The merry-go-round observatory.
The observatory is about 6.5 x 6.5 feet. The base is on eight 6" wheels, so the observatory as a whole can be rolled out of a garage that is its permanent home.
Attached to the base is the 'track', which in this case is a ring of three layers of 3/4" thick MDF. It is 5' in diameter. At the center is a bearing with a 3/4" steel shaft.
The observatory itself consists of a platform that rotates on the track. It has an 8" driven wheel that is connected to a commercial 1:100 gear reduction box, and two 6" semi-steel undriven wheels. A bicycle chain connects the driven wheel to the gear reducer, and a timing belt connects it to a servo motor. On the platform are the observing hut and the telescope.
The observing hut measures about 3 x 6 feet. It is insulated, has a door, and has a a two-part flat roof. One part lifts out, giving an 'open ceiling' for summer use. The other part is clear polycarbonate ("Lexan") that allows a view of the sky for the observer and the finder scope (as explained below).
Also on the platform is a 22" folded Newtonian telescope. The eyepiece from the telescope moves in an arc-shaped cutout in the observing hut. This allows heated indoor observing. The telescope itself only moves in altitude, since rotation of the observatory as a whole provides motion in azimuth.
The 22" newtonian has an 8" diameter flat secondary, and then a 3.5"
diagonal. This produces a very compact configuration:
-----------------------------------------------------
--> \
--> \
| \
| \ |
| /
--> /
--> /
-----------------------------------------------------
8" flat diagonal primary
Although the focal length is 99", the eyepiece is only 60" from the primary!
Designing a folded newtonian is tricky business because the relative size of the shadow of the diagonal on the flat needs to match the relative dimensions of the shadow of the flat on the primary. The final design was ray-traced.
There is an 8x50 finder that is adjacent to the eyepiece and also is on the inside of the hut; it peers through the transparent roof.
The telescope moves in azimuth, Dobsonian style. It uses two 1:30 gear boxes that drive 5/8" shafts on one side of the rocker box, while there are two passive 5/8" diameter bearings on the other side. The two gear boxes are linked together with a flexible shaft, and are driven by a small Pitman servo.
The observatory rotates using two surplus servo motors with a home-built driver. The hardware design is very simple, using just a couple of chips. Power is provided by a 12V, 5A surplus linear supply, and the control circuitry talks to a computer at 19.2Kbaud over a serial line. Custom software running under Linux makes it all work. The servos have encoder feedback, and can be positioned quite accurately. However, the azimuth motion is indeterminate within a couple of percent because of the ambiguity of the radius-of-motion due to the 2" width of the driving wheel. I have put an additional encoder on the center bearing to allow the azimuth position to be known within a couple of arc minutes.
The merry-go-round observatory.
This is a view from the inside. At the
top is the acrylic roof; straight ahead is the arc-shaped
cutout through with the eyepiece is visible. At the lower
right is a chair with a pillow.
Here are the altitude bearings. The
altitude is controlled by a servo motor connected to two
right-angle gear reduction boxes. The 24" radius altitude
bearing rests on the two 5/8" steel driven rods.
Here is the azimuth control. A
servo motor is coupled with a timing belt to a 1:100 gear
box, which is coupled to the driving wheel with bicycle
chain. The driven wheel is 8" diameter and 2 1/2" thick,
and rests on a 6-foot diameter circular track.
This is another shot of the
interior of the observatory.
This is a view down the tube.
The camera is next to the 8" diameter secondary (not
visible). You can see the tertiary (diagonal) and the
22" primary. The tube has many hexagonal cutouts (some
visible at the far left) to reduce weight.