Computer Controlled Dobsonian Telescope
The telescope at Schaller Observatory was significantly upgraded in the Fall of 2000. The new telescope has a main mirror diameter of 55.9cm (22 inches). The F/4.5 mirror was manufactured by Pegasus Optics, and is fabricated from 5.1cm (2 inch) thick annealed Pyrex. This amateur-built truss tube Dobsonian telescope was purchased used (as advertised in the Astromart Classifieds ) and underwent extensive modifications for this application. The two most significant modifications were: 1) Concentric optical relaying of the collected starlight to shorten the OTA (Optical Tube Assembly) length, and 2) Computer controlled "GO TO" automation.
The mirror's focal length of 2.5m (99 inches) created an OTA that was too long for installation into the existing observatory structure, which had previously housed a Coulter Odyssey 33cm (13.1 inch) motorized Dobsonian. Consequently, the optical path was "folded" using a central, circular flat mirror: Optical Path (Image, 94 Kb). The folding mirror diameter is 20.3cm (8 inches), polished to 1/10 wave, as purchased from Advanced Optics. The photometer is also mounted central to the OTA axis, residing in the shadow of the folding mirror. This arrangement is quite satisfactory for automated photometry or CCD photography, but clearly is not suitable for visual work at an eyepiece. The main detracting feature of this design is the 13% loss in collected starlight due to the presence of the folding mirror. Here is a picture of the stowed telescope, with protective guards on the main and folding mirror: Telescope Picture (Image, 126 Kb).
Dobsonian Telescopes are ideal candidates for "GO TO" automation per Mel Bartels' plans.
Building the stepper motor controller is something of a challenge, especially if you are not familiar with electrical projects. I ordered all the parts I needed from Digi-Key. It worked (first try!) and has not required my attention since. Here is a picture of the completed controller: Controller Picture (Image, 39 Kb). Completed controller boards are also available directly from Mel Bartel.
Locating suitable stepper motors for the project can be a challenge. I purchased used stepper motors from All Electronics for about $10 each by watching the catalog closely.
The telescope automation project also requires two high-ratio gear boxes. Now, if you read about the project at Mel's website, the gear boxes are constantly mentioned as difficult to find and expensive. I found my gear boxes used, scrounging through the junk at the American Science & Surplus store in Milwaukee, Wisconsin. Here's the secret: Don't look for gear boxes, look for electric motors that have gear boxes attached! The surplus gear boxes used here cost less than $5 each. Simply remove the extraneous motor and use the gearbox with your stepper motor . I have since seen similar gear motors at MECI, used, for less than $10. Don't be fooled by the hardware looking like an antique, it probably is antique... but so what! They work great and they are inexpensive.
Oak lumber was pressed into service as a structural members for mounting the dual axis drives. Conveniently sized oak boards are readily available and reaonably priced at most home improvement centers. Here are some pictures of my version of Mel's Automated Telescope Project: Altitude Drive (Image, 269 Kb) and the Azimuth Drive (Image, 225 Kb).