Here is my 8" f/7 telescope set up for solar observing. The azimuth engine and rocker box can be wheeled out of a storage shed for quick setup. It takes about 15 minutes from rollout to completion of initialization. I am then able to observe with the use of computerized goto and altazimuth tracking.

    After completing the Optical Tube Assembly (OTA), I initially mounted my 8" f/7 on my old Criterion German Equatorial Mount. The head was attached to a permanently mounted pier that that was sunk in 2 feet of concrete. This provided an extremely solid and vibration free mount. However, the mount lacked slow motion controls and so it could be extremely frustrating to first precisely center an object, say in the tiny field of view (FOV) of a small CCD chip. To alleviate this problem, I hobbled together a slow-motion control for the declination axis. This helped a bit, but I still had difficulty in centering objects in Right Ascension (RA). Finally, I decided to replace this mount altogether with a computer-controlled altazimuth mount. 

    A computer controlled altazimuth mount has a lot going for it:

     I purchased the gears, worms, stepper-motors motor mounts, and laptop from Lenord Stage. The gears are 15” diameter High Density Polyethylene (HDPE) produced by Andy Saulietis. These are used on both the azimuth and altitude axes. These are matched to worms driven by 400 step Vexta steppers with gear heads. The PCB and handpaddle were both manufactured by Mel Bartels. The motors are controlled by a laptop (a 486 WinBook XP running at 100 Mhz) loaded with Scope (provided free on Mel's website) and Guide 8.0, a planetarium software package that I purchased separately from Project Pluto. Lenord roughly-tuned the steppers for use with this laptop. I followed Lenord's examples of custom and no-harm dob goto mounts closely to come up with my own design. Please visit Mel's and Lenord's site for more information on computerized altazimuth scopes.

    The mount was constructed in my humble garage workshop that includes a table saw, router, and drill press. 1/2" and 5/8" Finnish/Baltic Birch plywood was used throughout. This plywood is vastly stronger than your typical home improvement plywood (see Kriege and Berry's book "The Dobsonian Telescope").

 Here is a side view of the telescope cradle attached to the altitude gear. 

The Vexta stepper, gear head, and worm are mounted on a plate. The plate can be adjusted so that the worm and altitude gear are properly aligned.

 The large, 13" radius altitude bearings ride on four bearings that are bolted to aluminum plates. The altitude bearing is lined with a 1/16" aluminum strip. This gives very low friction compared to a formica on teflon bearing. Each bearing actually consists of two - the inner bearing is a roller bearing, the outer bearing is a thrust bearing. The thrust bearing has a raised lip that helps guide the altitude bearing

The altitude gear is sandwiched between two aluminum plates lined with cork. This provides a friction clutch. Adjusting bolts can be tightened to apply more friction as needed.

The azimuth engine consists of a Vexta stepper, gearhead, and worm mounted to a plate (similar to the one on the altitude axis). This is bolted to the underside of the top plate. The gear is attached to the bottom plate. A 12" lazy suzan type bearing rides on spacers above the azimuth gear. Six rollout bearings attached to the top plate roll over a 1/8" thick aluminum plate that faces the top of the bottom plate. This provides additional stability. The bottom of the rocker box is faced with Ebony Star formica and rides on three squares of teflon like a traditional dobsonian mount. This functions as a clutch for the azimuth axis.

A Comment on GoTo

    Goto telescopes have been around for several years now yet letters are still routinely being published in S&T arguing for and against their use. I, for one, love goto. I have been star hopping for over 25 years and so know my way around the sky reasonably well. However, I live where there are light-polluted skies making star hopping difficult. I also have limited time for observing and so appreciate finding an object rapidly so I can spend time examining an object rather than laboriously looking for it. I don't find tracking down an object challenging so much as frustrating. Also, there are things a goto scope can do that are difficult with other mounts. For example, my goto allows me to find bright objects in full daylight without difficulty. I recently was observing the sun. As it was day, I initiallized the scope using the sun alone. Venus was near inferior conjunction and nearly impossible to see in the evening sky and so I decided to see if I could find it with my goto. I used Guide to select Venus. The telescope slewed just a few degrees elongation west of the Sun and THERE IT WAS! This is incredible, I thought. Lets try Mercury. Again, I selected Mercury in Guide and the telescope slewed to the east of the Sun and THERE IT WAS, less than 6 arc seconds in diameter and a mere 12 degrees elongation from the Sun . Finding and observing the inner planets in the daytime opens up some new opportunities for me for high-resolution imaging.

Day Time Initialization

    Initialization is usually straight forward. I usually use the two-star method for night time observing. Instructions for this can be found on Lenord Stage's and Mel Bartels' web sites. Here's how I initialize the scope in the daytime. I first place the scope horizontal and point it roughly south and enter the altazimuth coordinates 0, 180 (don't forget to then reset to the entered altazimuth coordinates). Then I launch Guide and note the Sun's (or Moon's) altitude and azimuth coordinates. I manually slew the scope to near the center of the sun and enter the altitude and azimuth coordinates for the sun's current position. I change the handpaddle mode to "Init 1 On". I depress the upper left handpad button. Then I change the handpaddle mode to "Init 2 On." After waiting for a few minutes, I slew the scope to again center the sun and depress the upper left handpad button again. I check the initialization. If it closely matches my site's latitude and longitude (this is usually to within about 2 degrees), I'm a happy camper. I may then turn tracking on (hit "T") or goto a new object. To then find a planet in the daytime, I launch Guide, "Go to Planet x" and then exit. The scope slews to the new target and starts tracking.

Minor Drawbacks

    Initially, I experienced some minor problems with my tracking system. Tracking was not as smooth as I would have liked. For example, at high powers of about 400x, there was a "heartbeat" movement detectable. This wasn't enough to blur details on planets during visual use or even during imaging at high focal lengths (typically at over 7,100 mm!) but it was somewhat annoying. Much of these complaints were eliminated after I applied quarter step correction (QSC) to the altitude motor. To make these adjustments, I followed instructions posted in the Excerpts from the Scope Mailing list maintained by Ben Davies. This consisted of measuring motor movement over a series of steps of the stepper motor as measured by reflecting a laser from a laser pointer off of a mirror attached to the motor shaft, and projected onto a distant wall. I will eventually also apply QSC to the azimuth motor.

    The mount is fairly quiet while slewing but makes a relatively loud and irritating sound in tracking mode. I may try to reduce  this noise by better tuning the steppers and by placing a housing over the altitude stepper motor. There is plenty of advice on house to reduce stepper noise on Mel Bartels' site. However, as time goes on, I find the noise bothers me less and less.

    One of the other problems inherent in a computerized altazimuth mount is field rotation. I often attempt to image the planets when they are near the meridian. This is where field rotation is at its greatest. I find that it isn't a problem as long as I restrict my selected frames from no more than about a 1 - 2 minute interval. Jupiter rotates so rapidly that its rotation limits usable intervals to less than 3 minutes anyway. Saturn  might benefit from combining frames from long intervals because longitudinally discrete features are rarely visible. The planet is relatively dim and can sometimes benefit from combining more frames to reduce noise. Creating multiframe mosaics of the Moon or Sun are also a problem, and frames from the mosaic must sometimes be rotated before assembly. A solution would be to install a field de-rotator. I'm not quite yet ready to tackle that project.

Field Rotation over a 7 minute period is apparent in this animation of Saturn from November 7, 2002. Saturn was 22 degrees above the celestial equator and near the meridian when these images were obtained. The images were obtained with an 8" f/7 newtonian with a Philips ToUcam Pro Webcam through an Edmund IR cutoff filter @ f/27 achieved with negative projection through a Televue 2x barlow.

Stability

    On the positive side, this mount is one of the most steady and stable that I've ever used. I can bump the mount while it is in use and this results in barely a jiggle at the eyepiece. Vibrations damp out very quickly. The results of my high-resolution imaging speak for themselves.

Links

Lenord Stage - Great service and advice on how to buy and/or build a working computer-controlled telescope.

Mel Bartels - A wealth of information on how to computerize a telescope plus links to sites of others who have completed their own scopes.

Books

Build Your Own Telescope by Richard Berry, Willmann-Bell, Inc.,

How to Make a Telescope: Second Edition, by Jean Texereau, Willmann-Bell, Inc., 440 pages.

The Dobsonian Telescope: A Practical Manual for Building Large Aperture Telescopes by David Kriege and Richard Berry, Willmann-Bell, Inc., 496 pages.

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