ATM My 8" Cave Astrola (very long winded)

["Bill Cotten" <billsas@worldnet.att.net>]

(WARNING, this is a LONG*winded* post)
Hi all,

As I mentioned in a previous post, I recently built a dob mount for my
(wife's) 8" Cave Newt. I've slapped a quick page of pictures up at:
http://www.spokaneastronomical.org/cave.htm. Please click the link now and
finish reading this message while the page loads.

It is an 8" f/8 Cave Astrola Newtonian with a coating date of 12-12-62. We
bought it in 1996 for $465 from a lady who had forgot she had it stored in
her closet for more than 20 years. This was the first telescope my wife and
I ever owned. The mount had been stored outside in a covered carport with a
canvas cloth over it. It was in excellent mechanical shape (RA driven) but
the paint was a lost cause. The OTA was in great shape. The only thing I
could find wrong with it was that the tube was slightly egg shaped because
it had been stored on its side for 20 plus years. I assume that blankets or
clothing must have been piled on top of it.

The primary coating looked ok but was dirty. After soaking and rinsing for
an hour or so I gently drug the first cotton ball across it. To my shock,
the cotton ball just shredded. Not knowing anything about star testing I
sent the mirrors out to QSP for new coatings. It never occurred to me to
look at
the sides of the mirror for a signature but there are no markings on the
back of
the primary. I doubt that this was an Alika Herring mirror since he had left
Cave to work for JPL about 14 months earlier. Next time I remove the
primary for cleaning I will have a good look for markings.

While the mirrors were out I quickly built a very crude and HEAVY dob mount
for the OTA. Three weeks later the scope was reassembled, collimated and
used for the first time. I was immediately disappointed with the images due
to vibration. You could not even touch the focuser. I ruled out the mounting
as
the cause and realized that the secondary/spider was the culprit. The
distance
from the secondary to the central hub of the spider was over 4" and the
slightest
touch would set it in motion for a minute or more. Correcting the problem
would
require drilling holes in that beautiful white tube and I wanted to avoid
that.

The only other problem I found with the OTA was the focuser. It was butter
smooth with no slop at all but the eye lens of most eyepieces was over 4"
from the surface of the OTA. The focuser had a slip fit extension tube in it
that had to be pulled way out to achieve focus. I noticed that the drawtube
was sagging under the weight of the eyepieces causing me to look above the
secondary as viewed through the focuser. To top it all off the focuser
drawtube was very nicely chromed on the inside!

After some thought and talking over my options with the fine people
(including Rick Singmaster) in the #ATM IRC chat room and my local ATM
group, I decided to install a new 3 vane Protostar diagonal/secondary holder
as well as a JMI mini-3 focuser. (I was not aware of the ATM list at that
time) In order to reduce the focuser height to accommodate the new JMI
focuser I had to either move the primary down the tube or the secondary and
focuser up the tube. Moving the primary would only gain ¼" of the needed
adjustment and in my mind that did not justify drilling three news holes for
the primary cell. A little more calculating showed that I could just barely
avoid drilling holes in the upper aluminum end ring to accommodate the new
spider. In the pictures you can see that I had to make some small aluminum
spacers for the spider vane bolts since they would rest half on and half off
the tube end ring. A small light shield (not pictured) that extends one inch
past the end of the tube opposite the focuser prevents stray light from
entering the focuser drawtube on the inside.

Small holes in fiberglass are easy to repair or hide but large focuser sized
holes are a different animal. So, with great trepidation I chucked up my
trusty hole saw and triple checked the location of the hole for the tenth
time then gently went to work. With the scope assembled, my 40mm TV plossl
ep in place and a clear sky I was very relieved to find focus with a 1/16"
out travel left to spare! I didn't screw it up! It turned out that the round
plug I had cut fit the original focuser hole so perfectly that it held
itself in place. When I installed the Telrad, the base perfectly hid the
original hole so I put the plug away for safekeeping. The original focuser
hole can be seen in the 5th picture down the page. To aid in aligning the
focuser with the optical axis of the OTA I used a JMI backing plate between
the tube and focuser. This allowed me to use the four leveling setscrews on
the focuser without them digging into the tube. The backing plate is covered
with thin felt to protect the tube.

Since the original paint on the mount was just about gone I decided to bead
blast the aluminum parts and paint it royal blue. This allowed me to take
the mount apart to clean and lubricate everything. While inspecting the worm
and ring gears I could not detect the slightest hint that they had ever been
used. There were no smooth wear marks on the gears and the grease was all
still very transparent. Because of its bulk, weight and "back corner"
storage location I have never even used the GEM. It was also difficult to
convert its dob mounting quickly for EQ use. With the new dob mount
altitude bearings I've built, conversion from dob to EQ use will be less
than a minute. My spray can paint job was adequate but I want to have it
powder coated some day. Now all I need to do is build an enclosure for the
mount in my yard.

The new dob mount was built in an unheated/freezing garage in January of
this year. It's made of ½" (9/16" actually) Baltic Birch plywood stained
with cherry stain and finished with Helmsman Spar Urethane. I borrowed the
basic design
from a picture I saw in a link posted to this list. If you know whose design
this might have been please let me know or thank him for me. The biggest
complaint my wife had about the original mount I built was it's weight and
when I saw this design I instantly knew this would accomplish every need we
had for the scope. The altitude bearing design let's us take full advantage
of the famous Cave rotating rings in both dob and EQ mount configurations.

In 1998 I gave my wife an Orion Sky Wizard III for Christmas but I had to
build a new rocker box for the old dob mount in order to use it. I never got
a round tuit so the SW3 sat in the closet until this mount was built. We
finally got a clear dark sky weekend and I was amazed at just how well the
SW works. Even with a less than perfect alignment, objects are very near the
center FOV with a 26mm TV Plossl. There was no noticeable drop off in
pointing accuracy after two hours of hopping all over the sky.

The optics in this scope are good as any I've ever seen. Planetary
performance borders on exquisite with deep sky performance limited only by
atmospheric conditions. Cool down times seem to be about 30 minutes. We
point the bottom of the OTA into any breeze that might be blowing and this
seems to help. Stability, even at high power is very good. Now the only
complaint my wife has is that the EP is too high when pointed near the
zenith. I might trade her straight across for an 8" f/6 Richard Fagin dob I
have if
she complains about it again. :o) We have saved all the original parts in
case
we ever decide to restore it to original. We have dreams of one day (when
we're very
old) giving this scope away to some young amateur who will truly appreciate
just
what a fine instrument it is. Thousands of men, women and children have had
their first look at the Universe though this scope and we hope that
thousands more
will as well.

I do have a few questions for list members about further improvements to
this scope
but will post them in a separate message to the list.

Thanks for reading and looking.
Bill Cotten