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[ATM] strange (collimation?) problem
Yesterday I startested my selfmade, not yet aluminiumized 300 mm f/5 30 mm
thickness (Pyrex) mirror (its my first mirror). To be able to do that I made
an extra collimatable platform with a 6-pointcell on the mirrorbox of my 300
mm f/6 telescope. Two round teflonpads at 90 degrees act as lateral supports
(see: http://home.wanadoo.nl/jhm.vangastel/Astronomy/scope3.jpg ).
What I saw inside of focus: nice round diffraction pattern with clear round
diffraction rings. No sign of astigmatism. Nice outer ring. Secondary shadow
well centered.
And outside focus: more turbulence, nice round diffraction pattern as well,
Rings a bit les clearly seen then inside focus. Secondary shadow breaks out
a bit later (but clearly less later then in my other 1/8 lambda P-V mirror)
as compared to inside focus (new mirror tested 1/13 lambda P-V; .98 Strehl,
using Figure XP; undercorrected edge).
BIG DIFFERENCE as compared to inside focus: secondary shadow not centered
but very clearly offset to one side. When I centered the secondary shadow
during the startest in the outside focus pattern, it became decentered
inside focus and was then also clearly off collimation as seen in the
cheshire.
I can't imagine I did the collimation wrong. Collimated the secondary with a
laser and the primary with a cheshire as I always do.
I have never expierienced this before and don't have a clue about the cause
of the problem.
Jan
http://home.wanadoo.nl/jhm.vangastel/Astronomy/
----- Original Message -----
From: "David Harbour" <scarab2@cox.net>
To: "Stan Truitt" <s.truitt@verizon.net>; "ATM List" <atm@atmlist.net>
Sent: Sunday, June 19, 2005 3:03 AM
Subject: Re: [ATM]: What do they offer:
> Re: [ATM]: What do they offer:Stan-
>
> You are too generous! All I can tell you is that this little bulb, the one
> that the hardward stores carry on their flashlight lamp charts (the only
one
> that is threaded, and for which radio shack has a socket and leads to a
two
> cell "D" cell battery holder), never caused me heat problems. In fact, I
> cannot believe how good my experience with this tester was, over the
years.
> I built five testers. This one was ready when I destroyed my McIntosh
tester
> (#4) by failing to put counterweights on the extension to the platform of
my
> very long optical bench, to test a very long mirror. Glad I had it ready.
> You can see it here:
>
> http://www.atmsite.org/contrib/Harbour/Tester.html
>
> My neat little (large, actually) McIntosh tester disintegrated in a shower
> of flying parts when it fell to the concrete floor.. But I was ready with
> #5!!! Someone kept it. (I expected this). It's okay; I was never going to
> use it again. I hope he loans it to others, whoever he is (the guy who
kept
> it).
>
> It is hard to believe that it was so bright that it caused me to have
> SERIOUS difficulty reading shadows on coated mirrors, but it was. Maybe we
> should form a club: "Not Happy with LED Illumination" club. On the other
> had, everyone on the list would probably just get mad at us, and shun us,
> forever.
>
> Thanks,
>
> Dave
>
> P.S.- Late stuff, since my website, courtesy my dear friend, Master GORDON
> TULLOCH:
>
>
> http://www.gordtulloch.com/harbour/index.html
>
> Dave
>
> All of you guys are the best club anyone could be a memeber of (ATM list)
>
>
> ----- Original Message -----
> From: Stan Truitt
> To: atm@atmlist.net
> Cc: scarab2@cox.net
> Sent: Saturday, June 18, 2005 6:35 PM
> Subject: Re: [ATM]: What do they offer:
>
>
> Aloha Dave,
>
> <Will someone offer me an explanation for what the "advantages" of LEDs
are,
> <compared to VERY BRIGHT incandescent filament lamps? I just have never
been
> <able to figure this out. I'm sure it's not a fad;' it's just probably
> <because I am stupid about these things.
>
> While being a fan of incandescent Tungsten, and to a lesser extent
Zirconium
> arc lamps, I would like to share an experience relating to LED sources.
In
> an ellipsoid being tested with the knife at it's long focus and the source
> at the focus closer to the mirror, convection currents from the heat
> dissipated within the light path seriously disturbed data repeatability.
> Efforts in ducting away the heated air, and water cooling the lamp
surround,
> improved the setup; but not without blocking an unacceptably large area at
> the mirror's center. Switching to the vastly more efficient LED source
> while retaining a bit of the water cooling resulted in excellent
> repeatability of the data, even with the water flow turned off.
>
> Perhaps you are not "stupid about these things", rather than highly
> enthusiastic about those things that worked for you. Patient forbearance
of
> others' curious investigation of alternate avenues seems, to me, a good
way
> to go! Pontificating is, at least for me, not.
>
> Tungsten ribbon lamps are a practical necessity in the double bounce from
> uncoated Cass secondaries in Hindle sphere setups. Not to mention
providing
> a fine place atop the lamp housing to set my pyroceram coffee cup!
> Coherence effects in porting my HeNe beam to this setup make the filament
> approach all the more desirable.
>
> I enjoy many of your messages to the ATM list. Some, for me, have been
> valuable. Your kindness and generosity garner my appreciation and
sometimes
> admiration.
>
>
> Sincerely,
>
> Stan Truitt
>
>
> _______________________________________________
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