[ATM] Star testing an uncoated mirror? No luck.
mbartels at bbastrodesigns.com
Fri Oct 23 14:28:24 JST 2009
I like your description very much.
If I might add one point on wishful thinking... My goal is not to wishfully
wish away wishful thinking. Instead, I accept that I'm fallible. A
successful strategy for me is to try on a second night. Psychologically I'm
much more accepting that I might need to spend more time figuring the
mirror, and consequently I see defects that I couldn't see before.
I've observed this phenomena multiple times with some of the best mirror
makers that I know. They literally are blind to defects that scream out.
Once I point them out, the mirror, almost as if by magic, changes shape and
now they too can see the issues. Interesting.
BTW, I'm seen same with Foucault testing, from time to time.
From: atm-bounces at atmlist.net [mailto:atm-bounces at atmlist.net] On Behalf Of
Sent: Thursday, October 22, 2009 6:45 AM
Subject: Re: [ATM] Star testing an uncoated mirror? No luck.
On Oct 21, 2009, at 6:57 PM, Guy Brandenburg wrote:
> We finally tried last night to use a quick-and-dirty all-purpose
> star-tester on a unsilvered mirror that was being worked on and
> tested. It was an adaptation, with modifications, of the universal
> star tester that was recently featured in Sky & Telescope, and was
> worked on and modified by myself and Bill B over a period of some
> Sad to report, our first try was a complete and utter failure.
> Couldn't see ANYTHING, no matter what we did. I felt like a fool. At
> our first location, right next to the Community Center building
> where we have the telescope making class, which is a sidewalk
> illuminated by numerous lights, all I could see in the eyepiece was
> a blur; if I removed the eyepiece I got a nice view of the plywood
> in back of the primary mirror.
I really like the method I star tested with on my first mirror. After
some exhaustive testing on the bench, I silvered it, and made an
extended test over a period of several months in it's actual OTA.
There were several reasons for this:
1. After exhaustive testing on the bench, I would not have wagered
large sums of money that my test results were all that accurate. There
is a strong desire to have a "good mirror", and just like P. Lowell,
there can be a great application of smarts on the viewing end of a
2. Multiple sessions of star testing allow us to have a better idea of
how the mirror performs. So we got our mirror all ready, and the
conditions are going to be just right to star test, eh? Wrong-a-roo.
Maybe in the Arizona desert, but conditions in my area are more likely
bad seeing than good. It takes a long time to be absolutely sure that
what is scope is cope, and what it conditions is conditions.
3. Nothing like being in the actual scope instead of a test bed.
Collimation, tube currents, and other deficiencies are pointed out. We
often get so engrossed by the mirror we are making that we forget that
we are building a system.
4. Finally, and by no means least important is that The experience of
silvering is fun. You have to learn how to do it correctly, but seeing
that beautiful surface come out of the bath when you get it right is
one of those magic moments you'll never forget.
p.s. After this extended test, which lasted almost a year, I concluded
that the mirror did indeed perform well. My bench testing was pretty
sound - although I still believe that I am capable of wishful thinking
- and I had a lot of fun testing and shaking down the scope. I
discovered tube currents, a design problem with my mirror mount, a
balance problem, and a couple small issues were found and corrected
By the time I sent the mirror out for it's Beral coating, the OTA and
mount awaiting it's return was pretty much ready to rock. The downside
was that it was another year before I could call my scope finished.
The upside was that I had a scope to use all that time.
-73 de Mike N3LI -
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