July 1,
2012: Undercorrection is not the answer
This story begins on the other side of the world, "down under", in Australia.
Frustrated
with the views he was seeing through one of the fine instruments he
built, Peter Read, owner of SDM Telescopes, went out and did something
that few telescope builders ever do - with the help of a friend he came
up with an accurate way to test mirrors that he suspected had problems.
Now that's what I call going above and beyond - a telescope builder that finds a way to quantify optical problems.
It
turns out he was right, it had a serious undercorrection problem, as I confirmed when
it was sent to me for testing.
While
some still believe that an undercorrected mirror will act properly
corrected while cooling, this does not turn out to be the case on 99% of
the nights that they are used. The undercorrection bloats stars and blurs
detail.
Why suffer bad images on most nights when you could have it the other
way around? An accurately corrected mirror will produce better
images on practically all nights that it is used, even when the seeing
is quite bad.
After
the refiguring was done and Peter got the mirror back in the scope, he
made the following short comment, which sums up the performance of the
refigured mirror quite nicely:
"Mike,
I could write a thousand words on how good this mirror is - pin point
stars in Omega Centauri, Saturn beautiful and crisp - you could drive a
truck through the Cassini division and detail on the Moon I have never
seen before. It was happy to take whatever power I threw at it and a
critical star test with a 5mm eyepiece showed near perfect
correction. But I think my wife summed it all up when she blurted
out 'I'm gob-smacked!! You've got to get yourself one of these!'"
I'm
proud to have been involved in gob-smacking someone.... I think....
Wait, let me google that.... yes I am indeed proud. (Disclaimer -
no one was hurt or harmed in any way during or by the gob-smacking.)
If
you're in the market for a large, accurately corrected mirror, please
send me an email. It might save you a future refiguring and
recoating charge.
Please check back for future installements of "In
the Shop".
Mike
Lockwood
Lockwood Custom Optics
