ATM First Fog

[Anthony Stillman <atmer@flash.net>]

When I'm asked about telescope making by would be ATMs, I always suggest
that the maker finish the mount and tube assembly before the mirror.
Adding that I have seen too many mirrors, done but scopeless, never see
first light, their makers' burned out.  What I haven't said (admitted to)
is that the suggestion stems from personal experience.

Twenty years ago I ground, polished and figured an 8 inch f/5.  On its
return from the coater it was examined and carefully repacked where it
could safely sit while I built a tube and mount.  Except for a brief
man-handling by my ex-wife the mirror has, all these years, filled its
packaging with distinction.

Three weeks ago, members of the mirror making workshop were asked to bring
their nice looking homemade telescopes to an event last Thursday evening at
the new Chabot Space and Science Center.  Unfortunately, I'm a component
carnivore.  That a part is currently included in a project means only that
it's easier for me to find.  And yea, the "nice looking" bit made me a
little uneasy too.  Just the same I decided to through something together
and to my amazement I completed it in the requisite amount of time.  To do
that, I kept it simple, opting for an old slightly modified Orion
alt-azimuth mount, a cardboard tube, manufactured optics mounts, and the
twenty year old mirror.  It took a bit of digging through various boxes,
bags and piles but over the course of a few evenings I a managed to scare
several spiders and collect all the pieces.  And yes, another project is
now missing its focuser.

While I was digging I coated the cardboard Quicktube from Home Despot
inside and out with epoxy.  I used 4 to 1 from Tap Plastics.  To the
outside sealing coat, sans color, I added 10 percent acetone to aid
penetration.  I did the same with the inside sealing coat but here I
included Tap's black pigment.  I coated the outside three more times
sanding between coats.  I had intended the tube to be purple.
Unfortunately the Tap Plastic purple separates and changes to gray-blue
with streaks of red and some purple.  It looks OK, but I may Monocoat it
sometime in the future.  I coated the inside only ones.  This after
stripping the inner layer of paper.  Not sanding leaves an irregular, very
rough and shinny black surface.  I let it bake in the sun for a day to help
blow off the slimy epoxy bloom and then spray painted the inside flat black
with Krylon epoxy enamel.  For scattering light its better than saw dust,
but not as good as walnut shells.

The placement of the various components was done with the aid of the
newtonian designer at  http://home.earthlink.net/~flyj/index.html  I've
never been comfortable with cutting holes in telescope tubes.  "I cut it
twice and its still too short"  Hence I went over the numbers three times.
They didn't change and unbelievably there are no patched or extra holes.

Once I had the optics in place I identified the center of mass by balancing
the tube.  I did this a few times and took the average.  To support the
tube in the rocker I bought two 7 inch diameter ABS drain couplings from my
favorite junk store.  Using a spreadsheet I derived the numbers for a paper
template which I cut out and wrapped around the drain couplings.  These
pseudo-sinusoidal curves allow the couplings to fit snugly perpendicular
against the telescope tube.  I used a jeweler's saw to cut the ABS and a
piece of what's called Dragon skin to rasp the shape as needed.   It was
messy but it looks nice.  In hind sight a coping saw would have been
better.  Incidentally, Orion's alt-azimuth mounts nominally use rather
small "latitude bearings", about four inches in diameter.  I corrected this
design flaw by prying up the plastic glides, removing the black
board-edging, sawing 7 1/8 inch diameter cuts outs, and then sticking the
edging and glides back in place.  At some point I'll replace the plastic
glides on the base with Teflon and I'd like to add levelers.

Both ends of the telescope tube are reinforced by an 1/8 inch thick, 1 inch
wide aluminum ring.  I bent and fit these by hand from straight stock.  To
attach the rings I used 60 second epoxy which takes about four minutes to
set up.  To do this I taped the metal that shows with masking tape for
protection and then used two hose clamps in series to draw in the ring once
I'd applied the epoxy to the metal that doesn't show.

To attach the ABS I used Supermend epoxy.  I identified the ABS bearing
center locations on the telescope tube carefully by measuring up from the
tube bottom and made sure they were directly opposite one another.  Then I
laid out on the tube with pencil where the ABS extremes should have been,
positioned the bearings and drew a line on the tube at the interface.
Finally I used short lengths of masking tape all along the line to protect
the tube from epoxy spill over.   Supermend is conveniently a sticky paste.
Unfortunately its white.  I didn't want to risk the bond by adding pigment,
hence I'm still detailing the bond line with Sharpie.  Mostly you can't see
it.

I was delighted to see that even after two decades the mirror coating was
in good shape.   Last Thursday I had yet to collimate the telescope.  It
didn't matter though, even the red airplane avoidance light was barely
visible in the fog.  Since then only one night has been clear and naturally
I was busy.

To collimate I normally mark the mirror center, but I was in a hurry.  So,
this time I've tried something different.  The last laser pointer I bought
(Costco) came with a set of computer generated diffraction gratings.  One
creates a circle of dots around a center dot.  I taped this grating to my
laser collimator and slipped it into the focuser.  The circle of dots forms
about an f/7 cone.  A little slow for this f/5 system, but close enough to
accurately orient the secondary.  In addition, in the dark,  I can clearly
see the secondary diffraction effects from the grating (multiple concentric
rings of dots) both projected on the mirror and there collimated and
reflected on the ceiling.  This made adjustments to the primary a snap.

Ronchii at the ready, I'm still waiting for first light and a chance to see
just how much more critical my eye has become.  However the clouds have
spoken, it won't be tonight.

Anthony