Re: ATM Beginner-Before I build this thing

["Bob May" <bobmay@nethere.com>]

The bolt through the spring works very well.  The angle that you need to
move the mirror is very small as you are trying to point the center of the
mirror directly back up the tube so any angle should really be 0 rather than
something else which is exactly on center for the spring over the bolt
system.  The nicety of that system is that you don't need to do push bolts
to hold the mirror backing plate in place as you adjust the primary.  With
the spring in there, you just move the nut and you are done.  With the
push-pull setup, when you tighten the push bolt you may end up moving things
about and then have to move the pull bolt to compensate.
As a side note on the adjustment bolts, make sure that they don't rub on
wood as that will allow for movement as the wood gives way.  Better to use
fender washers (the huge diameter ones relative to their hole) under regular
washers to insure that the forces get spread out well.
As to the offset, at F8, that offset will be quite small and can be ignored.
In much shorter scopes, the amount of light that will be missing on one side
can be irritating to those who worry about such little items.  In truth, the
effect is quite minimal and affects the image in that the brightness of the
image is brighter on one side and dimmer on the other.  If you want to have
some fun, sit down and figure out how much that happens on a F3 system (an
absurdly short scope) and you will see what I mean.  Express the brightness
change in mag. and it will be seen for what it is.
For a focuser, I've seen a bunch and the sink fitting one does well although
I have seen (and built) a lot of homemade Crayford ones from everything
probable.  Plastic, wood and metal have been used for some to all of the
designs I've seen!  I saw one that used buttons for the bearings (drilled
out the center and put on wood dowels!) with a plastic tube and wood base.
I would have used maple but he really went cheap and used pine!  It really
worked for the lightweight 1.25" EPs!
Bob May
http://nav.to/bobmay
bobmay@nethere.com