A "mount" is a glorified tripod. Astronomers often use equatorial or parallactic mounts, which have one axis parallel to the rotation axis of the Earth. This is very useful for astronomy as you can compensate for the stars rising and setting by moving only around the first axis, and moving at a constant rate. The setting of the second axis can remain fixed.
Many equatorial mounts have motors to turn around the first axis once a day, so the observer does not even have to do the turning herself. This is what you call tracking the object. Mounted on the mount is some sort of optics, usually a telescope. There are small motorised equatorial mounts designed only to carry a camera with short to moderate focal length.
Tracking is not terribly precise, so that for long exposure astrophotography the observer will use two sets of optics. The longer focal length is used with a reticle eyepiece and a star to check and correct the tracking, what we call guiding. The shorter focal length is used to bring light onto the detector, be that a photographic film, a CCD camera or in our case a webcam.
|Celestron 8 telescope. The frequency changer is here powered from a car battery.|
My telescope is a Celestron 8. This is a Schmidt-Cassegrain design with a D = 200 mm mirror and a focal length f = 2000 mm. The f ratio is f/10. The mount is a fork mount on an equatorial wedge, which in turn is on an aluminium column tripod. Two motors drive the hour angle axis. They are designed to track at solar rate from a 220 V, 50 Hz AC power supply. The power consumption is 1 or 2 W.
To use a stellar tracking rate, I purchased a long time ago a "frequency changer", which supplies 220 V AC with adjustable frequency. It uses a 12 V DC supply such as a car battery. The power intake is about 10 W so that an off-the-shelf 12 V 0.8 A regulated DC supply can just about cope. That of course works off the 230 V AC supply from Scottish Power.
When using the telescope for imagery, the mount can only track, there is no way to guide. Alternatively, and to increase the field of view, I may mount an ordinary photo lens piggyback. In that case the telescope is available for guiding. Guiding is a strenuous activity; usually I don't guide the webcam with the telescope, but still use the telescope mount for easy pointing and for tracking.
For good tracking and for precise guiding, the mount has to be aligned exeprimentally with the Earth's axis. This is time consuming and for webcams is not necessary. Rough alignment with Polaris is sufficient, because the individual frames are short exposures and are aligned before stacking anyway. However, bad polar alignment also causes field rotation. If the frames of a stack are taken over a long period, stacking becomes more complicated and software to do this may not be available.
Often I don't use an equatorial mount at all, but just a webcam with 50 mm lens mounted stationary (no tracking) on a standard photo tripod. Exposure times must then obviously be kept short, but with a webcam that is usually the case anyway. This makes the webcam very portable, I can take it on holyday without filling the car boot with a telescope.
Copyright © 2003 Horst Meyerdierks
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