planet killed! saga of an atm'r (long)
starburst
upon his name) offered a partially completed mirror kit for anybody who
was local to him and might actually finish the mirror. I was lucky
enough to be the one he chose to give the kit to. It seemed a good
project for me. The mirror would be a slow (f/8) 8", and since this neck
of the woods is better known for steady air rather than transparency, I
figured I'd give making a planet killer a shot. Plus, I'd wanted to try
making a mirror since I was a kid, and had never had the right push to
actually do it. Larry kindly gave me some terrific old manuals on
telescope making from edmund scientific, a rudimentary slitless foucault
tester (that turned out to work very well once I figured out the
concept), and some other assorted optical goodies (and a bit of junque,
too.) I also picked up Texereau's book from the library, too, and lazily
perused it for a while, trying to understand what I was about to try to do.
I let the glass sit for a few months before I had time to take it out
and look at it. The face was mostly polished out to the edge with some
cloudiness still very evident in the outer inch or so. I cobbled
together a little ronchi screen out of cardboard and wire and set the
mirror up to test. What a nightmare! Trying to find that teenie dot of
light in the darkness right at the radius of curvature, then getting the
tester *right* in the right place... Lots of frustration until - WOW!
You can see the whole face of the mirror with shadows and ronchi lines!
just like the book says! How freakin' cool! But then the air conditioner
would cycle on and I'd have to wait a few minutes for it to stop and for
the air to settle again. Amazing how sensitive the testing setup is. And
hmmm, there are some *serious* problems with the edge of this mirror,
which is substantially turned down.
So I fixed up the lap, hot pressed just as the instructions said, (it's
all in the manual) and started walking around the barrel. After a few
nights (maybe 4 hours polishing, or so, lots of zen contemplation) the
turned edge had pretty much disappeared and the glass looked smooth out
to the edge. A little while longer and I had convinced myself that I was
at a decent sphere - the ring around the edge of the mirror was perfect,
just like in the pictures. A few strokes away from a parabola... or not.
Not, in fact. My first shot dug a hole in the center of the mirror. Back
to walking around the barrel for another night or two... spherical
again... 30 strokes and... Blast! Turned the edge a little... etc. It
took about five tries before I got the donut shadows I was looking for.
And then it took me a while to convince myself that it was real. I made
up the masks and ran the tests and everything seemed about right, really
good, in fact. But I still wasn't sure what I was looking at.
Not wanting to aluminize a bad mirror (I'm kind of a perfectionist in
stuff like this) I got in touch with one of the retired astronomers down
here at UT, and he and I both ran the tests together, averaging the
numbers. when I got the numbers loaded into a mirror program and got the
results, it far surpassed my hopes. A .97 strehl with a 1/26th wave
error (1/13th on the wavefront). Root mean square of like 4.5nm or
something. I quickly emailed the man who had helped me test, and he shot
back a reply saying "don't touch it! Send it off to have it aluminized
at once!" Which I did. And I asked them to replace the 1.83 secondary
with a 1.55, which Newt said I should be able to get away with.
And then the mirrors sat while I tried to design and build a truss-tube
setup for it. Which was like another year. But the trouble and dustbins
and I were off to Colorado for vacation this last month and I really
wanted to have it ready to take with me so I finally hacked the thing
together and loaded it into the car.
I grew up in Colorado and had forgotten just how brutal the atmosphere
there can be on image quality. It took me a while to get everything
lined up and collimated, and once I did I was wildly disappointed. I
could barely bring the thing to a focus. Showed two elongated blobs on
epsilon Lyra. Two indistinct equatorial bands on Jupiter. Yuck. The
skies were transparent but I could see almost nothing through them. And
I lost my favorite eyepiece somewhere in the darkness, too, a celestron
17mm plossl that I'd had for years.
When I got back home I started tweaking the whole setup, and then
tonight... a drum roll please...
Holy Effing Heavens. Seeing tonight wasn't fantastic - I'd give it about
a 6 out of 10 - but good lord is this ever a good telescope. I let the
thing cool off for a couple of hours before turning it on the moon.
Stunning. Despite the heat bubbling off the edges, the detail was just
superb. I have to say here that I've been pretty lucky with my scopes -
my old orange tube C8 regularly outperforms newer (and larger) SCTs. No
joke. It's just a very solid telescope. But this new dob kicks it.
Beautiful contrast on the maria floors, razor sharp rills. I saw
something else tonight, too, that I'd never seen before - through a gap
in the eastern wall of Goldschmidt there was a straight line of light
right across the crater floor. It looked oddly man-made, like a
billboard. Neat.
Then I turned the scope on neighboring Jupiter, and just lost it. As I
say, the seeing wasn't that fantastic tonight, but in the moments when
it cleared, man. The absolute bar none best views of Jupiter I've ever
seen. Looked like a photo. The red spot was sitting right on the
meridian, like it was winking at me, and then stretched off eastward in
the south equatorial belt was a series of white festoons just as plain
as day. The belts were a lovely copper color with that iridescence you
notice on good, stacked webcam images. Every few seconds the polar belts
would resolve, zebra-like. The moons all resolved to tiny disks. It was
awesome. Literally. I raced in and hauled out my wife (whose interest in
astronomy is marginal) and sat her in the chair and gibbered at her,
"look! red spot!" She looked and said "... um.. Oh! There it is! That's
so cool! You can see a lot through this telescope." And she toddled off
to bed. I gazed for a while longer. This planet is officially killed.
I am a *very* happy camper. I couldn't have afforded a zambuto, but this
is as good as any I've seen. It's a terrific feeling of accomplishment,
and the views... beyond belief. Lovely night.
Clear skies, y'all - Chris
Chris.B
Excellent write-up!
Just like being there! :-)
Regards
Chris.B
Starlord
my Babylon 8, an f8 8 inch dob I use for Sidewalk Astronomy.
--
The Lone Sidewalk Astronomer of Rosamond
Telescope Buyers FAQ
http://home.inreach.com/starlord
Astronomy-Net Gift Shop
http://www.cafepress.com/astronomy_net
"starburst" <no...@nospam.net> wrote in message
news:db26la$fdr$1...@geraldo.cc.utexas.edu...
RMOLLISE
starburst wrote:
> So a couple of years ago in this forum, Larry Gilstrap (may peace be
> upon his name) offered a partially completed mirror kit for anybody who
> was local to him and might actually finish the mirror. I was lucky
> enough to be the one he chose to give the kit to. It seemed a good
> project for me. The mirror would be a slow (f/8) 8", and since this neck
> of the woods is better known for steady air rather than transparency, I
> figured I'd give making a planet killer a shot. Plus
Hi:
Great post. One of the best articles I've read on s.a.a. in a long,
long time. Congrats on the scope--your enthusiasm ALMOST makes me want
to start pushing glass again. ;-)
Peace,
Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_
<http://members.aol.com/RMOLLISE/index.html>
=============================================================
Like SCTs and MCTs?
Check-out sct-user, the mailing list for CAT fanciers:
<http://groups.yahoo.com/group/sct-user>
=============================================================
See: <http://journals.aol.com/rmollise/UncleRodsAstroBlog/>
For Uncle Rod's Astro Blog.
Roger Hamlett
> is as good as any I've seen. It's a terrific feeling of accomplishment,
> and the views... beyond belief. Lovely night.
>
> Clear skies, y'all - Chris
Having the patience to 'try again', sounds as if it has paid off. The
story makes a nice 'tale' to give an idea of just how important this is,
and the mirror will also be a sort of 'memorial' to it's original source.
Best Wishes
Larry G
Chris,
Congratulations on turning the potential of the 8" f/8 kit into a fine
performing reality. This excellent write-up of your achievement is most
gratifying. Such great work (and impressive results) may help revitalize
the sagging amateur telescope making hobby which has been
in decline over the past few decades. I may try again myself sometime
in the future.
Fortune has smiled upon us both. You were indeed the right person to
take over that mirror. I rest easier knowing that you have brought it
to such outstanding fruition.
Congratulations again, and thanks for sharing your experience.
Larry G.
--
Calling Austin HOME since 1982!
John Carruthers
results, it far surpassed my hopes. A .97 strehl with a 1/26th wave
error (1/13th on the wavefront). Root mean square of like 4.5nm or
something. I quickly emailed the man who had helped me test, and he
shot
back a reply saying "don't touch it! Send it off to have it aluminized
<<
Hi Chris,
It's such a good feeling isn't it ? I finished my 10" F6 off with a 1
1/2" sub dia small tool to get at the last annoying zone ;-) and
finished up with 1/12.6 wavefront.(1/25 wave).
I've only had a few nights since when I could use it to its full
capacity, but its sooo worth the effort.
I picked up a second hand 15mm TV wide on eBay, superb, and I have an
old 6mm Zeiss ortho which gives 'almost' as good results as the club's
Radians. Just waiting for the Mars opposition now.
Will you be doing any more mirrors soon ? ;-)
jc
starburst
>
> Hi Chris,
> It's such a good feeling isn't it ? I finished my 10" F6 off with a 1
> 1/2" sub dia small tool to get at the last annoying zone ;-) and
> finished up with 1/12.6 wavefront.(1/25 wave).
> I've only had a few nights since when I could use it to its full
> capacity, but its sooo worth the effort.
> I picked up a second hand 15mm TV wide on eBay, superb, and I have an
> old 6mm Zeiss ortho which gives 'almost' as good results as the club's
> Radians. Just waiting for the Mars opposition now.
> Will you be doing any more mirrors soon ? ;-)
Funny you should mention it...(grin). Oh yeah. I'm hooked. I'm kind of
contemplating the next one now, probably something in the 12 to 14"
range. I'll probably go with a 12 1/2" f/5, just because I'm afraid of
correcting anything bigger at this stage (I'm actually afraid of
correcting a 12 1/2, to be honest, but I want something that will show a
really big jump in improvement over the 8).
The weird thing is that it's something that I really want to do, but am
in no rush at all. Unless I decide to get into the big game with zambuto
and royce and Roland and all (and I'm sure that thought crosses the mind
of every kid who successfully pushes any glass, just as it crossed mine
when I saw the strehl figures) I probably will only make two or three
mirrors in my life, so I want the next one to be just as good. I can
afford to wait. It's a nice zen sort of thing, like making furniture or
fixing cars (both of which are hobbies of mine).
And thanks to all for your kind words on my post. I'd love to see more
stuff on ATMing in here, because there's something fundamentally pure
about it, but it's not for everybody. And while I'm an inveterate
tinkerer, this is the first mirror I've made, and I've been looking at
the stars with varying degrees of intensity since I was a kid thirty
years ago. Observing is pure, too. The moment a person looks up into the
night sky for more than 10 or 15 seconds it puts him on the road to
decency in my book. And optics are so cheap these days...
Still, there's something about completing a curve in three dimensions to
such insanely exacting tolerances that if the mirror were extended to
the scale of a mile it would be within a paper sheet's thickness of
mathematical perfection.
That's something.
Good luck with your next project, too, John. See you 'round the barrel.
Chris
Larry Stedman
Great write up.
Larry Stedman
Vestal
Scott Smith
Scott
"starburst" <no...@nospam.net> wrote in message
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