Field Flatterner w/out Focal Ratio changes
Paulie
Field flatteners are suposed to change the focal ratio, right?
Here's a mystery.
http://www.cloudynights.com/item.php?item_id=643
The astro-rubinar mak in the above reviews has a field flattener
at the back near the visual back. When you use a "straight
through", the focal ratio is F/10. When you use a star
diagonal, the focal ratio is F/16. Why does the focal ratio
when using a star diagonal becomes bigger?
But that's not the primary mystery. The following is the
main one.
http://www.telescope-service.com/maksutovs/MTO/MTO.html#MTO1000Zubehoehr
In the 4" MTO 1000 Mak with a field flattener behind the visual
back. There is no changes if you use a straight thru or a diagonal.
It would produce similar F/10.
In this scope, the field flattener doesn't change the focal ratio at
all and it is a cheaper scope compared to the astro-rubinar.
So a cheap field flattener that doesn't change the focal ratio
can be created?
Owners of MTO 1000 Mak. You can confirm that the focal ratio
is still f/10 when you use a 90 degree diagonal than using
straight thru, right?
Thanks.
paul
Chris L Peterson
<jones....@yahoo.com> wrote:
>Field flatteners are suposed to change the focal ratio, right?
No. Field flatteners reduce field curvature. Focal reducers (and
barlows) change the focal length. Some focal reducers (mainly for SCTs)
also incorporate field flatteners. And some field flatteners may
slightly change the focal length, even though that isn't their purpose.
>In the 4" MTO 1000 Mak with a field flattener behind the visual
>back. There is no changes if you use a straight thru or a diagonal.
>It would produce similar F/10.
>The astro-rubinar mak in the above reviews has a field flattener
>at the back near the visual back. When you use a "straight
>through", the focal ratio is F/10. When you use a star
>diagonal, the focal ratio is F/16. Why does the focal ratio
>when using a star diagonal becomes bigger?
The effective focal length of a system with a focal reducer depends on
the distance between the focal reducer and the focal plane. Adding the
diagonal increases the distance and changes the focal length. Even if
this is just a field flattener, using it with the wrong spacing might
change the focal length. It also probably means that the field isn't
being properly corrected.
>In this scope, the field flattener doesn't change the focal ratio at
>all and it is a cheaper scope compared to the astro-rubinar.
>So a cheap field flattener that doesn't change the focal ratio
>can be created?
Is it the same field flattener (or focal reducer) as in the previous
example?
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
Paulie
> On Fri, 23 Jan 2009 16:08:01 -0800 (PST), Paulie
>
> >Field flatteners are suposed to change the focal ratio, right?
>
> No. Field flatteners reduce field curvature. Focal reducers (and
> barlows) change the focal length. Some focal reducers (mainly for SCTs)
> also incorporate field flatteners. And some field flatteners may
> slightly change the focal length, even though that isn't their purpose.
>
I know what field flatteners do which is to flatten the field. What I
meant to say was that field flatteners can change the focal ratio
as a side effect of the design which may be slightly as you said.
> >In the 4" MTO 1000 Mak with a field flattener behind the visual
> >back. There is no changes if you use a straight thru or a diagonal.
> >It would produce similar F/10.
> >The astro-rubinar mak in the above reviews has a field flattener
> >at the back near the visual back. When you use a "straight
> >through", the focal ratio is F/10. When you use a star
> >diagonal, the focal ratio is F/16. Why does the focal ratio
> >when using a star diagonal becomes bigger?
>
> The effective focal length of a system with a focal reducer depends on
> the distance between the focal reducer and the focal plane. Adding the
> diagonal increases the distance and changes the focal length. Even if
> this is just a field flattener, using it with the wrong spacing might
> change the focal length. It also probably means that the field isn't
> being properly corrected.
The Astro-rubinar of Ed has a field flattener at the back. I wonder
why
it can change the f/10 to f/16 when a star diagonal is used.. unless
you are saying that it is actually a focal reducer? I wonder if all
focal reducer automatically serves as a field flattener. If not. Then
the field flattener of the astro-rubinar is a combined reducer/
flattener?
>
> >In this scope, the field flattener doesn't change the focal ratio at
> >all and it is a cheaper scope compared to the astro-rubinar.
> >So a cheap field flattener that doesn't change the focal ratio
> >can be created?
>
> Is it the same field flattener (or focal reducer) as in the previous
> example?
I don't know if the astro-rubinar and the mto 1000 has the
same field flattener at the visual back. Both are produced
by Lzos in Russia
paul
Paulie
>
> - Show quoted text -
http://www.cloudynights.com/item.php?item_id=643
Oh. Another thing. If the author above removes the field flattener
and use his star diagonal, the system would become f/10. If
he put the field flattener right at the visual back (where you
can hit it with a longer diagonal tube), it becomes f/16. So
what kind of field flattener is it... something that can function
as focal extender.. I wonder if all focal extender automatically
flattens the field. It is a doublet designed field flattener.
paul
William Hamblen
>The astro-rubinar mak in the above reviews has a field flattener
>at the back near the visual back. When you use a "straight
>through", the focal ratio is F/10. When you use a star
>diagonal, the focal ratio is F/16. Why does the focal ratio
>when using a star diagonal becomes bigger?
The effective focal length of a cassegrainian telescope changes with
the spacing from the secondary mirror to the focal plane. The
increased distance means increased effective focal length means longer
focal ratio.
Bud
Paulie
wrote:
> On Fri, 23 Jan 2009 16:08:01 -0800 (PST), Paulie
>
A light cone from the secondary mirror can only change in
length if it diverse or converge halfway. I wonder why the
focal length can change after passing thru a star diagonal
which only has a mirror inside which doesn't converge or
diverse the beam. Any ideas?
optic novice
paul
Chris L Peterson
<jones....@yahoo.com> wrote:
>A light cone from the secondary mirror can only change in
>length if it diverse or converge halfway. I wonder why the
>focal length can change after passing thru a star diagonal
>which only has a mirror inside which doesn't converge or
>diverse the beam. Any ideas?
You focus an SCT by changing the separation between the primary and
secondary mirrors. When you add backfocus, as by using a diagonal, you
need to change the focus, which changes the focal length of the system
somewhat.
Paulie
> On Fri, 23 Jan 2009 18:43:56 -0800 (PST), Paulie
>
In the russian astro-rubinar, one focuses by turning the
front part of the tube itself so you move the miniscus
corrector. In the SCT, you move the primary. In the russian
mak, the diagonal and the primary is same distance always.
In the sct, the diagonal and the primary distances vary with
the focusing knob where you move the primary.
I wonder how this can explain the mystery of why when
a diagonal is used in the rubinar, the focal ratio changes
from f/10 to f/16 and when the field flattener is removed,
the focal ratio becomes f/10. Strange beast.
paul
Paulie
>
If this is true, it means the common 8" SCT is also affected. I wonder
what would happen to the focal ratio of an advertised 8" Celestron SCT
f/10 when
1. No diagonal used. Eyepiece directly connected to visual back
2. Mirror diagonal used
3. erecting Prism diagonal used
4. 3" tube extension put in between the star diagonal and visual back
In each case, the focal ratio changes to..... ?
paul
Chris L Peterson
<jones....@yahoo.com> wrote:
>In the russian astro-rubinar, one focuses by turning the
>front part of the tube itself so you move the miniscus
>corrector. In the SCT, you move the primary. In the russian
>mak, the diagonal and the primary is same distance always.
>In the sct, the diagonal and the primary distances vary with
>the focusing knob where you move the primary.
>I wonder how this can explain the mystery of why when
>a diagonal is used in the rubinar, the focal ratio changes
>from f/10 to f/16 and when the field flattener is removed,
>the focal ratio becomes f/10. Strange beast.
As previously noted, focal reducers do not provide a fixed degree of
reduction. They are designed to provide a certain reduction for a
certain distance between them and the focal plane. If you change that
distance- which you can do by refocusing the telescope- you also change
the focal reduction.
Assuming this so-called field flattener is acting also as a focal
reducer, there's nothing mysterious about this behavior at all. What
does the device look like? A field flattener by itself looks almost like
a piece of glass- you might see a little distortion looking through it,
but not much. A focal reducer (or a field flattener/focal reducer) will
show a significant positive power- like a magnifying glass.
dke...@hotmail.com
Hi
Most field flattners are negative lenses. The farther the eyepiece
is from the flattner, the more the magnification ( change in f/
ratio ).
SCT's don't actually have a fixed focal ratio. The ratio depends
on how far the eyepiece is from the secondary ( since the
secondary is the same as a negative lens ).
This change is small because of the large distance from the
secondary.
The field flattner lense multiplies this effect, being a negative
lense.
Changing the magnification changed the f/ratio.
Dwight
dke...@hotmail.com
Hi
To answer your questions. The visual back is about 1.5 feet
from the secondary. It magnifies about 3X at this distance.
meaning each 6 inches would be another 1X or 4X total.
( You need to make some actual measurements as these
are just numbers off the top of my head ).
Dwight
Paulie
>
> - Show quoted text -
Duh. What you mean "it magnifies 3X at this distance".
No eyepiece is used, yet you mention magnification,
are you talking about increase in the linear size at
the focal point? Magnification is dependent on focal
length only. How does the 3X fit into this?
optically inept,
Paul
Quadibloc
> I wonder
> why
> it can change the f/10 to f/16 when a star diagonal is used.
I wondered that too. After all, a star diagonal doesn't contain any
lenses.
But Chris L. Peterson's post did supply an explanation. The star
diagonal changed the distance between the field flattener and the
focal plane. So, when you brought an image into focus in the two
cases, the position of the field flattener was different, and this is
what changed the optical system. It still sounds a bit strange to me,
I admit, but it's not totally implausible.
John Savard
Quadibloc
> A light cone from the secondary mirror can only change in
> length if it diverse or converge halfway. I wonder why the
> focal length can change after passing thru a star diagonal
> which only has a mirror inside which doesn't converge or
> diverse the beam. Any ideas?
Ah, but you're forgetting something that happens before the field
flattener that can converge or diverge the beam.
Without the star diagonal, it's true the star is the same distance
away, optical "infinity". But you focus an SCT by moving the primary
mirror by turning that knob on the back. So you adjust it so that the
beam converges where your eyepiece is.
Add the diagonal, and you have to focus the telescope differently to
see the star with your eyepiece so much further away. So now the light
going through the field flattener is a less strongly converging cone.
What is strange, though, is that if it changes the speed from f/10 to
f/16 when used with the star diagonal, it must be like a divergind
lens. So even without the diagonal, it should change it upwards, even
if just from f/10 to, say, f/10.5, one would think.
John Savard
Paulie
If the field flattener is removed, his star diagonal with eyepiece
would become a f/10 system. I wonder what kind of field
flattener is it. Also the scope is not exactly a mak nor s.
cassegrain but some kind of hybrid. See the following
url for full description:
http://www.cloudynights.com/item.php?item_id=643
Here's a brief description of the lens system:
"106mm Clear aperture
F10 - 1000mm when used "straight-through" near the visual back.
F15 (1600) when used with a 90-degree mirror star diagon"
"1. The optical system contains two full aperture lenses which
surround the secondarly mirror baffle, two mirrors and a cemented
field-corrector doublet. Thus light passes through eight optical
surfaces
(not 18) and off of two mirror surfaces on the way to the eyepiece.
2. The field flattener doublet lens can be removed. This does not
affect
the quality of views through eyepieces at all. However, it does
reduce the focal length of the system from something like F16
(using a diagonal and eyepiece) to under F10 (using a diagonal
and eyepiece). This means that with a 32mm plossl, the AFOV is
c. 1.5 degrees rather than 1 degree. At the optical back (without
the diagonal) the 32mm now gives an AFOV of 2.4 degrees.
3. However, with the field-flatterner lens removed, only two of
my five eyepieces come to focus on celestial objects when
using the diagonal."
-------------------------------
I wonder what kind of catadioptric is it. Also why removing
the field flattener can turn it from f/16 to f/10 and why
putting a diagonal with the field flattener in place can also
turn it to f/10 yet becoming f/16 if straight thru without
diagonal. It's the mother of all mystery. Wonder if someone
has a clue to this russian masterpiece.
paul
Chris L Peterson
(Clarification: the description says the system is f/15 or f/16 with the
diagonal and f/10 without it.)
Okay, with your added information, there's nothing mysterious at all.
The field flattener is also a focal magnifier (a barlow). That is, it's
a lens system with a negative focal length. When it is in the system,
the effective focal length of the objective is increased by ~1.1 to
1.6x, depending on the distance between it and the focal plane. That
explains why the focal length changes when you use a diagonal, since the
amount of magnification a negative lens introduces depends on the
distance between that lens and the focal plane. Increasing the distance
increases the magnification (or if you prefer, increases the effective
focal length).
So you have the three cases from the system description:
(1) No corrector, FL < 1000mm (doesn't matter whether you use the
diagonal or not)
(2) Corrector, no diagonal, FL = 1000mm (focal plane close to the
corrector)
(3) Corrector, diagonal, FL = 1600 (focal plane far from the corrector)
Quadibloc
> (1) No corrector, FL < 1000mm (doesn't matter whether you use the
> diagonal or not)
That probably isn't the case. I've looked at the original article -
the Astro-Rubinar is a camera lens that can also be used as a
telescope. The field flattener isn't somethiing one adds on, like
those f/10 to f/6.3 field flatteners one can get for the typical SCT,
it's apparently just part of the lens.
Focusing the lens by twisting one of the rings on the barrel actually
changes its focal length, apparently, so the fact that this changes
the focal length from f/10 to f/16 is not mysterious. And that the
field flattener might have changed the focal ratio to a lower value is
entirely possible as well.
John Savard
Chris L Peterson
wrote:
>That probably isn't the case. I've looked at the original article -
>the Astro-Rubinar is a camera lens that can also be used as a
>telescope. The field flattener isn't somethiing one adds on, like
>those f/10 to f/6.3 field flatteners one can get for the typical SCT,
>it's apparently just part of the lens.
>
>Focusing the lens by twisting one of the rings on the barrel actually
>changes its focal length, apparently, so the fact that this changes
>the focal length from f/10 to f/16 is not mysterious. And that the
>field flattener might have changed the focal ratio to a lower value is
>entirely possible as well.
Without knowing more about the optics, there's no way to say how much
focusing changes the focal length. But a factor of 1:1.6 is very large,
and I doubt this happens. Certainly, with an SCT, there is a small focal
length shift associated with focusing, and a very large one associated
with using a barlow or focal reducer with the wrong focal plane spacing.
The description does say the corrector is removable, and that the lens
can be used with or without it. I'd still bet that most of the focal
length variation comes from using a negative power corrector at two very
different focal plane distances.
Paulie
> On Sat, 24 Jan 2009 08:40:38 -0800 (PST), Quadibloc <jsav...@ecn.ab.ca>
It is said that for a typical F/10 SCT, moving the primary 1mm, will
move the focal point about 25mm, and change the focal length by
about 4mm. In the case of the Rubinar with f/10 to f/16 changes
which Savard attributed to mere focusing. Well. The rubinar has
this internal optics description:
"The Astro-Rubinar is a catadioptric design; but it is not a
Maksutov or Schmidt telescope. The system includes three
full aperture lenses, a primary mirror, a rear surface secondary
mirror and a small field flattener lens. The secondary mirror is
affixed to the outermost aperture lens. A set of two full aperture
lenses is situated between the secondary and primary mirror.
The primary mirror is spherical. The field flattener lens is
situated in the optical back, behind the primary mirror. The
central obstruction of the secondary mirror is 13% of the
area and 34% of the diameter of the clear aperture"
-----
Now what scope have you encoutered wherein there are 2
full aperture lenses in between the primary and secondary
mirror?? The cones may be so altered inside that it is
possible moving the focus knob changing the mirror
distances can produce such f/10 to f/16 changes as
Savard theorized.
But the reviewer said if he removed the field flattener,
there is f/16 to f/10 changes with same diagonal/eyepiece
setup making it looks like the field flattener is the main
factor for it although the mirror spacing and optics
used can complicate it.
paul
Paulie
I have an unrelated inquiry to this thread about airy disc. For
a certain aperture, there is a correspding angular airy disc size
and 84% of the energy is in the center and the diffraction
patterns have the rest of the energy. To see the airy disc
at the eyepiece, you have to view a point of light where it
will cause parallel rays beam to the objective and the
corresponding refractions that will produce the pattern. Now
what if you are viewing a star or point of light which has 4x
or 10X and so on the size of the angular airy disc size of
that particular aperture. Would it also produce diffraction
patterns with rings around the center bright spot (composing
of many points correspoding to the airy disc spots)? or would
the diffraction patterns be around each point correspding to
the aperture angular airy disc size.. meaning to say for
some point of light that is 4X bigger than the angular
airy disc, it's diffraction patterns would be lost among
the smaller points inside the bigger blob of light. If this
is the case, would the bigger blob of light containing
the many smaller points with its diffraction pattern produce
another sets of bigger diffraction rings corresponding to
that blob of light that you can see with the eyepiece?
Thanks.
Paul
dke...@hotmail.com
>
> - Show quoted text -
Hi
On most SCT's, the focus is done by moving the primary,
as Chris P. had stated earlier. This means where the
secondary intersects the light from the primary changes
the effective focal length of the system.
The secondary mirror works like a Barlow lense. It
stretches out the focal length of the complete systems.
How else did you think they got a 2 meter focal length
crammed into a telescope that was only about a foot
and a half long.
The secondary magnifies the primaries focal length.
In the case of the SCT this is about 3X. Of course,
if one moves the primary relative to the secondary,
this changes the length of the cone of light from
the secondary to the focal point. Adding about 6 inches
to the system increases the effective focal length
of the system by about 1X or to a total of about 4X.
Of course, this may cause vignetting at the visual
back of the cone of light getting to the focus.
I think what you need to do is get the book called
"All About Telescopes". It explains all about such
things.
Dwight
Quadibloc
> I'd still bet that most of the focal
> length variation comes from using a negative power corrector at two very
> different focal plane distances.
I won't argue, as you know more about this sort of thing than I do,
but I'll note that if the corrector were one with a *positive* power,
and used in an inverting fashion, changes in magnification would be
large and the reason for them obvious. Of course, the same basic law
applies with a Barlow lens as well; and the fact that the lens is
focused means, at least, that it is moved relative to the corrector.
John Savard
William Hamblen
The Rubinar mirror lens focuses by changing the spacing between the
primary mirror at the rear and the secondary mirror at the front, like
most SCTs and MCTs. When the distance between the mirrors is close to
the difference between the absolute values of the focal lengths of the
mirrors a small change in the distance makes a big change in the
effective focal length. When the distance is exactly equal to the
difference in focal lengths, you have the condition for a galilean
telescope and the effective focal length is infinite.
Bud
William Hamblen
>Focusing the lens by twisting one of the rings on the barrel actually
>changes its focal length, apparently, so the fact that this changes
>the focal length from f/10 to f/16 is not mysterious. And that the
>field flattener might have changed the focal ratio to a lower value is
>entirely possible as well.
You need an additional lens near the mounting flange so you don't get
dark corners in your pictures.
This web page had a diagram of a Rubinar lens:
http://ixbtlabs.com/articles2/rubinar/
Some eyepiece adapters that attached to camera lenses had a weak
negative lens at the front. The lens in the adapter acted as a barlow
lens and pushed the focal plane out far enough for eyepieces to focus.
Bud
William Hamblen
On a Celestron C8 you get the nominal f/10 focal ratio with the image
plane about 4 inches from the back of the 2 inch outside diameter rear
port. When you increase the extension the focal ratio goes up. You
could work out the effective focal length and focal ratio if you had
the prescription for the telescope. I haven't taken my C8 apart to
measure, but here are estimates. Roughly speaking the secondary and
the image plane are about 24 inches apart at f/10. Increase the
spacing to 27 inches you would get about f/11. It isn't an enormous
change.
It says in the instructions for the Celestron focal reducer/corrector
that they took into account the change in focal length of the main
telescope when you refocus after adding the focal reducer.
Bud
Paulie
I know that. An 8" SCT has a F/2 primary and F/5
secondary. So the focal length of the primary
is 200mm x 2 = 400mm. Since it's a roughly
2000mm focal length scope. The secondary
is F/5. And 2000mm/400mm is 5 so the secondary
magnifies it 5X.. not 3X as you mentioned.
paul
dke...@hotmail.com
>
> - Show quoted text -
Hi
Sorry if I got the numbers wrong, I was just using
those numbers as examples. Be careful about using
any F ratio numbers of the secondary. This has
nothing to do with the actual magnification, only
the potential light lost by vignetting.
Magnification is determined by where in the primaries
light cone the secondary is placed and the focal length
of the secondary.
In any case, we'll use your numbers. The visual
back of the telescope is about 16 inches from the secondary
( please measure and correct this number ). The total
change is 5X so the effective magnification from
the secondary must actually be 4X ( 5 - 1 = 4, closer
to my estimated 3X. It is 1X at the secondary ).
Since this distance is about 16 inches( you really should
measure this ), every 4 inches change in focal position
would change this by 1X.
( do remember the my original numbers were guestimates,
just as my 16 inch number is ).
Dwight
dke...@hotmail.com
Hi
If I use William Hamblen's 24 inches, it would be 24/4 = 6.
This is 6 inches for each 1X increase.
Dwight
Paulie
Ic. Say. Aren't you bothered with the curvature in the focal
plane and the corresponding eyepiece image? Do some
scope designs show more curvature such as refractor
having less curvature than SCTs? I wonder why most
scope makers like Takahashi or even Astrophysics doesn't
automatically put field flatteners or given as option for
flat field across the view.
Maybe field curvature is more noticeable or of more
conern at terrestrial views than the sky. What is your
experience on this dude?
paul
dke...@hotmail.com
Hi
I find the secondary obstruction to be the bigest problem
for low power viewing during daylight.
Do remember that most eyepieces have some curvature
correction built in. Even a refractor has some curvature
but not as much as an SCT.
One disadvantage to putting a corrector in is that
it distorts the field of view. This may be OK for somethings
but not others. For astronomical viewing, one also
loses some light with another optical element.
Dwight
Paulie
What's your theory why refractor has less curvature
than SCT when the light cones and focal length are
the determining factor in magnfication and the same in
both of them. Do you think a refractor with longer focal
ratio like f/8 has less curvature than one f/4? These
details seem not to be available in the book "Telescope
Optics" which I owned and read repeatedly.
> One disadvantage to putting a corrector in is that
> it distorts the field of view. This may be OK for somethings
> but not others. For astronomical viewing, one also
> loses some light with another optical element.
> Dwight- Hide quoted text -
Well. A field corrector corrects the field.. how can you
say the correct distorts the field of view? Also I think
our eyes accomodating the curvature is the same as
when our eye lens contract or expand courtesy of
the ciliary muscles which control accomodation of
near object to far object. If our eyes can accomodate
it, this means the curvature is not there or does this
curvature means images you see in the eye are not
where they are but shifted in position or sorta?
Thanks dude.
Paul
Chris L Peterson
<jones....@yahoo.com> wrote:
>What's your theory why refractor has less curvature
>than SCT when the light cones and focal length are
>the determining factor in magnfication and the same in
>both of them.
Refractors do not necessarily have less field curvature than SCTs.
Curvature is introduced by each curved surface; positive and negative
elements combine to reduce curvature. The more surfaces you have to work
with, the more control you have in managing this aberration. Refractors
with two surfaces (that is, a single lens objective) have severe field
curvature, as do SCTs with just two spherical surfaces (we can ignore
the corrector, which just compensates for spherical aberration). Most
_practical_ refractors have objectives with 4-6 surfaces, which gives
the designer far more opportunity to manage field curvature and other
aberrations.
Some refractor designs include an integral field corrector to minimize
or eliminate field curvature and astigmatism- the Tak FSQ series being
the best known of these. And of course, field flatteners are common on
SCTs. Field curvature is not such a big problem for visual observers
(subject to individual taste); it is something that has to be controlled
when you are imaging, however.
Martin Brown
> On Fri, 23 Jan 2009 16:08:01 -0800 (PST), Paulie
>
> <jones.pau...@yahoo.com> wrote:
>
> No. Field flatteners reduce field curvature. Focal reducers (and
> barlows) change the focal length. Some focal reducers (mainly for SCTs)
> also incorporate field flatteners. And some field flatteners may
> slightly change the focal length, even though that isn't their purpose.
You can usually get a small range of zoom around the nominal stated
behaviour by adding to the drift length with extention tubes. SCTs can
accomodate quite a long back focus.
>
> >In the 4" MTO 1000 Mak with a field flattener behind the visual
> >back. There is no changes if you use a straight thru or a diagonal.
> >It would produce similar F/10.
> >The astro-rubinar mak in the above reviews has a field flattener
> >at the back near the visual back. When you use a "straight
> >through", the focal ratio is F/10. When you use a star
> >diagonal, the focal ratio is F/16. Why does the focal ratio
> >when using a star diagonal becomes bigger?
>
> The effective focal length of a system with a focal reducer depends on
> the distance between the focal reducer and the focal plane. Adding the
I think in this case it is actually because without a matched barlow
lens before the diagonal there simply isn't enough back focus (~25mm)
on the photographic MTO 1000/10 to use with a diagonal. Mine requires
an eyepiece very close in to the back plate to get focus at f10 and
can only be used straight through with normal eyepieces.
It is a formidable compact 4" finder scope with a wide field eyepiece.
Shame about the crick in the neck problem.
> diagonal increases the distance and changes the focal length. Even if
> this is just a field flattener, using it with the wrong spacing might
> change the focal length. It also probably means that the field isn't
> being properly corrected.
I am pretty sure there is a Barlow in their eyepiece adapter with
diagonal kit. Hence the altered focal ratio.
>
> >In this scope, the field flattener doesn't change the focal ratio at
> >all and it is a cheaper scope compared to the astro-rubinar.
> >So a cheap field flattener that doesn't change the focal ratio
> >can be created?
>
> Is it the same field flattener (or focal reducer) as in the previous
> example?
Could well be the same mirror but with a lot more back focus intended
to be used as a telescope rather than as a a T2 mount mirror lens. You
don't need anything but a mirror if the lens can focus with a diagonal
at f10.
Regards,
Martin Brown
Paulie
wrote:
Is your MTO 1000/10 the same one as shown in the following
web site (or is your "MTO" a rubinar?):
http://www.telescope-service.com/maksutovs/MTO/MTO.html
Long time ago. I saw a person use it with a 45 degree erect
prism diagonal with a 25mm eyepiece giving 40X or so
magnification. I think a star diagonal and
45 degree prism erector has somewhat similar
backfocus so a star diagonal can be used without
problem. I think what you didn't do is to remove
the screw which can make you rotate the front
corrector all the way out until it disengage from the
main body which I've seen it done in actual. This
would also enable you to focus at an object 1
meter way or so with a star diagonal and any
eyepiece.
paul
Martin Brown
Mine is most nearly like
http://www.telescope-service.com/maksutovs/MTO/MTO.html#MTO1000
Except that it came in a cardboard box with Russian on the outside and
was still in a black leather carrying case. It is at least 20 years
old now. I think there was also a red 25A full aperture filter as well
as the ones shown.
>
> Long time ago. I saw a person use it with a 45 degree erect
> prism diagonal with a 25mm eyepiece giving 40X or so
> magnification. I think a star diagonal and
> 45 degree prism erector has somewhat similar
> backfocus so a star diagonal can be used without
> problem. I think what you didn't do is to remove
> the screw which can make you rotate the front
> corrector all the way out until it disengage from the
> main body which I've seen it done in actual. This
> would also enable you to focus at an object 1
> meter way or so with a star diagonal and any
> eyepiece.
I use it mainly as a very long focal length camera lens so I don't
really want close focus.
Regards,
Martin Brown
Paulie
> Except that it came in a cardboard box with Russian on the outside and
> was still in a black leather carrying case. It is at least 20 years
> old now. I think there was also a red 25A full aperture filter as well
> as the ones shown.
>
>
>
> > Long time ago. I saw a person use it with a 45 degree erect
> > prism diagonal with a 25mm eyepiece giving 40X or so
> > magnification. I think a star diagonal and
> > 45 degree prism erector has somewhat similar
> > backfocus so a star diagonal can be used without
> > problem. I think what you didn't do is to remove
> > the screw which can make you rotate the front
> > corrector all the way out until it disengage from the
> > main body which I've seen it done in actual. This
> > would also enable you to focus at an object 1
> > meter way or so with a star diagonal and any
> > eyepiece.
>
> I use it mainly as a very long focal length camera lens so I don't
> really want close focus.
>
> Regards,
>
> - Show quoted text -
There are 2 kinds of MTO 1000. The new one is manufactured by
LZOS. The old one looks like the following with shallow part in
the middle:
http://www.ussrphoto.com/wiki/default.asp?ImageID=3348&ParentID=2&WikiCatID=28&ContentID=921
Is it the one you own?
So you use it as a 1000mm Telephoto lens. It should produce equivalent
magnification of 20X. But if you use a 6" Intes or even 8" SCT. The
equivalent magnification would be 40X. What camera are you using
with the MTO? Is it digital? They say mirror telephoto would produce
donuts like objects in out of focus background. What do you think.
paul
Paulie
>
> Is it the one you own?
>
> So you use it as a 1000mm Telephoto lens. It should produce equivalent
> magnification of 20X. But if you use a 6" Intes or even 8" SCT. The
> equivalent magnification would be 40X. What camera are you using
> with the MTO? Is it digital? They say mirror telephoto would produce
> donuts like objects in out of focus background. What do you think.
>
>
> - Show quoted text -
The following is a clearer picture of the early MTO 1000. Is it
like the one you own?
Martin Brown
>
> Is it the one you own?
No that one predates mine. Mine looks quite similar to the one still
called MTO1000 T2 mount for photographic use.
>
> So you use it as a 1000mm Telephoto lens. It should produce equivalent
> magnification of 20X. But if you use a 6" Intes or even 8" SCT. The
> equivalent magnification would be 40X. What camera are you using
> with the MTO? Is it digital? They say mirror telephoto would produce
> donuts like objects in out of focus background. What do you think.
On a digital camera you get an extra 1.4x magnification for free. The
sensor is 0.7x the size of 35mm film. I have used it over the years
with various Pentax film cameras and now with an IstDS. It doubles as
a spotting scope too by carrying an eyepiece adapter.
And yes out of focus highlights do become donuts. So you either live
with them or try to avoid having any out of focus specular reflections
in the field of view. Sometimes the effect can be quite pleasing too
if there are lots of them.
On a low contrast natural scene the effect of limted depth of field
just concentrates attention on the subject because the background is
soft. You do have to get the target in focus though it isn't at all
forgiving. And it is a lot cheaper and more compact to carry around
than a conventional 1000mm lens so you pays your money and takes your
choice.
Regards,
Martin Brown
Paulie
So you didn't just remove the screw the limits the corrector
focusing.
>
>
> > So you use it as a 1000mm Telephoto lens. It should produce equivalent
> > magnification of 20X. But if you use a 6" Intes or even 8" SCT. The
> > equivalent magnification would be 40X. What camera are you using
> > with the MTO? Is it digital? They say mirror telephoto would produce
> > donuts like objects in out of focus background. What do you think.
>
> On a digital camera you get an extra 1.4x magnification for free. The
> sensor is 0.7x the size of 35mm film. I have used it over the years
> with various Pentax film cameras and now with an IstDS. It doubles as
> a spotting scope too by carrying an eyepiece adapter.
>
> And yes out of focus highlights do become donuts. So you either live
> with them or try to avoid having any out of focus specular reflections
> in the field of view. Sometimes the effect can be quite pleasing too
> if there are lots of them.
Know the optical principles why the donuts or shadows from the
secondary mirror can be seen in the out of focus background.
We certainly can't see the donuts with the eyes.
paul
>
> On a low contrast natural scene the effect of limted depth of field
> just concentrates attention on the subject because the background is
> soft. You do have to get the target in focus though it isn't at all
> forgiving. And it is a lot cheaper and more compact to carry around
> than a conventional 1000mm lens so you pays your money and takes your
> choice.
>
> Regards,
dke...@hotmail.com
---snip---
>
> Know the optical principles why the donuts or shadows from the
> secondary mirror can be seen in the out of focus background.
> We certainly can't see the donuts with the eyes.
>
> paul
>
>
Hi
It is because your eye is seeing the light from all angles
as it crosses the focal plane. If you were to put a
piece of paper at the focal point and look at it with
a magnifying glass ( what you do with an eyepiece ),
you'd clearly see the same effect. This is because
the image of even the out of focus things are formed
at the focal plane and then viewed. When looking
through the eyepiece, the out of focus things are
not formed at the focal plane of the rest of the image.
In otherworks, still out of focus.
Dwight
Helpful person
wrote:
I don't understand the difference between viewing with the eye and
viewing with a camera. After all, the retina is exactly equivalent to
the camera image plane. The only difference with the eye is that it
can easily refocus and (probably) does not have as high a resolution.
It would seem that the doughnut illumination patch should be present
for both.
dke...@hotmail.com
Hi
I should correct my self here. There are two effects involved here.
One is that the eyepiece magnifies more, making things that
are out of focus more diffuse. This is because of reduced
depth of field.
The next is that the eye is a little more forgiving on contrast.
If seen against a dark field such as a star field, defocused
point source ( stars ) are clearly seen as donuts.
We were talking about taking daylight pictures and comparing
effects. So the statement that they are not seen is not
quite true.
Dwight
Helpful person
wrote:
So the statement that they are not seen is not
> quite true.
> Dwight
You mean false.
L.A.T.
L.A.T.
"L.A.T." <to...@sci.net.au> wrote in message
news:ltJfl.15044$cu.1...@news-server.bigpond.net.au...
>I wish I knew what you are all talking about.
No I don't
>
>
dke...@hotmail.com
Hi Helpful
If you like.
Dwight
Paulie
>
> - Show quoted text -
Don't forget that the effects of the donuts are only seen
in a scope with central obstruction when using a
camera. If you use a refractor with camera. No donuts
can be seen. Now the mystery is why the donuts
can be seen in a camera only using scope with
central obstruction and not refractor.
paul
Helpful person
>
> - Show quoted text -
No mystery. It's quite straightforward to calculate the diffraction
pattern with solely defocus for a circularlty symmetrical pupil.
Fo a more "intuative" explanation try drawing a set of rays coming to
a perfect focus. Now imagagine what they would look like with a
central obstruction at an out of focus plane.
Paulie
>
> - Show quoted text -
Such brilliance. You are better than Newton. So the donuts is
because of the defocused appearance of the central obstruction
compared to without wherein there is equal distribution of
the defocused diffraction patterns as seen in the focal plane.
Pages in Suiters book illustrate this.
paul
Paulie
So if you can get a sensor that is 0.5x the size of 35mm film. A 500mm
focal length refractor used as telephoto would appear similar to the
MTO 1000mm for similar magnification of 20X. I wonder what cdd
you know that can do that for general terrestrial view. Also when
you use the 0.7x sensor. Does the image have the same brightness
as the 35mm since it is the sensor that gets smaller? Wonder why
you didn't get a 80mm refractor such as the Megrez 72 FD or similar
that can produce the same or even better image than the MTO 1000.
Another good. I noticed 10 years ago that the MTO 1000 corrector
is uncoated. What do you think is the reflectivity of the mirror used
there? 89% or 80%? After 20 years.. don't you think the mirror
has lowered to something like 70% reflectivity? Also is the
field flattener in the MTO a doublet? I'd like to know to estimate
total light throughput of the scope before I purchase a new one
at LZOS if there is no smaller refactor that can beat it.
paul
>
> And yes out of focus highlights do become donuts. So you either live
> with them or try to avoid having any out of focus specular reflections
> in the field of view. Sometimes the effect can be quite pleasing too
> if there are lots of them.
>
> On a low contrast natural scene the effect of limted depth of field
> just concentrates attention on the subject because the background is
> soft. You do have to get the target in focus though it isn't at all
> forgiving. And it is a lot cheaper and more compact to carry around
> than a conventional 1000mm lens so you pays your money and takes your
> choice.
>
> Regards,
Martin Brown
There is no mystery here. Significantly out of focus you see the shape
produced by naive geometrical optics rather than a diffraction pattern.
You might like to try masking a refractor with a triangular mask and
take the scope from one side of focus to the other and watch what
happens. It is more fun with an assymetric triangle shape than a donut.
A mask with two or more holes is sometimes used as a focussing aid see
Hartmann Mask and the lines below for an explanation.
http://www.astropix.com/HTML/I_ASTROP/FOCUS/METHODS.HTM#HM
Regards,
Martin Brown
Martin Brown
The image formed is a property of the lens as is what size of sensor the
lens can illuminate without vignetting. The choice of sensor is up to
you. Most digicams are using a 25mm sensor rather then 35mm film.
> you didn't get a 80mm refractor such as the Megrez 72 FD or similar
> that can produce the same or even better image than the MTO 1000.
I had the option to buy a particularly good specimen from a local dealer
and tested it with a good eyepiece before buying.
>
> Another good. I noticed 10 years ago that the MTO 1000 corrector
> is uncoated. What do you think is the reflectivity of the mirror used
> there? 89% or 80%? After 20 years.. don't you think the mirror
> has lowered to something like 70% reflectivity? Also is the
> field flattener in the MTO a doublet? I'd like to know to estimate
> total light throughput of the scope before I purchase a new one
> at LZOS if there is no smaller refactor that can beat it.
I would not obsess about light transmission. Suffice to say that it is
good enough - photographically an f10 lens is always going to be pretty
dim. Stop down a normal camera lens for manual preview to f8 or f11 to
see what I mean.
Regards,
Martin Brown
Bhogi
for m42 cameras which have 45.5mm flage distance. That is not enough
space to mount a diagonal. So to achieve infinity focus you must focus
the lens beyond its designed infinity (from f/10 to f/16). This is the
reason why you must remove the focus stop to be able to do this.
As others said the focal ratio decreases as you focus this lens from
near to infinity and beyond. F/16 gives you 1600mm focal length
instead of 1000.
Field flattener used on rubinar is indeed a mild extender.
I guess the second page is just more descriptive while the first is
more technical and therefore more acurate.
On 24 jan., 01:08, Paulie <jones.pau...@yahoo.com> wrote:
> Hi,
>
> Field flatteners are suposed to change the focal ratio, right?
>
> Here's a mystery.
>
> http://www.cloudynights.com/item.php?item_id=643
>
> The astro-rubinar mak in the above reviews has a field flattener
> at the back near the visual back. When you use a "straight
> through", the focal ratio is F/10. When you use a star
> diagonal, the focal ratio is F/16. Why does the focal ratio
> when using a star diagonal becomes bigger?
>
> But that's not the primary mystery. The following is the
> main one.
>
> http://www.telescope-service.com/maksutovs/MTO/MTO.html#MTO1000Zubehoehr
>
> In the 4" MTO 1000 Mak with a field flattener behind the visual
> back. There is no changes if you use a straight thru or a diagonal.
> It would produce similar F/10.
>
> In this scope, the field flattener doesn't change the focal ratio at
> all and it is a cheaper scope compared to the astro-rubinar.
> So a cheap field flattener that doesn't change the focal ratio
> can be created?
>
> Owners of MTO 1000 Mak. You can confirm that the focal ratio
> is still f/10 when you use a 90 degree diagonal than using
> straight thru, right?
>
> Thanks.
>
> paul
Paulie
> I have this rubinar lens and know why this happens. Rubinar is a lens
> for m42 cameras which have 45.5mm flage distance. That is not enough
> space to mount a diagonal. So to achieve infinity focus you must focus
> the lens beyond its designed infinity (from f/10 to f/16). This is the
> reason why you must remove the focus stop to be able to do this.
> As others said the focal ratio decreases as you focus this lens from
> near to infinity and beyond. F/16 gives you 1600mm focal length
> instead of 1000.
>
> Field flattener used on rubinar is indeed a mild extender.
>
Not according to the reviewer of the rubinar at:
http://www.cloudynights.com/item.php?item_id=643
He specifically wrote near the end of the article:
"2. The field flattener doublet lens can be removed. This does not
affect the quality of views through eyepieces at all. However, it
does
reduce the focal length of the system from something like F16
(using a diagonal and eyepiece) to under F10 (using a diagonal
and eyepiece). This means that with a 32mm plossl, the AFOV is
c. 1.5 degrees rather than 1 degree. At the optical back (without
the diagonal) the 32mm now gives an AFOV of 2.4 degrees."
So it is possible the field flattener itself can change the focal
ratio from f/10 to f/16 as Mr. Peterson mentioned earlier.
Have you tried removing the field flattener and seeing how
the focal ratio changes?
paul
> > paul- Hide quoted text -
Bhogi
Yes I tryed that. You are confusing something.
The article AGREES with what I said and observed although I could
never focus to infinity without the field flattener because the focal
point moved sof far forward.
I used an inexpensive diagonal and machined it to be as close to the
lens as possible and I actualy don't understand how the author managed
to focus the stars.
When I said the field flattener is a mild extender that's what it
means it changes focal ratio by a factor 1.6 from F/16 (with) to F/10
(without).
BUT the lens was designed F/10 for photographic use with the
flattener. To be able to mount the diagonal and use an eyepiece you
have to focus beyond infinity which by itself changes focal ratio from
F/10 to F/16. If you THEN remove the field flattener, which is a
negative lens ("barlow", "1.6x tele extender"), the focal ratio
increases back to F/10.
I think what's confusing you is the change of focal ratio from
designed F/10 to F/16 when used with the diagonal. If you removed the
field flattener for photographic use the lens would be F/6.25 (not
exactly, becuse the focal point is different, but you get the point).
I didn't measure any of this I'm just going with the number mentioned
in the article.
Paulie
>
> - Show quoted text -
Ok. Got it. A barlow power has a lot to do with how one
position the focal plane.
Just wondering why you get the rubinar.. as a telephoto lens?
But f/10 is just dim. Why not get a 500mm f/3 nikon with a
2X teleconverter that can give you same magnification of
20X (1000mm focal length) with no donuts in out of focus
background and much less cooldown time.
Do you happen to plan to sell your rubinar? Know where
I can get a low priced used one (or the MTO 1000) too?
paul
Martin Brown
> On Jan 29, 1:23 pm, Bhogi <bh...@siol.com> wrote:
>>
>> I think what's confusing you is the change of focal ratio from
>> designed F/10 to F/16 when used with the diagonal. If you removed the
>> field flattener for photographic use the lens would be F/6.25 (not
>> exactly, becuse the focal point is different, but you get the point).
>>
>> I didn't measure any of this I'm just going with the number mentioned
>> in the article.- Hide quoted text -
>>
>> - Show quoted text -
>
> Ok. Got it. A barlow power has a lot to do with how one
> position the focal plane.
Yes. Changing the drift length between the field flattener and the focal
plane alters the effective focal length of the combined system when it
is brought back into focus.
A quick guide to what happens with different drift lengths on an LX200
f6.3 used with a 2x teleconverter and a 0.63x telecompressor and a
distant terrestrial target is on my website:
http://www.nezumi.demon.co.uk/astro/zoom/zoom.htm
>
> Do you happen to plan to sell your rubinar? Know where
> I can get a low priced used one (or the MTO 1000) too?
Scan the secondhand photographic lists regularly for M42 and T2 gear.
Manual focus but otherwise optically very good long focal length lenses
like this come up from time to time. I got my 300mm f4 lens and a very
eexpensive when new fisheye lens that way.
Regards,
Martin Brown
Bhogi
> On Jan 29, 1:23 pm, Bhogi <bh...@siol.com> wrote:
> > I think what's confusing you is the change of focal ratio from
> > designed F/10 to F/16 when used with the diagonal. If you removed the
> > field flattener for photographic use the lens would be F/6.25 (not
> > exactly, becuse the focal point is different, but you get the point).
>
> > I didn't measure any of this I'm just going with the number mentioned
> > in the article.- Hide quoted text -
>
> position the focal plane.
>
> Just wondering why you get the rubinar.. as a telephoto lens?
> But f/10 is just dim. Why not get a 500mm f/3 nikon with a
> 2X teleconverter that can give you same magnification of
> 20X (1000mm focal length) with no donuts in out of focus
> background and much less cooldown time.
Yes I got it as a telephoto lens when I started with dslr photography.
If I knew then what I know now I'd still buy it but only for observing
the moon. It has too low contrast for planetary observations. While
you can clearly see the jupiter bands on a refractor, they are only
detectable with the rubinar.
Haha, yes nikkor 500 f/3, that would be rich, I mean I'd have to be
rich :)
I got the rubinar from www.rugift.com, I think it was about $400 back
then.
> Do you happen to plan to sell your rubinar? Know where
> I can get a low priced used one (or the MTO 1000) too?
>
> paul
The rubinar is said to be optically superior, at f/10 it is
diffraction limited. I don't plan to sell it. It's one of those
things, too cheap and too good to not have around once you have it.
Try ebay, I got a canon fluorite 300mm f/2.8 that way.
Paulie
> On 29 jan., 09:38, Paulie <jones.pau...@yahoo.com> wrote:
>
>
>
>
>
> > On Jan 29, 1:23 pm, Bhogi <bh...@siol.com> wrote:
> > > I think what's confusing you is the change of focal ratio from
> > > designed F/10 to F/16 when used with the diagonal. If you removed the
> > > field flattener for photographic use the lens would be F/6.25 (not
> > > exactly, becuse the focal point is different, but you get the point).
>
> > > I didn't measure any of this I'm just going with the number mentioned
> > > in the article.- Hide quoted text -
>
> > Ok. Got it. A barlow power has a lot to do with how one
> > position the focal plane.
>
> > Just wondering why you get the rubinar.. as a telephoto lens?
> > But f/10 is just dim. Why not get a 500mm f/3 nikon with a
> > 2X teleconverter that can give you same magnification of
> > 20X (1000mm focal length) with no donuts in out of focus
> > background and much less cooldown time.
>
> Yes I got it as a telephoto lens when I started with dslr photography.
> If I knew then what I know now I'd still buy it but only for observing
> the moon. It has too low contrast for planetary observations. While
> you can clearly see the jupiter bands on a refractor, they are only
> detectable with the rubinar.
>
> Haha, yes nikkor 500 f/3, that would be rich, I mean I'd have to be
> rich :)
> then.
>
> > Do you happen to plan to sell your rubinar? Know where
> > I can get a low priced used one (or the MTO 1000) too?
>
> > paul
>
> The rubinar is said to be optically superior, at f/10 it is
> diffraction limited. I don't plan to sell it. It's one of those
> things, too cheap and too good to not have around once you have it.
>
> - Show quoted text -
In the world of SCTs. It's sufficient to follow rules of thumbs
and formulas but in a telephoto converted to telescope. Its
a bit more complicated as one have to master ray tracing
to even understand it.
Anyway. Last inquiry I'd like to make in this thread is why is
the barlow at f/10 diffracted limited as designed in the scope.
I mean. If we are focusing something less than 10 meters
away. I understand how the image can't be diffraction
limited. But in a scope designed for f/10 with frange
focus depth of say 45.5mm but extended to f/16, the image
is not diffraction limited? A barlow just refocus the rays to
longer focal length. It should still form airy discs similar
to the original.. unless the airy discs always differ when
barlow is used? What's the tolerance or the limit of
the curves of the barlows before the airy discs get
worse (and hence not diffraction limited)? Or maybe
it has to do with how the barlows is positioned that
affects or change the spherical abberations? Anyone
familiar with this? Does an image gets worse if barlow
is put before the diagonal compared to after it (how
much worse or changes in airy discs)? I can't find the
details in the net after searching it a while ago or in the
book "Telescope Optics".
paul
"One doesn't become enlightened by memorizing rules
of thumbs but by making conscious the Light" - Optics Zen
Martin Brown
No you don't. In a sense (apart from most camera lenses being a lot
faster than telescope eyepieces can handle) a camera lens is simpler
than a telescope because they are all designed with a focal plane for
infinity focus at a fixed position (for a given brand of camera).
That distance is usually too short to get a diagonal in so if you want
to use a diagonal you have to add a barlow lens to extend the light
path. Or have optics that were designed to focus a long way beyond infinity.
>
> Anyway. Last inquiry I'd like to make in this thread is why is
> the barlow at f/10 diffracted limited as designed in the scope.
> I mean. If we are focusing something less than 10 meters
> away. I understand how the image can't be diffraction
> limited.
Diffraction limited means that the image quality is determined primarily
by the size of the optical aperture and the wavelength of light. It says
nothing at all about the distance to the object under study.
A good microscope is usually diffraction limited and the subject can be
so close to the front of the objective lens that immersion oil is used
to cushion it and avoid two air glass discontinuities.
> But in a scope designed for f/10 with frange
> focus depth of say 45.5mm but extended to f/16, the image
> is not diffraction limited? A barlow just refocus the rays to
> longer focal length. It should still form airy discs similar
> to the original.. unless the airy discs always differ when
> barlow is used? What's the tolerance or the limit of
Putting an extra optical surface in always has a small cost in image
quality. The further away from the focal plane it is the more sensitive
to small errors it becomes. The primary needs to be 1/4 wave or better.
Modern Barlows are pretty good you should be able to find plenty of
testimonials. There are some poxy ones come with toy scopes.
> the curves of the barlows before the airy discs get
> worse (and hence not diffraction limited)? Or maybe
> it has to do with how the barlows is positioned that
> affects or change the spherical abberations? Anyone
> familiar with this? Does an image gets worse if barlow
> is put before the diagonal compared to after it (how
> much worse or changes in airy discs)? I can't find the
Very slightly. Any small errors get magnified with drift length, but it
really isn't worth worrying about. Quite a lot of top quality eyepieces
these days use an internal matched barlow type arrangement to improve
eyerelief.
> details in the net after searching it a while ago or in the
> book "Telescope Optics".
An ideal Barlow just changes the effective focal length. You can work
out the effect of putting a weak negative lens in the optical train from
first principles if you want. Geometrical optics gives the right results.
Regards,
Martin Brown
Paulie
wrote:
>
> - Show quoted text -
Hi, This is my really last inquiry after hours net research... or
just
for confirmation especially as it pertains to your MTO 1000 and
optical physics.
When using a barlow/refractor.. increasing the distance to
the eyepiece increases the magnification. This needs
a corresponding decrease in the spacing between
the barlow and the objective lens.. so you move the
focuser inward. Now in an SCT where the secondary
acts like a barlow. If one moves the focus backward from
the rear. One has to make the mirrors (between primary
and secondary) move closer, right??
Now when one is focusing an object just 2 meters away.
The mirrors have to be move closer too. Right? So all
catadioptrics mirrors are adjusted closer in most applications.
Now going to http://www.telescope-service.com/maksutovs/MTO/MTO.html
The MTO 1000 is supposed designed as f10 at a point maybe just 2"
from the rear. If one put a diagonal and move the focal point
backward,
one moves the mirrors closer (by removing the screw stop) and
the focal ratio changes to something like f/12, right?
In the above url. They said that using a 20mm plossl in the diagonal
(with screw stopper removed) would create magnification 1000/20=50X.
Since the f10 of the MTO 1000 is designed for one without diagonal.
The 20mm plossl plus star diagonal would produce something like
f12-13 and 60X magnification contrary to the web site data, right?
Lastly. Is there another screw stopper to prevent one from moving the
corrector tube out of the main primary tube? I remembered someone
just removed the corrector tube out of the MTO 1000 10 years
ago to make me see the inside primary mirror. Does all MTO 1000
do this without any screw stopper? Have you tried removing
the corrector tube from the main primary tube at least once??
Many thanks.
Paul
Bhogi
> Hi, This is my really last inquiry after hours net research... or
> just
> for confirmation especially as it pertains to your MTO 1000 and
> optical physics.
>
> When using a barlow/refractor.. increasing the distance to
> the eyepiece increases the magnification. This needs
> a corresponding decrease in the spacing between
> the barlow and the objective lens.. so you move the
> focuser inward. Now in an SCT where the secondary
> acts like a barlow. If one moves the focus backward from
> the rear. One has to make the mirrors (between primary
> and secondary) move closer, right??
Yes.
> Now when one is focusing an object just 2 meters away.
> The mirrors have to be move closer too. Right? So all
> catadioptrics mirrors are adjusted closer in most applications.
Wrong. You move the secondary out.
> Now going tohttp://www.telescope-service.com/maksutovs/MTO/MTO.html
>
> The MTO 1000 is supposed designed as f10 at a point maybe just 2"
> from the rear. If one put a diagonal and move the focal point
> backward,
> one moves the mirrors closer (by removing the screw stop) and
> the focal ratio changes to something like f/12, right?
>
> In the above url. They said that using a 20mm plossl in the diagonal
> (with screw stopper removed) would create magnification 1000/20=50X.
> Since the f10 of the MTO 1000 is designed for one without diagonal.
> The 20mm plossl plus star diagonal would produce something like
> f12-13 and 60X magnification contrary to the web site data, right?
Yes.
> Lastly. Is there another screw stopper to prevent one from moving the
> corrector tube out of the main primary tube? I remembered someone
> just removed the corrector tube out of the MTO 1000 10 years
> ago to make me see the inside primary mirror. Does all MTO 1000
> do this without any screw stopper? Have you tried removing
> the corrector tube from the main primary tube at least once??
If I recall correctly, in rubinar there is only one focus stop, which
prevents the focus ring from turning full 360 degrees.
I removed the tube, cleaned the focus threads, put in a less sticky
lythium grease (which as I hear is a mistake but after a few years the
optics are still clean), and disassembled the secondary group with
both corrector plates. In short the lens was totaly disassembled,
greased and put together working impecably.
Without the focus stop you can focus to about 1m, before the tube is
unscrewed. The lens can then serve as a long distance microscope :)
> Many thanks.
>
> Paul
Paulie
>
> - Show quoted text -
Here's a great mystery or the mother of all mysteries.
The MTO 1000 as I know has only one focus stop which is to
prevent it from unscrewing the corrector out... which also
prevents close focus of object 2 meters away as you
need to make the mirror separation greater. But how
come Martin Brown can't focus using a diagonal in
his MTO when what must be done is to make the mirrors
closer rather than farther while the MTO in the above
url can use a diagonal?
paul
Paulie
>
> - Show quoted text -
btw... knowing that you have completely disassembled the rubinar.
There is a description in the cloudynight review that mystify me.
It is described "The secondary mirror is affixed to the outermost
aperture lens. A set of two full aperture lenses is situated between
the secondary and primary".
I have never heard of a catadioptic with lens *between* the primary
and the secondary mirror. In SCT. There is no lens between them.
Do you know what is the function of each of the 2 lens in between the
primary and secondary mirrors of your rubinar? Note I know the
third lens is the corrector itself. I'm not talking of the corrector
but of the lens that lies between the mirrors which is described
above.
Also I need to know something. If the rubinar is designed for f/10
with 45.5.mm flange focus depth. I want to know how many times
is the diameter of the diffraction pattern of the airy disc compared
to the reference 1/4 wave at f/10 when the focus is moved further
back (becoming f/16) and the mirrors move closer (which can affect
the spherical aberrations). How do you calculate it. It's for purely
theoretical understand purpose because the eyes may not detect it
significant in practical usage. Thanks.
paul
Bhogi
> Here's a great mystery or the mother of all mysteries.
> The MTO 1000 as I know has only one focus stop which is to
> prevent it from unscrewing the corrector out... which also
> prevents close focus of object 2 meters away as you
> need to make the mirror separation greater. But how
> come Martin Brown can't focus using a diagonal in
> his MTO when what must be done is to make the mirrors
> closer rather than farther while the MTO in the above
> url can use a diagonal?
>
> paul
The lens needs some extra space to focus beyond infinity - to move the
secondary towards the primary. Rubinar has enough, the MTO might not.
On the other hand rubinar 500mm f/5.6 doesn't have enough space with
some eyepieces. Depends on the lens design.
But Martin also said he can't focus at f/10, which is a designed focal
point of 45.5mm behind the flange. That is not enough space for a
diagonal and an eyepiece to work correctly.
If mechanics allow it he may be able to do this at f/16, or maybe not.
No mistery here.
Bhogi
> btw... knowing that you have completely disassembled the rubinar.
> There is a description in the cloudynight review that mystify me.
> It is described "The secondary mirror is affixed to the outermost
> aperture lens. A set of two full aperture lenses is situated between
> the secondary and primary".
This is how the optics of rubinar look like:
http://www.ejarm.com/rubinar.jpg
> I have never heard of a catadioptic with lens *between* the primary
> and the secondary mirror. In SCT. There is no lens between them.
The secondary is a mangin mirror, which is a lens with one reflective
side. This is what it means between the primary and secondary. The
light reflected from the primary goes first thru the mangin and
reflects from the reflective side of the mangin back thru the same
surface.
> Do you know what is the function of each of the 2 lens in between the
> primary and secondary mirrors of your rubinar? Note I know the
> third lens is the corrector itself. I'm not talking of the corrector
> but of the lens that lies between the mirrors which is described
> above.
Obviously for additional corrections.
> Also I need to know something. If the rubinar is designed for f/10
> with 45.5.mm flange focus depth. I want to know how many times
> is the diameter of the diffraction pattern of the airy disc compared
> to the reference 1/4 wave at f/10 when the focus is moved further
> back (becoming f/16) and the mirrors move closer (which can affect
> the spherical aberrations). How do you calculate it. It's for purely
> theoretical understand purpose because the eyes may not detect it
> significant in practical usage. Thanks.
1.6 times the diameter. No hard math here.
With the original straight-thru telescope adapter at f/10 and 100x
magnification I can clearly see the diffraction pattern around bright
points of light, it is small but noticable.
With central obstruction the diffraction pattern is more complex,
larger and more noticable than without the obstruction.
Paulie
> On 31 jan., 04:35, Paulie <jones.pau...@yahoo.com> wrote:
>
> > btw... knowing that you have completely disassembled the rubinar.
> > There is a description in the cloudynight review that mystify me.
> > It is described "The secondary mirror is affixed to the outermost
> > aperture lens. A set of two full aperture lenses is situated between
> > the secondary and primary".
>
> This is how the optics of rubinar look like:http://www.ejarm.com/rubinar.jpg
>
> > I have never heard of a catadioptic with lens *between* the primary
> > and the secondary mirror. In SCT. There is no lens between them.
>
> The secondary is a mangin mirror, which is a lens with one reflective
> side. This is what it means between the primary and secondary. The
> light reflected from the primary goes first thru the mangin and
> reflects from the reflective side of the mangin back thru the same
> surface.
Thanks for the info. I could never have imagine that it uses
a mangin mirror where a lens is part of the mirror put in
front that is never heard in the world of SCTs, Mak or even
New-Mak.
>
> > Do you know what is the function of each of the 2 lens in between the
> > primary and secondary mirrors of your rubinar? Note I know the
> > third lens is the corrector itself. I'm not talking of the corrector
> > but of the lens that lies between the mirrors which is described
> > above.
>
> Obviously for additional corrections.
>
> > Also I need to know something. If the rubinar is designed for f/10
> > with 45.5.mm flange focus depth. I want to know how many times
> > is the diameter of the diffraction pattern of the airy disc compared
> > to the reference 1/4 wave at f/10 when the focus is moved further
> > back (becoming f/16) and the mirrors move closer (which can affect
> > the spherical aberrations). How do you calculate it. It's for purely
> > theoretical understand purpose because the eyes may not detect it
> > significant in practical usage. Thanks.
>
> 1.6 times the diameter. No hard math here.
Thanks for this equally valuable information.
Say. Maybe you know about close focusing calculations.
I'm studying the following website.
http://www.telescope-optics.net/SCT2.htm
In the above situation. Close focusing wavefront
error is calculated for the SCT. At distance of 15
meters for the F/10 SCT Mirror Focusing (see
the graph in the url) the wavefront error is 0.8 P-V or
1.25 wave (noting diffraction limited is 1/4 wave).
Now if the target object is mere 1.1 meter away from
objective (for example, analysis of dangerous substance),
I wonder what is the wavefront error. The formula
in the web site seem to involve the symbol for psi
or wave function in quantum mechanics) and it needs
the program OSLO. If you can calculate it or know
a good estimate of how the wavefront error is roughly
for a target object at distance of mere 1.1m from objective
(like is it 2 wave or 10 wave). Let me know before I
close thinking about this exotic optical device called the
rubinar or MTO. Also do you think the rubinar and MTO
are the only device in the planet that can focus at
1.1 meter beside the $9000 3.5 Questar long distance
microscope? What other catadioptics do you know
that can make you adjust focusing by rotating the
corrector tube itself until it just unscrews from the tube.
Thanks man. You save hours of mental agony.
paul
Bhogi
> On Jan 31, 8:58 pm, Bhogi <bh...@siol.com> wrote:
> > This is how the optics of rubinar look like:http://www.ejarm.com/rubinar.jpg
>
> > > I have never heard of a catadioptic with lens *between* the primary
> > > and the secondary mirror. In SCT. There is no lens between them.
>
> > The secondary is a mangin mirror, which is a lens with one reflective
> > side. This is what it means between the primary and secondary. The
> > light reflected from the primary goes first thru the mangin and
> > reflects from the reflective side of the mangin back thru the same
> > surface.
>
> Thanks for the info. I could never have imagine that it uses
> a mangin mirror where a lens is part of the mirror put in
> front that is never heard in the world of SCTs, Mak or even
> New-Mak.
Clever design indeed.
Glad to help when I can.
Unfortunately I'm not an optician so I can't help you here, but
remember that camera lenses (rubinar and MTO) are designed also for
closer than infinity focus. 1m focus is of course not optimal, but
with small focal ratios these lenses may still be diffraction limited
even at these close focuses. Just a guess.
SCTs are designed only for infinity focus. Questar on the other hand
is obviously designed for close focus.
I can tell you just from my observations that image quality doesn't
suffer a lot when focused at 1m. Apart from more pronounced
diffraction effects working at perhaps f/20 or even less, I didn't
notice any image degradation.
If you are looking for a long distance "microscope", I'd say rubinar
is your best bet, it's cheap, it's good, it has very long focal length
and it's small. 1000mm refractive microscope would be MUCH larger, and
the only other cheap reflective option is the MTO. I don't know of any
other ~1000mm or more cheap camera lens.
Paulie
The data in the site is for the 8" SCT. If the aperture gets smaller.
I'm thinking how the optical train would behave and whether
the wavefront error would get smaller in the smaller catadioptric.
>
> SCTs are designed only for infinity focus. Questar on the other hand
> is obviously designed for close focus.
>
> I can tell you just from my observations that image quality doesn't
> suffer a lot when focused at 1m. Apart from more pronounced
> diffraction effects working at perhaps f/20 or even less, I didn't
> notice any image degradation.
>
> If you are looking for a long distance "microscope", I'd say rubinar
> is your best bet, it's cheap, it's good, it has very long focal length
> and it's small. 1000mm refractive microscope would be MUCH larger, and
> the only other cheap reflective option is the MTO. I don't know of any
> other ~1000mm or more cheap camera lens.- Hide quoted text -
>
> - Show quoted text -- Hide quoted text -
>
> - Show quoted text -
Well. The Rubinar is being sold for $800 now while the MTO $450
because the U.S. dollar gets lower in value versus the Euro.
So I may just get the MTO. Unfortunately, there seems to be
batches where the mirrors separation can be adjusted lesser
and batches where the mirror separation has a certain
limit. A decade ago I have seen an MTO 1000 focus with
a 45" degree erect prism diagonal (which has somewhat
similar light path as the star diagonal) using a 20mm eyepiece.
Martin Brown probably got the version where the mirrors
separations can't be closer while other batches (later) have
probably been redesigned. So have to research for the
internet for more info. About the rubinar. It just have
too many surfaces that can affect the image quality.
The MTO has lesser surfaces.
paul
Bhogi
http://www.rugift.com/photocameras/rubinar_1000_lens.htm
you may have to copy the address. Both for much less than $800.
About the number of surfaces. With VERY bright sources of light like
overexposing the lightbulb I do get some flare which i suspect come
from the second corrector which is almost flat. So you do have a point
there.
Paulie
>
> you may have to copy the address. Both for much less than $800.
>
> About the number of surfaces. With VERY bright sources of light like
> overexposing the lightbulb I do get some flare which i suspect come
> from the second corrector which is almost flat. So you do have a point
>
> - Show quoted text -
What is "overexposing the lightbulb"? How can you overexpose
a lightbulb? Assuming its not very strong of light.. What if
the surrounding air is filled with really filled with photons. You
may see this as glare when it's really in the object and background a
and form on-axis part of the image. Whats your definite test to
check for glare in a lens or mirror system? I can't differentiate
if glare came from the shiny wall of a tube or from reflections
or scattering of imperfect coating. Both of which can cause
contrast loss in large degrees of the view or large scale. Add
this to the fact that the glare may come from surrounding air
as of the object as it distorts the light beam. It seems difficult
to distinguish.
About close focusing of object say 1 meter away. Supposed
you used 50X for it. At 1/2 meter, you simply need 25X to see
same detail (this is equivalently speaking bec you can't focus that
close). At 1/4 meter, you need 12.5X magfication. At 5" distance,
you need about 6.25X. At 2.5", you need 3.125X and at 1.25"
distance, you need 1.6X. Now what is this? A magnifying glass!
It acts like a 3X or less magnifying glass. It can't equal any
real microscope which has 20x-300x magnification. I wonder
what applications can you mention wherein one needs a
telescope magnifying glass that must be used a meter away,
why not just use a cheap $3 magnifying glass to see the
object instead of using the rubinar combo.
Anyway. I'm going to calculate and compare the airy discs
of the magnifying glass and how badly it behaves compare
to focusing at 1 meter distance with the rubinar. Do you
know how the airy discs of a magnifying glass looks like
or how to calculate for it? Books don't talk of airy discs
of the magnifying glass.
Paulie
Oh. I just thought of this. Why not increase the magnification
of the 1 meter target object to 1000X or so. At a focal length
of 1600. If you use a 3mm eyepiece, the magnification would
be 533X. Now use a 2X barlow for magnification of 1000X or
so. In a 3" distance equivalent. This would be like a microscope
of 100X magnificant. So you can see cellular processes. But
do you think the scope has enough separation distance to put
a 3mm eyepiece and a 2X barlow. This may produce exil
pupil of 100/1000 or 0.1mm. But the target object
would be illuminated with very strong light source like
a foot from the car headlights. Do you think this would work.
This would be useful for biology students to
study anthrax without accidentally smelling them or other
hazardous material or other exotic use
paul
> Anyway. I'm going to calculate and compare the airy discs
> of the magnifying glass and how badly it behaves compare
> to focusing at 1 meter distance with the rubinar. Do you
> know how the airy discs of a magnifying glass looks like
> or how to calculate for it? Books don't talk of airy discs
> of the magnifying glass.- Hide quoted text -
Bhogi
> What is "overexposing the lightbulb"? How can you overexpose
> a lightbulb? Assuming its not very strong of light.. What if
> the surrounding air is filled with really filled with photons. You
> may see this as glare when it's really in the object and background a
> and form on-axis part of the image. Whats your definite test to
> check for glare in a lens or mirror system? I can't differentiate
> if glare came from the shiny wall of a tube or from reflections
> or scattering of imperfect coating. Both of which can cause
> contrast loss in large degrees of the view or large scale. Add
> this to the fact that the glare may come from surrounding air
> as of the object as it distorts the light beam. It seems difficult
> to distinguish.
You overexpose with a camera. I tested this with a flashlight in the
lens direction in a dark room, expose for the dark room. With rubinar
you get a large purple donut shaped glare. But the flashlight is REALY
overexposed, much too bright to be a meaningful part of the picture.
Something like this also happens with very long exposures of the night
sky with very bright stars in the picture.
> About close focusing of object say 1 meter away. Supposed
> you used 50X for it. At 1/2 meter, you simply need 25X to see
> same detail (this is equivalently speaking bec you can't focus that
> close). At 1/4 meter, you need 12.5X magfication. At 5" distance,
> you need about 6.25X. At 2.5", you need 3.125X and at 1.25"
> distance, you need 1.6X. Now what is this? A magnifying glass!
> It acts like a 3X or less magnifying glass. It can't equal any
> real microscope which has 20x-300x magnification. I wonder
> what applications can you mention wherein one needs a
> telescope magnifying glass that must be used a meter away,
> why not just use a cheap $3 magnifying glass to see the
> object instead of using the rubinar combo.
I never said I used it as a microscope. How about for observing a wasp
nest? Also I think there's something wrong with your math. At 1.25"
distance a loupe gives you 10x magnification.
> Anyway. I'm going to calculate and compare the airy discs
> of the magnifying glass and how badly it behaves compare
> to focusing at 1 meter distance with the rubinar. Do you
> know how the airy discs of a magnifying glass looks like
> or how to calculate for it? Books don't talk of airy discs
> of the magnifying glass.
Because you can't see them. Looking thru the loupe it's like wearing
glasses of say +40 diopter. Do you see any airy discs when you look at
the stars?
Bhogi
> Oh. I just thought of this. Why not increase the magnification
> of the 1 meter target object to 1000X or so. At a focal length
> of 1600. If you use a 3mm eyepiece, the magnification would
> be 533X. Now use a 2X barlow for magnification of 1000X or
> so. In a 3" distance equivalent. This would be like a microscope
> of 100X magnificant. So you can see cellular processes. But
> do you think the scope has enough separation distance to put
> a 3mm eyepiece and a 2X barlow. This may produce exil
> pupil of 100/1000 or 0.1mm. But the target object
> would be illuminated with very strong light source like
> a foot from the car headlights. Do you think this would work.
> This would be useful for biology students to
> study anthrax without accidentally smelling them or other
> hazardous material or other exotic use
No this would not work. First, when you focus close the focal length
reduces with rubinar for the same reason it increased when focusing
beyond infinity. Second to achieve useful 1000x magnification you'd
need to have 1m (a guess) aperture. For a microscope it doesn't matter
what focal length you use, but the focal ratio should remain the same.
For high magnification you need very fast lenses f/1 or even more
regardless of focal length.
Paulie
When you use say a 25mm eyepiece in your rubinar at f/16
with a star diagonal. It should produce roughly 1600/25= 64X
magnification. If you are focusing at a wasp nest at 1.1
meter distance from the objective. What is the estimate of
the magnification you would get with the same eyepiece/diagonal?
30X? 20X? 10X? What do you think?
The reason big exit pupil is brighter is because there
is more light intensity that is converging from your
cornea to the retina. Now imagine a 0.1mm exit
pupil. In normal star light or even daylight. It
may produce small light intensity that would make
the image dim. Now imagine intense lux of light
source like football searchlight illuminating the
target wasp nest. The 0.1mm exit pupil would
be like a laser and the light intensity reaching
the point in your retina would be similar to
say 2mm exit pupil at daytime. It is a matter
of intensity.
pupil
Paulie
What I mean to say is. To see the same detail at 1.25"
away using the same system which can see 40X at 1 meter
away. You need 1.6X magnification at 1.25".
Each of the following would produce the same detail:
1 meter away = 40X
1/2 meter away from target object = 20X
1/4 meter away = 10X
5" or 1/8 meter away = 5X
2.5" away = 2.5X
1.25" away = 1.626X or 1.6X
That is.. using the same system equivalently speaking.
What the above means is that a $5 loupe can make
you see more detail at 1.25" than using the rubinar
using 40X at 1 meter way. Ponder on this.
>
> > Anyway. I'm going to calculate and compare the airy discs
> > of the magnifying glass and how badly it behaves compare
> > to focusing at 1 meter distance with the rubinar. Do you
> > know how the airy discs of a magnifying glass looks like
> > or how to calculate for it? Books don't talk of airy discs
> > of the magnifying glass.
>
> Because you can't see them. Looking thru the loupe it's like wearing
> glasses of say +40 diopter. Do you see any airy discs when you look at
> the stars?
What i mean to say is. If the diameter of the airy discs in
the magnifying glass is say 30X (intense diffraction effects)
from the size of the supposedly normal airy disc (for
the magnifying glass aperture). You may see the loss
of image quality with the average resolution of your eyes.
paul
Bhogi
> The reason big exit pupil is brighter is because there
> is more light intensity that is converging from your
> cornea to the retina. Now imagine a 0.1mm exit
> pupil. In normal star light or even daylight. It
> may produce small light intensity that would make
> the image dim. Now imagine intense lux of light
> source like football searchlight illuminating the
> target wasp nest. The 0.1mm exit pupil would
> be like a laser and the light intensity reaching
> the point in your retina would be similar to
> say 2mm exit pupil at daytime. It is a matter
> of intensity.
At f/10 and 100x magnification you can see diffraction rings, at f/100
(your 0.1mm exit pupil) you'd see 10x bigger diffraction rings, nice
laser bright bulls eye diffraction rings.
I think you should get yourself some cheap optics, start with a cheap
telescope and test your theories.
Paulie
> On 1 feb., 16:29, Paulie <jones.pau...@yahoo.com> wrote:
>
> > The reason big exit pupil is brighter is because there
> > is more light intensity that is converging from your
> > cornea to the retina. Now imagine a 0.1mm exit
> > pupil. In normal star light or even daylight. It
> > may produce small light intensity that would make
> > the image dim. Now imagine intense lux of light
> > source like football searchlight illuminating the
> > target wasp nest. The 0.1mm exit pupil would
> > be like a laser and the light intensity reaching
> > the point in your retina would be similar to
> > say 2mm exit pupil at daytime. It is a matter
> > of intensity.
>
> At f/10 and 100x magnification you can see diffraction rings, at f/100
> (your 0.1mm exit pupil) you'd see 10x bigger diffraction rings, nice
> laser bright bulls eye diffraction rings.
Given an aperture and focal ratio, changing the magnification
doesn't change the focal ratio. For example. a 4" refractor
at f/8 with focal length of 800. Using eyepieces of say 20mm
and 25mm would change magnification to 40X and
respectively. The focal ratio would remain f/8. In an SCT.
The focal length varies a little if there is no barlow at the
rear end. So using a 2mm eyepiece in an f/10 SCT
wouldn't change the focal ratio to f/100. Where did you
get the f/100 figure anyway? You mentioned earlier
"when you focus close the focal length
reduces with rubinar for the same reason it increased
when focusing beyond infinity"
Well. If the focal length reduces, the focal ratio must
get be f/5 or so instead of f/100 from the normal
f/10 or even f/16. So how you derive the f/100 is
an enigma.
paul
Bhogi
> > At f/10 and 100x magnification you can see diffraction rings, at f/100
> > (your 0.1mm exit pupil) you'd see 10x bigger diffraction rings, nice
> > laser bright bulls eye diffraction rings.
>
> Given an aperture and focal ratio, changing the magnification
> doesn't change the focal ratio. For example. a 4" refractor
> at f/8 with focal length of 800. Using eyepieces of say 20mm
> and 25mm would change magnification to 40X and
> respectively. The focal ratio would remain f/8. In an SCT.
> The focal length varies a little if there is no barlow at the
> rear end. So using a 2mm eyepiece in an f/10 SCT
> wouldn't change the focal ratio to f/100. Where did you
> get the f/100 figure anyway? You mentioned earlier
> "when you focus close the focal length
> reduces with rubinar for the same reason it increased
> when focusing beyond infinity"
> Well. If the focal length reduces, the focal ratio must
> get be f/5 or so instead of f/100 from the normal
> f/10 or even f/16. So how you derive the f/100 is
> an enigma.
You are right, but you seem to think you can achieve any
magnification. F/100 was to make you think. Using the same 10mm
eyepiece with your over the top magnifications (0.1mm exit pupil)
would require a 100mm (aperture) f/100 telescope, same as 100mm
(aperture) f/10 with 1mm eyepiece. Good luck with that.
This is my last post.
I realy suggest you try all of this, you don't even need barlows etc.
just cut the aperture masks out of cardboard for chosen focal ratios
for your chosen telescope and eyepiece and see what happens.
Paulie
>
> - Show quoted text -
Many thanks for your help. I just realised something yesterday.
Calculations and theoretical principles are enough to understand
it. Our eyes have resolution of 60 seconds of arc, and the
telescope's resolving power is 120/D arcseconds. The maximum
magnification is 60/(120/D) or 0.5D (in millimeters). This
would enable our eyes to detect the airy discs especially
in target object with high contrast such as wasp nest illuminated
by strong light source. This means for the 100mm Rubinar. We
only need 50X to see resolve it. Magnifying it to 1000X would
produce empty magnification. I knew all years many years
back. But the idea of the rubinar focusing close messed up
my mind as I was thinking about microscope principles.
In a microscope. Resolving power is determined by the
numerical aperture which depends on the focal ratio of
the objective beam and not on the aperture size or focal
length. Thought the rubinar can do this at close focus
but realized that it doesn't. Hence the earlier 1000x confusion.
In conclusion. A 50X resolving power in the rubinar would
produce 1.6X magnfication at the equivalent distance of
1.25" or so from target. Therefore a $5 loupe with 10X power
would produce better resolution than the $500 MTO. To
see at the cellular level. One needs a microscope which
can pull the 1000X trick because its light path uses numerical
aperture principles different from telescopes.
paul