[ATM] secondary mirror help

[Jay Kirkland <jaykirk2@compusmart.ab.ca>]

You'll probably want tp go with a 1.8" secondary, rather than a smaller
diameter. The 1.83 (from Discovery Telescopes) on my 8" f8 with a fairly
standard height focuser gives a 1/2" diameter of 100% illumination, which
is fairly average for a general purpose scope. You might want to go smaller
if your primary observing interest is the planets, but as the October issue
of S&T says, the smaller f6 with a 2" focuser is a fine RFT.
	9.6" is the smallest tube ID you want to use if you extend the tube about
one mirror diameter past the centre of the diagonal. But if you don't want
to extend your tube that far, you can easily wrap Kydex, black cardboard,
etc around the tube OD to make a fine light baffle.
	On all of my scopes, I build a crude Bopp-style tube first, to test the
focal length. Just take a 2x4, carefully, securely, mount your primary to
one end, roughly square, mount your diagonal carefully, securely, at
roughly 45 degrees, to something at the other end that you can slide
coarsely back and forth. It doesn't have to be anything even approaching a
spider. It can even be a 1x2 or 2x4 edge on. You don't really need to care
about diffraction. And on this diagonal morror holder, at the right height
from the centre of the diagonal to where the base of your focuser will be,
mount your focuser. By siding the diagonal/focuser assembly back and forth,
you can find the exact place you will want your focuser to be that will
also allow you to focus all your eyepieces. Remember, all your eyepieces
now will not be all the eyepieces you'll have later. Make sure you leave
1/2" of in-travel behind the eyepiece (or barlow) that has the closest
focus point. If you ever plan on using the 31mm Nagler 5, (another reason
for a 1.8" diagoinal) leave a full inch. 
	Can't stress how important it is to secure the mirror carefully. For a
primary mirror cell, I nail on a board at 90 degrees to the 2x4 and use
lots and lots of black electrical tape. Also black tape for the secondary. 
	Jay 


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