Imagine aiming something only to watch it slowly and constantly move off target. Frustrating! Yet that is the position that many telescope users find themselves in. Objects slowly and constantly move out of the view, necessitating a dash back to the eyepiece every minute or so (every few seconds if at very high magnifications!) to recenter the object, lest it be lost.
Dobsonian telescopes offer big thin mirrors with stable, inexpensive altazimuth mounts. Motorizing a dobsonian adds automatic tracking and finding capabilities.
With the object remaining centered in the eyepiece, you enjoy the relaxing view. In addition, you can use higher magnifications to bring out detail. Finally, you can greatly extend your reach into the universe by attaching a camera to your telescope.
The rapid locating of very faint objects is easier thanks to computerized slewing. In addition, computerized motors allow for complex telescope movements such as smooth high power 'fly-over' scrolling scans of large extended objects and fields, and high speed satellite tracking.
With a computer controlled motorized mount, you have several ways to locate an object. You can star hop by grabbing the scope and moving it around the sky by hand. The optional encoders will keep track of the scope's position. You can also star hop using the motors, adopting a hand's off approach. In addition you can move the scope either by hand or by motor until the coordinates displayed on the screen match the object's coordinates. Finally, you can let the computer find the object for you either from one of the many object lists included, or from Guide, or another controlling planetarium program. The most important point is that however you arrive at the object,the scope will track, keeping the object centered in the eyepiece.
In fact, for my 20" f/5, the field where coma does not exceed 1/4 wavefront is exceedingly small - a couple of Jupiter diameters from the center of collimation. This calls for very accurately reposition every few seconds if hand tracking.
Because altazimuth drive rates vary constantly, we will need a microprocessor a.k.a. PC or laptop to operate the motors. With steppers, the software, via the PC's parallel port, directly controls the waveform of each stepper motor winding, resulting in an inexpensive and simple drive of great versatility. The steppers are made to track smoothly by microstepping and to slew at high speed by overvoltage halfstepping. With servos, the low level servo controller chip takes care of rotating the motor at an accurate velocity, while the higher level software calculates drive rates, corrects for errors, and watches for commands.
Optional encoders can be interfaced to the control program. This allows automatic updating of coordinates when hand-slewing, and automatic recentering of an object if the scope is accidently bumped.
A note about field rotation: a field de-rotator is optionally included in the project, however, it is only needed for long exposure astrophotography on an altazimuth mount. For most areas of the sky using an altazimuth mount, one can image with a CCD camera for a few minutes, and expose film for a couple of minutes before field rotation shows in the extreme corners.
I love amateur astronomy because it transports me to the heavens; the poetry of the stars, the peacefulness of a high mountain meadow at sunset, the thrill of discovery of starlight millions even billions of light years away when I peer into the eyepiece. True enough, I love to build telescopes and grind mirrors, but ultimately, the telescope is but a vehicle to span the incredible distance to the stars. Computerized finding, and particularly tracking, is important enough when using a telescope that I am willing to put up with the complexity of a computerized drive. But only a limited amount of complexity. The design of this project has as one of its most fundamental tenants the minimization of complexity and fussiness. It is as simple as I can make it: a single motor per axis, a very simple drive circuit, minor modifications to the standard telescope mount design, and the use of inexpensive older PC/laptops to control the scope.
Changing to an equatorial mount:
advantages: if carefully polar aligned, a single drive motor operating at a constant speed can handle the tracking,
drawbacks: a heavier more complicated mount that is trouble to transport, eyepiece can be awkwardly placed unless a rotating tube cradle is added, and the traditional sling can no longer be used to support the thin mirror's rim.
Adding an equatorial table is a good solution:
advantages: a platform is relatively easy and inexpensive to build, no modification to the telescope itself is necessary, no field rotation at the eyepiece, no computer needed,
drawbacks: a platform is best used within a latitude band of +- 5 degrees, polar alignment takes time, a heavy scope requires the platform to be designed to rotate the scope around its center of gravity, guiding a scope on a platform will induce field rotation unless you are imaging near the meridian, the platform must be rewound every hour or so, and one does not have motorized 'goto' functionality.
Computerizing an altazimuth mount is the solution I choose:
advantages: the mount can be placed at any latitude, and after a quick alignment, the telescope accurately tracks and finds objects across the sky, the system can be programmed for complex touring motions, altazimuth mounts are more transportable, it is also easier to mount the thin primary without causing undue flexure, coupled with a motorized focuser a total hands off approach can be taken resulting in more accurate focusing, and vibration free viewing,
drawbacks: modification of the mount to add two motors, field rotation can be handled but at the expensive of a third motor rotating the focuser, extra setup time for portable scopes, and in this system, a laptop or PC is required.
I have found over the years that my computerized newtonian has transformed my observing experience, bringing it to a new exciting level, allowing me to see objects and do things I could not have done otherwise. That alone makes the project worthy.