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I read Mr. Sherman's email about his Galaxy mirror with great
interest. The 15" mirror that originally came with my "premium" commercial
dob was a very poor mirror until I sent it to Galaxy for refiguring. The
original manufacturer was unwilling to do anything to it unless I paid him to
refigure it, and that's what led up to it being sent to Galaxy. The
original mirror foucault tested at 1/5 on the wavefront with a Transverse
aberration of 3.03! Now it yields superior high power planetary images and
tests at 1/10 on the wavefront with a Transverse aberration of .65.
Clearly, in this case Galaxy did a more than competent job. I concur with
one respondent that the mirror may be over corrected, but from Mr. Sherman's
description I believe there is more going on here. His description of the
diffraction rings inside focus(or the lack thereof) is EXACTLY what
my 2" thick primary looks like inside focus until it cools off. Until I
get the mirror to within a reasonable degree of the ambient air temperature, it
looks horrible! However, once it cools, I get what are for all practical
purposes text book diffraction rings inside focus. Mr. Sherman's 2.25"
thick mirror is one heck of a big chunk of glass to get cooled off, and I highly
suspect it is serious thermal problems that MAY be preventing him from being
able to get ANY diffraction rings inside focus. I'd have to know more
about what measures he's taken to abate the thermal issues with such a large
mirror, but I've found it takes a lot of "horsepower" to cool my 15" full
thickness mirror. I use two 26 cfm fans blowing on the face of
the mirror, and one 85 cfm blowing on the back, and the early
indications are it's the best modification I've ever made to my scope. It
took nearly 140 cfm to get my 15" to cool down in a reasonable period of time
and perform up to it's potential. Before I took these steps I was PLAGUED
by thermal problems and substandard imaging. Finally, I don't
discount the possibility his mirror may be substandard, but I strongly believe
anyone who has a large aperture thick mirror must use a serious amount of
cfm for their optic to perform at all.
Submitted for your consideration
Alan Garcia
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