80-cm Telescope
The biggest telescope in our inventory has a 80 cm mirror with a 3200 mm focal length (F/4), A Paracor coma corrector stretches the effective focal length to 3600 mm (F/4.5). Designed and built by Al Kelly and Andy Saulietis, Houston, TX, the telescope is on an alt-az mounting with a computerized drive system to follow the motions of celestial bodies as the Earth rotates. There is a four-position eyepiece turret holding a 32 mm wide angle eyepiece, a 20 mm Nagler, a 13 mm Nagler and a 9 mm Nagler (all two inches in diameter). For astrophotography, there is a built-in field de-rotator which is necessary with an alt-az drive.
The telescope drive was renovated with a Sidereal Technology dual servo telescope controller and two dc servo motors from BBAstrodesigns (bbastrodesigns.com). This was a simple replacement of the two stepping motors used previously with the two servo motors and connecting the appropriate wiring to the controller. The shaft encoders on the telescope were integrated into the new system for enhanced capability. After calibrating the motors and encoders, the telescope can now slew at about 2 deg/second, pan at 2 arcmin/second and guide at 2 arcsec/second.
There is a "Drag and Track" mode for manual use of the telescope. In this mode, the telescope is initialized on the NCP (North Celestial Pole) by pointing it at the NCP and pressing one of the hand pad buttons for 4 seconds. Index marks set up after a careful NCP alignment let the user get a pretty good alignment even in daylight. Once this is done the motor controller relies on the shaft encoders on the telescope (and their calibration) to set the correct drive speeds and directions for the two motors to track an object as it moves across the sky. The scope can be slewed with the hand pad. This makes the telescope an ideal platform for working on, say, the Messier Club certificate. The user will still use the Telrad, finder scope and "star hopping" to locate objects. There is a search mode in which the telescope can be commanded to do a concentric circle search around the last slew position.
The go-to mode uses a computer to interface with the motor controller. There is a "master" computer located in the base of the telescope that is controlled by a remote "slave" computer via a wireless connection. The remote computer can be located most anywhere (even via the Internet if we so choose), but for most use it is on a shelf riding on the side of the rocker box. A planetarium program is used to show where the telescope is pointing and to select new targets. We are using Cartes du Ciel. The control program is able to measure the errors in the mount and to correct for them. At the end of an observing session, the telescope is "parked" which means it is put into a position that will be "remembered" by the software. This park position is, of course, the storage position for the telescope. At the next session, the telescope is "unparked" and knows where it is pointing (not perfectly because we release the drive shafts for storage and there can be, and often is, some slight movement of the telescope from the parked position). This position is good enough to slew to a bright star which can be centered and synced. From there on, targets can be selected from the planetarium program and slewed to automatically. This mode would be used primarily for group viewing where we don’t want to waste time star hopping, especially for prepared presentations of specific target lists.
The go-to mode will also be used for imaging (when we get a suitable camera) but the remote computer will most likely be located away from the telescope (in a warm or cool room?) and the targets will be controlled by scripts (as will the camera, filters, focusing, etc).
All in all, the 32-inch is proving to be a remarkable resource that accommodates the most advanced amateur to rank beginners. ASEM members need only be "qualified" to schedule personal time on the telescope, non-members can remedy that status quite easily. 
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| Jim R. (l) with Al K. at the Danciger, Texas Observatory |