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Astronomy

Telescope Mounts

A telescope is of little use without a suitable support/mount. It would be wise to spend as much time and effort in deciding on a mount as is spent in choosing a telescope. A decent mount can make the difference between a system that is a pleasure to use and one that is not.

There are two main classes of mount. ie Alt/Az (Altitude/Azimuth), and Equatorial. They can be manually adjustable, motor driven, or even computer controlled. In all of these cases, adjustment is typically done via gear/worm drives so that movements can be easily controlled.

Pan and tilt tripod heads as used by photographers provide Alt/Az control, but do not include the gear/worm drives needed for effective control of a telescope.

At first sight, Alt/Az mounts might appear to be very simple to use. There's a horizontal plane of rotation and a vertical plane of rotation. Simple! Well, actually not so simple. If you want to follow the movement of a star across the sky with a manually adjustable Alt/Az mount, you must keep readjusting in both planes of rotation. Not so simple, especially after the subject has drifted out of the field of view!

Motor driven/computer controlled Atl/Az mounts can follow an object across the sky, but all Alt/Az mounts suffer from 'field rotation' (see below).

Equatorial mounts provide two working planes of rotation, known as RA (Right Ascension) and DEC (Declination), but also include manually preset and lockable Altitude and Azimuth settings. The latter two are used to do an initial 'polar alignment' in which the RA axis of rotation is set to point to the celestial pole. In the Northern Hemisphere, this means aligning to Polaris, the pole star (actually to a point very close to Polaris). The Alt and Az settings are then locked down. The intended subject is initially brought within the field of view by adjusting both the RA and DEC settings. Then, to follow a star across the sky, ONLY the RA need be adjusted. In practice, this is very much easier than using a simple manually driven Alt/Az mount!


For long exposure astrophotograpy, a Motor Driven Equatorial Mount Is Essencial. Typically both RA and DEC axes are motorised, but given an accurate polar alignment, only the RA motor should operate to track the subject.

Mounts of the type shown on the left include a built-in polar alignment telescope and a computerised handset that not only provides for control of tracking, but also for 'Goto' operation. 'Goto' allows a user to automatically slew a telescope to view a subject selected from a built-in catalogue of celestial objects.

Other advantages of the type of mount shown on the left include :
-- Easy fitting/removal/exchange of telescopes
-- Adjustable counterbalance weights to allow for a different payloads.
-- Auto guiding port for use with a secondary 'guiding' telescope.

There are a number of variants of equatorial mount. The German Equatorial Mount, often referred to as a GEM mount is the most common and is what is shown and referred to here.

Alt/Az mounts can be automated to provide tracking of a chosen target and 'Goto' operation. HOWEVER, all Alt/Az mounts suffer from 'field rotation' which means that although a star at the centre of the field of view can be tracked accurately, long exposure photographs will show other stars within the field of view as streaks or arcs around the central star.

Alt/Az fork mounted Schmidt Cassegrain and Maksutov Cassegrain telescopes from Celestron and Meade are popular. Telescope and mount are supplied as one unit which cannot be easily separated. These mounts are motorised/computer controlled and can provide very well for visual astronomy.

If these fork mount Alt/Az systems are to be used for long exposure photography, an 'equatorial wedge' must be included. The wedge allows the azimuth axis of the mount to be aligned to the celestial pole, effectively becoming an RA axis. The mount thus becomes an equatorial mount. This solution is widely used, but does not present as stable a platform, or as flexible a system as that provided by a (GEM) equatorial mount. A tilted fork mount/wedge setup imposes heavy stresses on the relatively short azimuth bearings of the fork mount and there is no facility for balancing out these stresses as there is on a (GEM) equatorial mount.

      

Dobsonian mounts: The Dobsonian mount was made popular by John Dobson in the 1960's. It is an Alt/Az mount and in it's original form had no motors or gear/worm drive at all. The mount could be made by any DIY enthusiast from easily obtainable materials and in sizes to carry Newtonian telescopes from say 6" up to very large telescopes of 20" or more. Setting up such a mount was very easy. The mount would be placed on the ground. The telescope would then be lifted into the Alt bearing cradles, and it was ready to go. No alignment was needed. The telescope was simply pushed to point at the subject to be viewed. The Internet can provide a lot of information about the history, design, and construction of Dobsonian mounts.

This original form of Dobsonian mount is best suited for use with telescopes at low magnification so that the inevitable drift of objects across the field of view is not too much of a problem. At high magnification, one is likely to spend most of the time 'chasing' objects as they drift out of the field of view. Basic Dobsonian mounts with Newtonian telescopes can be obtained from most suppliers of astronomical telescopes and are becoming populer on account of their simplicity in use and low price when compaired with conventional Alt/Az and equatorial mounts. However, some of these offerings have sacrificed the convenience of the original 'drop-in' Az bearings for rather less convenient screw together shaft bearings.

More recently, Dobsonian mounts have been made to include motor drives that can be controlled via a computerised handset to provide 'Goto' operation. The cost of these motorised Dobsonian kits is obviously higher than the basic non-motorised mounts. In all cases, Dobsonian mounts remain as Alt/Az mounts (cannot use an optical wedge) and so are not well suited to astrophotography.

Barn Door mounts: Folks wishing to do some 'wide sky' astrophotography on a tight budget might consider making up a 'Barn Door mount'. Find this in the section on Astro Projects.

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19/02/13