Why?: The real reason I built this telescope is because it's so much fun... but I should come up
with a better explanation than that. Actually it started out as a result of
my 20" telescope project. The 20" is supposed to be my full-time piece of equipment, but it needed
accessories! I had seen pictures of Steve Swayze's 40", complete with a 12"
finder, and thought how much fun and useful it would be to have a good sized finder on mine. I was considering a 6", but I came
across a piece of 8" glass from Richard Schwartz on the ATM list. He was very helpful in getting me
started and I appreciate his help & advice very much!
Primary Mirror: The mirror is just short of 8" in diameter. It is roughly 3/4" of an inch thick plate glass.
It looks like it might be two pieces of glass fused together. In addition, the mirror was slumped
to a 44" focal length. I started with 120 grit through 5 micron and polished using cerium oxide.
This was my first mirror, and although it was challenging the results were beyond my expectations.
Tube Assembly: The
optical tube is a serrurier truss structure. There is a central ring that defines the center
of gravity for the scope in typical use (with a 2" eyepiece). From the ring is a truss structure
that supports the secondary cage on the upper end, and the mirror & its cell on the lower end.
The secondary cage has two rings that were cut using a router. These are connected by 4 tubes, and
the focuser board. The wood is all 1/2" baltic birch. It's very easy to work with and looks very nice.
I installed a JMI DX3 2" crayford focuser. I also use this focuser on my 20",
and have found it to be superb... I regularly get comments from other telescope users about how well
it works. My wife couldn't believe it costs $150 :)
The mirror cell is made from 2 pieces of 1/2" Baltic
birch cut into circles. The trusses are fixed to the bottom piece, and the top piece supports the mirror. The plan is to have the mirror RTV'd
to this upper plywood cell, and have it adjusted by two lengths of all-thread threaded rod that protrude
from the front of the mirror cell to a place that is convenient to reach from the eyepiece.
The third collimation screw is fixed... This means you may have to move the mirror in relation to
this fixed bolt. It might have a bit of a learning curve, but should be a less work in practice.
Updated 8/2/2000 - The collimation is very nice. I can view from the eyepiece, and two bolts is not a problem compared to three.
This whole truss assembly will rotate in a hexagonal structure. This will permit are more comfortable
viewing position when the location of the eyepiece requires rotating the entire optical assembly.
The truss structure will be attached to the truss structure behind the mirror cell in the typical
"Dobsonian" fashion: with a pivot bolt and Teflon vs. Ebony Star formica. The ring in the center of the
optical tube (center of gravity) will be faced with ebony star, and also ride on
Teflon inside of the hexagonal structure to permit rotating by hand. This should permit easy turning at a low
cost. We'll see how it comes out!
Update 8/1/2000 - I got back from Stellafane and after using the scope the tube rotation works very easily. It was probably the
best feature I put into the telescope.
Mounting: I guess this is the trick behind the whole telescope. The mount says the most about what kinds
of observing you will do; at least with this sized telescope. For example, if you're a star-hopper
and are not planning on photography the likely mount is a dobsonian mounting. If, however, you are
planning on imaging it would be ideal to have an equatorial of some kind. But this telescope is
supposed to be versatile. Remember, it's supposed to ride alongside of my 20" and be able to be
used stand-alone for both visual & imaging.
Here's what the untold future may hold:
Alongside the 20": The best way I thought this scope could be mounted on the 20" was
alongside on the center of gravity. This would prevent rebalancing everything the 8" was removed.
I'm currently trying to figure out how to mount it & provide for adjustments. You know the routine,
first you align the Telrad, then the finder, and then the 8"! Further info to come...
Standalone: The other need is for this telescope to be able to be used stand-alone both
visually & with a CB245 cookbook CCD. Ideally I would be able to build the mount myself, including
the tracking system. I've got in mind a split-ring mount. This should be fairly straight forward
to construct, and allow for what I consider to be a pretty unique method of tracking. I'm planning
on utilizing a piece of nylon threaded-rod to drive the telescope. The telescope's split ring will
ride on rollers, and on the southern bearing will be another sector. Attached to this sector will
be a length of nylon threaded rod. This will rigidily attached to and be driven by a motor, and coupled
with a nut on the sector. As the rod turns, it pulls the sector towards it thereby creating the
counteraction to the sidereal motion taking place. This will be an inexpensive way of creating
the required tracking motion, and I'm hoping will give acceptable results. To keep periodic error
low, I will use a coupling nut (basically a really long nut with many more threads than usual) to
help "average" out periodic errors on the threaded rod. The threaded rod is
flexible enough to ride on the circumference of the sector, possibly in a groove. More details coming...
Updated 8/2/2000 - As you can see from the pictures I went with a split ring. The ring is about 32" in diameter,
and is covered with 1/8" aluminum strip. It is currently supported by rubber wheels. On the south end I have a 1"
aluminum bar that is in a wooden bearing lined with Teflon. An aluminum plate on the far south end acts as a flange
bearing.
The declination assembly is 1" aluminum rod coming from the tube and rotating in wooden blocks lined with teflon. The motion is very smooth, but I'm having a problem with vibrations due to the small bearing sizes. I'm going to build a few circles, and probably use
Teflon vs. Ebony Star or similar to increase the bearing sizes in declination. This should do wonders. Finally, I'll begin working on the drive system. If you're
knowledgeable in this area and have some suggestions, please feel free to let me know your thoughts.
Overall the mount is nice. It's a little larger than a Dobsonian, but when the declination motion gets redone it will be very stable and will also have the major advantage of being an equatorial mount.
Why?
