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Subject: Re: (LX 200) Beginner could use some direction
From: John Martellaro
Reply To: mapug@shore.net
Date: Wed Apr 03 15:48:32 1996
.... The image never seems to get very sharp. I have
>noticed the same thing when I use the scope to look at other objects during
>the day.
One way to test your optics is to point the system at a 3rd or 4th
magnititde star and use about 100x or 150x. You should see what's called
at Airy disk. This is a disk of light with about 80% of all the light from
the star. That's surrounded by concentric rings, each fainter and fainter.
If you can see the disk, that's great. Your scope is fine. If you cannot
obtain this disk with concentric rings at any magnification, that doesn't
mean the scope is bad.
I learned recently that Schmidt-Cassegrains are manufactured by vacuum
forming the (heated) corrector plate against a master metal plate. When
cooled, it is matched to what ever primary seems to give the best results
on an optical bench. (Some of the MAPUG gurus will have additional
information.) If the primary-secondary-corrector-plate meets the in-house
optical test, whatever that may be, then the system is shipped.
This is contrast to a system like a Questar where the pair is
hand-refigured until it can produce an Airy disk. So there is some
considerable extra optical craftsmanship and expense there.
I have been asking LX-50/200, 7-inch Maksutov owners whether their system
can produce an Airy disk because I'd like to buy one. Clearly, a Maksutov
corrector plate cannot be fabricated in the same fashion as the S-C system,
so that's been a topic of interest. There have been some comparisons
against the Questar 7 in sci.astro.amateur. Several have reported that it
can produce one. No doubt, the fact that it is f/15 helps.
I don't believe that a system like this can be tested in the daytime.
That's how most casual binocular buyers do it. I tell them to test them on
Jupiter. If the binoculars can put all the light inside a near perfect
circle and the moons are all points with no flares, the binoculars pass.
I think a good optics test is the Trapezium embedded in the Orion Nebula. I
don't recall the magnitudes, but I think they are about 4 or 5th magnitude
-- and form a trepezoid. If not Airy disks, then they should be at least
fairly good pinpoints with no flaring at, say, 200x. You should see
delicate filaments from the nebula, suggesting a 3-Dimensional effect.
Everyone has a favorite test.
If you have a friend who's a fairly experienced amateur, he or she can
probably tell, in a few minutes (at night), whether your system is good
enough, or for some reason is somehow out of alignment or fails customary
QC. Occassionally, one of these scopes gets shipped that is borderline,
and it's up to the customer to determine if the system is acceptable.
Taking it to a star party also a good idea - you can do some side-by-side
comparisons.
John Martellaro Internet: marty104@usit.net
The SimSmythe Company http://www.usit.net/public/marty104/
Voice: 1-500-HiDrLea
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