Re: ATM 4" scope plans...

[Brett Schaerer <schaerb@home.com>]

Ron-
    I've made a similar scope similar to what you're pondering in the
past (currently it's permanently loaned to my young
cousins--unfortunately I didn't take any pics before I gave it to
them--otherwise I'd have it up on my web page).  The mirror was a
homemade 4.25" f/5, in a 6" diameter black ABS tube (covered with green
shelf paper to hide it's scuffs and scrapes) on a small dob mount (put
on top of an old camera tripod, whose head had boken).
    ABS pipe is a bit more expensive than PVC (~$6-7 per foot), but is
much sturdier and lighter (doesn't flex anywhere near as much, and the
filling inside the wall is full of tiny air bubbles).  I found 6"
diameter pipe at George Morlan Plumbing in Tigard.
    From my notes, the mirror (my first) took quite a while to polish
out completely (about12 hrs.) but was very easy to parabolize after that
(got it in the first 5 minutes of figuring).  If you want to make it
easier on yourself for a few of them, you could make the mirrors f/7-8
or so and leave them spherical with no ill effects on the image (you'll
lose out on the wide fields a bit, but have a little more power for
lunar and planetary viewing with any given eyepiece).  I'm working
(slowly--haven't touched them since I before moved in April) on a pair
of 4.5" f/8 spherical mirrors (Bed Bath & Beyond candleholders) which
might become a bino or a couple of small scopes, depending on how
ambitous I am when I finish them.
    I wouldn't try to make your own diagonals, unless you need the
experience for some reason. You will spend a lot of time trying to get
them flat, (unlike the primary which can be a revolution of any
parabola, the secondary has to be exactly flat) and the appropriate
sized diagonal can be had from Orion for $13 plus shipping.
    For mirror cells on small scopes, I like to glue the mirror (with
silicone) on three elevator bolts (1/4-20 bolts, about 2" long with a
~1" flat disc on one end, used for leveling refrigerators and the
like).  These bolts are tensioned by springs, pass through a wooden disc
that fits inside the tube, and are held by wingnuts (for easy
collimation).  This arrangement is solid, easy to build and use, and
leaves a lot of airspace around the mirror.
        Others will hopefully have some better ideas with what to do on
the focuser.   I made a PVC fitting focuser for the 4.25" f/5, but I
cheated by using my grandpa's lathe to de-taper the threads to extend
the range of smooth focusing action (seems like it should be easy to do,
but I wouldn't do it again!).  (Teflon thread-sealing tape was
superglued on the male threads to take up the slack and smooth out the
motion.)
   I don't know exactly what you have in mind for the mount--equatorial
fork or dob-type with two pieces of wood sticking straight up (no
stabilizing third piece in the front of the rocker box).  I would
definitely go with a dob-type mount with front stabilizing board (it's
not that much more work or wood or weight).
    Well, I think that's about enough advice for now.  Sounds like a fun
project(s).  Good luck (I'll probably run into you in person if the
you're going to be involved with the RCA telescope making group).
--

-Brett