The two tasks are almost orthogonally related, and are
> distinct specializations in which optimal techniques are often
> entirely different in approach and execution.
>
Yes, I think so. In fact I haven't sold the MN78 and probably won't just yet!
Regards
John Moore
AstroApp
Still my ISP has not updated s.a.a. so I am taking the Google archive
post and responding to it --
>Thank you for your lengthy and helpful reply. I'll try to respond:
>The gist of your comments is that the C11 is actually expected to give a worse
>image than the MN78 on a planet under such poor observing conditions.
That tends to be something close to what I think I would claim, when
comparing just average to below average seeing conditions on Jupiter
or Saturn, with my 120 mm f8 achromat, and my C-11, side by side.
Of course one never quite gets exactly the same magnification in both
scopes, not having enough eyepieces to get even within 5% of the exact
same magnification; so allowing for this variance, the small aperture
unobstructed scope -- even one with only a decent achromat objective,
not an apo -- tends to look sharper.
Then on rare occasions the C-11 will "do its stuff" and reveal a very
detailed planet, with loops, festoons, ovals, etc. When the second
little red spot appeared, it was seen in both of these telescopes
though much more distinctly in the C-11.
That being said, a 7" StarFire apo blows them both away, so much so as
to cause me almost to want to shed tears! Yet, since I cannot see
15th-16th magnitude galaxies in such a scope, but CAN see them in the
C-11 (with great diligence and care), I bought the scope I really
*need* for making such observations. I never intended the C-11 to be
my all-time greatest planetary scope and in an amusing way I had that
"anti-wish" granted.
Damian's and others' pictures with the C-11 are as amazing to me, as
they are to you. In my own case there was an order of magnitude
improvement when going from my own modified, obsolete webcam to a
Meade LSI CCD, which I have documented in an article about novice
attempts to do digital imaging. The LSI had so much more critically
sharp detail registration and color reality, compared to the mushy,
strangely hued and washed-out rendering of the webcam (mine is sadly
not the equivalent of a ToUCam) that my own 'mediocre' Jupiter shots
were probably compromised more by the webcam's limitations than the
seeing or the telescope. My Mars shots with the LSI were in fact
equal in many ways to the ones done by an expert, a friend of mine who
works for the company selling the American "Orion" planetary imager.
So, if I were to want to get a huge step forward I'd merely invest the
small amount for a new Philips webcam with modifications, or the even
smaller cost of a Meade or Celestron planetary imager...
>I think in turn you have to accept that I do have some experience in planetary
>imaging.
Well, no significant background about this was initially forthcoming
in your request for advice; only a specific link direct to a JPEG
picture of the resulting telescope picture of a defocused image of the
primary showing a light source of some sort. Eventually after the
thread had grown by some amount I did some research on the Net and
went to your personal website and looked at your excellent pictures
with the smaller scope, which confirms what you say. You must accept
my apologies for not intuiting this all at once. Furthermore, you did
not indicate until just now exactly how your comparative pictures of
Jupiter were recorded or processed. Since the C-11 picture was
inferior even to the wretchedly amateur, novice one that I did with
the cheap webcam and C-11, one can further be forgiven for not
understanding that you have indeed taken all appropriate steps to
understand the complexities of all the issues related.
But, your name was unfamiliar to me at the outset of your first post
of inquiry, and your extant images of excellent quality with the small
scopes too were unfamiliar until I found and examined them by my own
efforts to locate them.
Those of us who have come to photography and/or digital imaging after
decades of observing visually (in my case back to the late 1950s for
the solar system, and to about 1976 when I started exploring the deep
sky with a 10" telescope) more or less expect to find an understanding
of the seeing conditions, and optimizing them specifically for the
planets, learned over a great deal of time at the eyepiece, to be a
pre-requisite for being able to do imaging that is world-class in
quality.
The idea of trying to image Jupiter when it is up only 20 degrees
above the horizon would frankly not occur to a visual planetary
observer. Only one time in my entire LIFE have I had an occasion to
see Jupiter down this low, and to be able to discern the belts, ovals,
and colors. It was around thirty years ago, using a friend's 14" f/6
Newtonian from a mountain top site at about 2,500 feet above sea level
on the California coast. A rare stillness of autumn air near the
horizon was present and despite the denseness of the atmosphere,
Jupiter was simply amazing: perhaps the second best view of my life,
the best being with the StarFire right after the collision of
Hale-Bopp. Views of Jupiter that I've had with the 36" refractor at
Lick Observatory, during some years in which my wife was the
co-director of Lick's Music of the Spheres concert series (and thanks
to which we both had some extra opportunities to use the scopes) never
equalled either experience.
That being said, Damian's pictures rival those recollections that are
still brilliantly etched in my memory. Little wonder that his C-11
pictures of Jupiter are so enchanting and bewildering.
I personally have a fairly clear idea of what would be necessary to
START to get results that would lead me in that direction, using the
C-11.
First, of course: change the imager. And the Meade LSI or Celestron
equivalent, at only about a hundred pounds' cost, are toys compared to
what the "professional amateurs" will be using, I suppose.
Second, quit using my free graphical processing software --
PhotoStudio 5, which came with my scanner -- and opt for Photoshop
with the various plugins devised for astronomical use by experts.
Perhaps use something like Maxim for acquiring and doing intial
processing of the data from the imager.
Third, unlike your experience with Hartmann masks, mine has been more
hopeful. I have not yet built one for my C-11 because, frankly, I
haven't taken the necessary steps to do high grade planetary work.
But I've worked alongside another amateur who does his focusing that
way, and saw the improvement that it wrought. Perhaps this does not
work well on a 6" scope. With a 10" scope or larger, it helps a lot.
Instead I use the Windows magnifier to blow up the screen display
while focusing; but the processing overhead in my slow PC causes a
delay that leads me often to overshoot the right focuser adjustment.
Fourth, my 667 MHz Pentium III will *have to go!* It does use XP Pro,
but is slow despite having a whole gig of RAM. I did make an
improvement by installing a USB 2.0 card; but later motherboards with
built-in USB 2 have better drivers and superior performance. I get,
for example, a VERY slight artefact of horizontal fringing, even thru
the USB 2 adaptor; with the motherboard's default USB 1.0 connector it
is in fact so terrible that I would not use it ever for serious
imaging with my USB 1-era webcam.
My friend who helped test and develope the Orion planetary imager too
had this experience with his HP Laptop: the same imager with USB 1 had
the artefact, which completely goes away with USB 2 (as many people
have noted in web articles that have recently appeared.)
Fifth, I find that my Celestron GPS 11 alt-azimuth mount, no matter
how carefully I do the alignment procedure, does after a while reveal
tracking errors on the planets. I suspect that the algorithms in the
software are rudimentary. Tracking on planets *seems* to me to be a
little worse than on deep-sky objects; that, or perhaps it's just that
one uses more magnification on the former and notices it quicker. My
German equatorial scopes, carefully aligned but using only rudimentary
drives, could hold (say) Jupiter or Saturn steady for hours and hours.
I have never been able to get the C-11 to do so for more than about a
half-hour to 45 minutes; so there is constant creep that must surely
have to be dealt with. I believe I read somewhere in the discussion
that you, John, are using a Losmandy German equatorial mount so you
are way ahead of me on this point...
As far as the actual telescope itself is concerned, the locale where I
live -- at 90 meters altitude in the famed silicon valley -- is
inadequate except one or two rare nights per year, and one is seldom
lucky that Jupiter or Saturn are optimal then, if visible at all. The
mountain site that I use for deep-sky observing, at 900 meters, is
vastly better; Lick Observatory's Mt. Hamilton site at 1100 to 1200
meters, is better still. But the agony of driving there, with 350+
hairpin turns, makes me so nauseous that I have no inclination to fuss
with a telescope when I get there! I frankly don't understand how
anybody who does not STOP there for two days to get over the trip, can
do either any observing or imaging with satisfaction!
The only time I have had a "state of the art" Jupiter view here in San
Jose, at or near my house, was once about 18 years ago at a nearby
large grassy park, when Jupiter was being shown in a 3.5" aperture
Questar at an amazing 300x magnification: it was a bit shimmery on the
limb but had so much detail that I was speechless. Of course, as
usual, other people at the star party -- with their big Newts and C-8s
-- had the usual "Jupiter mush" that is overmagnified in an attempt to
impress the novice members of the public. Only the view in that tiny
Questar was good -- in fact, as I said, it was staggeringly anomalous,
unexpected, and -- for the next decade or more in my own life --
unrepeatable by ANY instrument I had access to!
Maybe Damian has a rare confluence of site, seeing, and equipment that
gives him more than the usual opportunities of nights like that...
>The MN78 image is indeed noisier. I think this is because the much smaller
>telescope aperture forced the use of more gain to achieve the same level from
>the webcam. I read no more into it than that.
>
I do think that the presence of noise in an image leads the brain to
*believe* that it is perceiving more detail. High end audio
restoration engineers, working with original shellac disk 78 rpm
recordings, have long known this. If all traces of the high frequency
background hiss are totally suppressed -- even if the bandwidth up
through the fundamental frequency regions is linear -- the listener
tends to think that the recording sounds DULL, muddy, and indistinct.
When the signal is examined it is often found that anything above 9
kHz or so is merely transient noise impulses, unrelated to musical
information or harmonics. Yet: take this OUT totally, and the brain
thinks the signal is "dull" and will, in many cases, prefer a wideband
signal INCLUDING the hiss. I have a great deal of experience in this
field as it was part of my profession for decades as an audio engineer
working with noise reduction system developments professionally.
I know a neurophysiologist who is a professor at NYU, working in the
area of studying the neurophysics of sight. He is also an audiophile
and absolutely, completely agrees with me about the hypothesis that
NOISE is under some conditions -- critical threshold, frequency
response, rise time -- intepreted as being part of the actual
information signal, and can even assist in the mental correlation of
patterns and discernment. Ergo, filtering out noise will not
necessarily improve the perception of detail in either a visual, or an
aural, representation.
Furthermore, noise either added, or intrinsic, in a signal otherwise
lacking in details in the noise frequency spectral domain, may be
perceived as being helpful! It tends to increase the sensation that
the representation has more, not less, detail. And if without it, the
signal (visual or aural) seems lacking in detail, then the brain will
interpret a certain amount of added noise as being IMPROVED DETAIL.
This sounds preposterous to people who haven't either studied the
experiments and literature, or who haven't done them themselves. But,
remarkably, it's true.
I believe that your small telescope image that has more noise also --
for various reasons -- seems to have more contrast as well. Perhaps
it was the seeing; perhaps the scaling; perhaps the processing was a
little different. But the totality of the two pictures, as
represented, causes the viewer to conclude that the small telescope
Jupiter is more detailed, and more vivid, than the C-11 Jupiter.
So, by definition, the small scope image merely IS more impressive,
since the viewe thinks so at once.
But, the only way to KNOW what the actual data are would be to examine
one single frame of each picture, totally unprocessed. However, this
would be extremely hard to do scientifically and accurately, since the
image scale is greatly different; you'd have to try to look at
different sized pictures and compensate mentally (impossible to do
repeatably, with certainty) or to scale the big one down, or the small
one up. In any of these three cases, an absolutely equivalent
impression cannot be gained; some difference related to the image
scale will have to be re-interpreted.
So even here there is no exactly scientific, equivalent way to compare
the two pictures in order to quantify, repeatably, how people will
critique the detail in the two pictures; and more importantly, to
extrapolate meaningful information about the EXACT intrinsic data of
each one.
My guess is that despite your best efforts to match them, that the
small telescope picture DOES indeed have a difference caused by
processing that "washes out" other differences in the data with
respect to the seeing, the image scale, and the ultimate S/N ratio.
Ergo, one can only crudely decide that the C-11 data are actually
INFERIOR to the data from the smaller telescope. Yes; in this one
experiment -- with all its myriad steps of integration -- the pictures
look different. But, what does this tell us SCIENTIFICALLY about the
C-11's performance?
I would claim that it can be judged repeatably, for the purposes of
making systematic improvements, ONLY by using an artificial light
source in a carefully controlled experiment where you can at least
attempt to find most stable thermal conditions of the air between the
telescope aperture, and the light source. You cannot do that when
photographing a planet, because not only does the seeing vary over
milliseconds' time, it also varies unpredictably over longer periods
since the planet is constantly changing altitude, going through
patches of air with different temperature and wind conditions, and
being affected by local conditions near the aperture (and in the
instrument itself.)
You can of course test by making dozens or hundreds of photographs,
spaced as closely as possible, on MANY different occasions, and then
trying to piece together a mental construct of what you perceive as
general trends. To quantify these with precision would require an
extraordinary degree of precisely referenced data gathering and
weighting. In fact, such tests were indeed done by professional
astronomers when they chose sites for observatories (and I've read
about them in professional journals.) But nothing an amateur has done
can compare, in my experience researching the subject.
So, ultimately, the planetary images are done more as achievements of
art than strict science. They obey a general law of randomness. That
some tend to be better than others is reflected in analyzing the
techniques of individuals. I just don't quite feel convinced that
your examples here show enough data about the performance of the C-11
to convince me that *the telescope is flawed*.
- We don't know if you have verified the collimation; indeed I believe
you say that you haven't done so, critically;
- We have only one example done with Jupiter was only up from 20 to 23
degrees above the horizon, comparing two telescopes with radically
different focal lengths, light gathering, and image scale;
- Since the data were gathered by instruments with significantly
different optical performance (much dimmer light, over a smaller area
of the CCD, from the small scope), it was necessary to adjust the
images to the same size for your comparative pictures. Are you
absolutely certain that this process did NOT alter the contrast, the
edge regions, the overall sharpness, either for the better or worse?
We only have this one example so can't get further ones to see any
trends.
- We do know now that you are observing in an area of population where
there are heat sources, and possibly variances in local near-ground
humidity. You say you try to do this late at night, when things have
stabilized for the better, but how are you able precisely to control
this? One cannot. Again, one gets random results and can only judge
trends from many experiments.
- We don't have any assurance that you have also taken the effort to
compare another person's near identical C-11. We know that you
purchased yours USED. What measurements have been done to verify that
when you got it, it was delivering the optical performance that
Celestron would have guaranteed to the original purchaser when it was
brand-new? What istruments were used to test the figure, the
collimation? Is it fair to expect that a used C-11 is not going to
vary -- or that YOUR specific, particular used C-11 won't vary -- from
the performance of Damian's C-11?
I don't want to drive this discussion into the ground but might only
urge patience and diligence as you extend the experimental process
further. And I hope you are not offended if I don't *appear* to be
doing justice to your experience, knowledge, and skills. I have only
looked at a few of your pictures, for a minute or two, and have only
your words IN THIS THREAD to go by, in order to judge your lifelong
involvement in observing and imaging.
I will, I assure you, spend more time on your articles and in studying
your examples.
Best,
AstroApp
AstroApp
>> blobs of cement, in order to lock the mirror for long exposures, I
>> would want to do a careful examination of the Airy disk concentricity
>> with, and without, the locking mechanism, to make sure that the cure
>> is not worse than the disease.
>The screws aren't on the mirror, just small plates of stainless steel. But yes I
>agree, I will be making a comparison of the star pattern with and without the
>locks done up.
There certainly will be a different temperature coefficient of the
glass, the glue, and the steel. How are you going to control that?
Furthermore, how did you determine, when you glued the plates on, that
the mirror had relaxed all stresses and was ABSOLUTELY thermally
stabilized over its entire surface, or that the application of the
glue and plates did not introduce the same?
I can guarantee you, judged from my own personal experience acquired
during the Foucault testing of my home made 8 inch scope, and in the
testing of the Carl Zambuto primary that replaced the original optics,
that even BREATHING on the mirror will be seen as rippling effects in
the Foucault test. Touching the mirror -- I shudder to think about
it! What's even worse is GLUING something directly to the back of the
mirror, at three places no less! As the glue dries there will no
doubt be differential stresses that are not canceled. In fact, some
people claim that glass over long periods of time will acquire these
stresses permanently.
I tend to agree with that theory. The 8 inch home made mirror in my
f/5.2 photographic Newtonian looked very good when first figured. By
the time I replaced it with a mirror made by my friend Carl Zambuto,
in the early 1990s, the mirror's figure looked really awful. Somehow,
over a ten year period, the mirror "deteriorated". My conclusion was
that the original blank was unstable; and that years of being used in
a cheap three-point cell had caused its original good surface figure
to deteriorate until it had various aberrations including many waves
of error.
So, I'd be afraid that, over years of time, things glued to the back
of your Celestron primary *could* arguably affect its figure,
permanently. I'd want to consult with experts at Celestron and also
-- to get corroboration -- other makers, both professional and skilled
amateur -- before I'd ever decide to glue things to MY mirror's back.
Then, I would wonder what efforts are used to insure that the pressure
applied at several points is identical, or to even KNOW what the
stress related figure-altering properties would be? My guess is that
if you set the scope up as is it now modified, with the corrector
plate/secondary assembly removed, and adjust these "locks" while you
look at the Foucault pattern, that you will be *greatly surprised*,
shocked, and disturbed by the way it is degraded when any of the
screws touches the back of the plates on your mirror. That is, if the
glued down plates have not already introduced permanent stresses.
And, these will perforce CHANGE continuously due to the temperature
coefficient differences in the glue and the steel. The more I think
about this, the more uncomfortable I get about it.
AstroApp