Altazimuth problems

Observers who have not previously used an altazimuth telescope may find some aspects of the operation of the WHT to be counter-intuitive. The two main problems are long slews in azimuth and the rotation of the field of view.

The WHT has to move through a large angle in azimuth as it tracks close to the zenith. In fact, the velocity in azimuth exceeds the mechanical speed limit within 0.21 of the zenith, so objects in this area (the ``zenith blind spot'') cannot be observed. Only objects between declinations of 28 30 \ and 29 are affected, but time may also be wasted by alternating between northern (Dec > 28 45 , roughly speaking) and southern (Dec < 28 45 ) objects . The WHT can only move in elevation by about 78 (between horizon and zenith) --- it cannot move through the zenith. Consequently, moving between objects on the meridian but north and south of the zenith takes three minutes (180 in azimuth at 1 s). Programmes should be planned so that groups of northern and southern objects are observed together.

The times until horizon, rotator and azimuth limits, if any, are displayed by the TCS.

The WHT can rotate by a total of 500 in azimuth and the azimuth displayed by the telescope computer is in the range -175 to +355 (approximately). Objects with azimuths in the range 180 to 355 can therefore be observed with the telescope in one of two positions. On source change, the telescope will always drive to the nearer of these azimuths consistent with the limits.

The field of view of an altazimuth telescope rotates as the telescope tracks. At the Cassegrain focus of the WHT, this rotation is compensated by a turntable, driven under computer control. The turntable has 500 of travel. The mode of operation of the turntable depends on the precise requirements of the observation. The options are:

1. A given position angle on the sky (e.g., if a particular slit PA is required, or if a guide star is only available at one PA).

2. Compensation for field rotation, but with no constraints on PA, so that the sky PA is allowed to alter on source change in order to minimise rotation, but is thereafter held constant.

3. Rotator tracking the sky, but with the position angle reset so that the slit is vertical whenever a new source is selected.

4. Rotator stopped, so that the field maintains a constant orientation with respect to the vertical.

The last option may be suitable for spectrophotometric observations in which the slit has to be kept close to the vertical to minimise the effects of atmospheric diferential refraction, but guiding is only possible by viewing the reflection of the object itself from the slit jaws (off-axis guide stars move). An easier alternative is to set the PA of the rotator equal to the parallactic angle (or parallactic angle -- 180, whichever is closer) at the beginning of an observation, and to allow it to track thereafter.

The turntable suffers from the same problem as the telescope close to the zenith: the parallactic angle changes rapidly with time (see Appendix ).

At Prime focus the range of rotator travel is only 358, so some position angles cannot be acquired, and much more care needs to be taken at the start of an exposure that the turntable will not reach one of its limits before the end.

Summary of Altazimuth problems:

• Zenith blind spot (telescope will not track within 0.21 of the zenith).
• Avoid long slews in Azimuth by grouping objects north and south of Dec 28 45 together.
• Azimuth limits: -175 to 355.
• Rotator options: specific sky PA; floating sky PA; stopped.