Southern declination limits

At low latitudes, fork mounts in equatorial mode cannot point at the southernmost part of the sky (Figure 4.14). The reason is that a fork-mounted telescope cannot point directly away from the pole; if it did, it would be looking through its own base. Accordingly, there is a limit to how far south the telescope can be aimed, and if you live in the tropics or subtropics, part of the sky may be blocked.

This limit can be expressed as a declination, independently of the observer's latitude. For example, most Schmidt-Cassegrain telescopes experience some blockage when aimed south of declination -50° or -55°, which is not a problem in the continental United States. (At New York, declination -50° is on the horizon.)

Latitude of New York -No problem

Latitude of Miami -Can't see south horizon

(view blocked by base of telescope)

Latitude of Miami -Can't see south horizon

(view blocked by base of telescope)

Figure 4.14. A fork-mounted equatorial telescope cannot see the far southern sky at low latitudes because the base blocks its view.

Maksutov-Cassegrains, with their longer tubes, have more of a problem. For example, the Meade ETX-90 "bottoms out" at just -35°, which means that even in New York, part of the southern sky is not viewable in equatorial mode.

German-style equatorials do not have this problem, nor do fork mounts in altazimuth mode.

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