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Re: [ATM] Flat grinding the back of the primary mirror
Bob,
Thanks for the advice. Especially about comfortably viewing by standing
with the scope at 75 degrees.
My goal is to make two distinct scopes that would be good planetary viewers.
But, more importantly, my goal is to learn the mirror grinding skills that I
can apply to larger projects.
My other thought process with the longer focal length is that I can possibly
further minimize the m.a. of the secondary. I'm also considering spiral
spiders.
Quality is my chief objective, so if grinding the back flat on the backs
would provide any percentage of improvement, then I would like to do that.
I want to make a good a scope as you can get with a 6 inch aperture. It may
be a good idea to do it anyway, just to get some practice on how to grind a
flat surface.
My plan is to start out with a couple of 6 inch f/10's (one for me, and one
for my brother), and then (perhaps) a few 6 inch f/8's (for my three sons).
All of the scopes would be identical, with the exception of the differing
focal lengths. Nice brass nameplates with our names would be a nice
finishing touch to differentiate the owners. :-)
Then, I would move on to bigger mirrors/projects.
I purchased two 6 inch pyrex blanks off ebay which are 6:1, and grit/tool
kits from GotGrit.com. Just need to make the table to perform the grinding.
I'm planning on grinding both mirrors at the same time, progressing both
through the successive grits at the same time, and try to make them as
identical as possible.
A number of years ago, I went to Stellafane, and saw some octagonal OTA dobs
made of wood, and thought they looked very aesthetically appealing (a nice
combination of form and function). Of course, the Obsession design also
looks cool.
Regards,
Terin
Grinding the back and sides is only done to gain a regular
surface. Theoreteciall, they should be polished. However
realith sets in and polishing such surfaces really doesn't do
anything to improving the action of the mirror. If you plan in
selling the resultant mirrors, you will be wanting to make a lot
more than two of them and ground surfaces do tend to indicate
"attention to detail" that otherwise would be missing. Then
again, if you 're making a lot of mirrors, you will have the
machines to grind surfaces flat real fast.
If you're just making the scopes for yourself, I'd check for your
eye height and build a scope that would allow you to stand
straight up when the scope is at about 75deg. or so, depending
upon where you are on the earth (really just set the scope for
about the highest that a planet can get) and build your planetary
scope to that length. The longeer the focal length of the scope,
the better the magnification of the planets would be.
Bob May
bobmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net
Replace the obvious words with the proper character.
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