SALT/PFIS Observer's Guide

1. Introduction

1.1 The Southern African Large Telescope

SALT Coordinates:
Lat: -32 22 46 Long: 20 48 38.5 E El: 1798 m

The Southern African Large Telescope (SALT) is closely modeled on the Hobby-Eberly Telescope (HET) concept. The design comprises a spherical primary mirror mosaic of 91 identical 1 m wide hexagons, tilted at a constant zenith distance (37 degrees for SALT), with azimuthal rotation only for target acquisition. The target is then tracked by moving the instrument payload at the primary focus. The payload tracker has a range of +/- 6 degrees. The spherical aberration corrector (SAC) provides an F/4.2 beam with an 8 arcminute field of view at prime focus.

1.2 Observing Constraints

SALT is a fixed altitude telescope, and therefore observing with it is more complicated than observing with most ground-based telescopes. SALT can access ~70% of the sky observable at Sutherland, but only during specific "windows of opportunity". Objects are not always accessible by SALT, even though they may be above the horizon. However, the dates an object can be observed during the course of a year are almost identical to that of a more traditional telescope.

Figure 1 shows the region of the sky visible to SALT. This is the annulus on the sky with elevations of 53 +/- 6 degrees. The airmass over this elevation range varies from 1.17 to 1.37 with a mean of 1.25. Prominent astronomical objects are marked at their respective declinations. Objects at southerly declinations are visible for longer periods (several hours) compared to those at northerly declinations, where the average time for a single track is only 50 minutes. For all except the most northerly or southerly declinations, objects can potentially be observed twice a night at favorable times of the year.

Figure 1. SALT observing annulus as a function of declination and hour angle. The hashed regions show the range of motion for the tracker at two different declinations.

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