transit telescope

A transit telescope is a telescope mounted to allow change of its tilt between "more vertical" and "more horizontal", but that supports no rotation around the horizontal. They are typically positioned so vertical rotation is north-south, i.e., positioned to view anything directly above the local meridian.

The passage of a star across the field of view of a telescope is termed a transit and the timing such transits is an astrometry technique to determine precise right ascensions. The term transit telescope was coined for optical telescopes specifically designed to carry out such measurements: using a north-south configuration, it can view a meridian transit of any star observable from its location, a meridian transit being the star passing the celestial meridian, the great circle around the celestial sphere that passes through the celestial poles and the zenith.

Radio telescopes have been configured with this restriction (i.e., as transit radio telescopes) but not with the purpose of such high-precision astrometric measurements. Given their daytime usability, such a radio telescope can observe any target within their reach within any 24 hours, and for all-sky surveys, they can cover all the sky observable from their location even with just that single degree of freedom. For very large radio telescopes, available resources can be dedicated to the size of the dish rather than a mechanism to horizontally rotate it. The lack of east-west tilt limits the integration time of an observation, but repeated viewings are in some cases a practical way to provide more.

Some past large radio telescopes were built that way, and more recently, some telescopes dedicated to certain all-sky investigations have been built this way. Example transit radio telescopes:

• Northern Cross Radio Telescope.
• Atacama Cosmology Telescope.
• Nançay Radio Telescope.
• Ooty Radio Telescope.
• Canadian Hydrogen Intensity Mapping Experiment (cylindrical).
• Tianlai Cylinder Array (cylindrical).
• Green Bank 300ft Telescope (past).
• Lovell Transit Telescope (past)

The cylindrical telescopes do not move their reflector at all, but use phased-array methods with a row of receivers to identify the declination of radio sources. Some radio telescopes that physically aim over just one dimension handle the other dimension using phased-array methods. This includes the Northern Cross Radio Telescope and also the Ooty Radio Telescope, which is physically aimable with an equatorial mount. Radio telescopes that truly view across just one dimension have the disadvantage that they are nearly useless for VLBI.