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.
Because of the novel design, namely the fixed altitude (53 degrees) of the telescope and a moving prime focus tracker with a range of only 12 degrees in Hour Angle, SALT can only observe objects that fall within a certain zenith distance (31 degrees < z < 43 degrees). This means that the telescope can access a viewing 'window' in the shape of an annulus, centred at the zenith (i.e. straight up), but with radii 31 and 43 degrees respectively.
For SALT to follow targets along their sidereal paths the tracker and its instrument payload, suspended above the primary mirror, must move appropriately, as shown in the following diagram:
The tracker can move in both the X and Y directions by 3.25 metres in total, while the payload itself can tilt by 8.5 degrees in from the Z-axis in the two directions perpendicular to X and Y.
An object can be acquired and tracked so long as the tracker and payload stay within the above translational (1.625 m in either direction in X or Y) and rotational (8.5 degrees in either direction in tip/tilt) limits. SALT can therefore track an astronomical object once it is inside the viewing annulus. Objects rising in the east can be observed as soon as they are ~43 degrees from the zenith and can be followed as they rise until they either leave the annulus at ~31 degrees zenith distance, or, alternatively, when the tracker hexapods are at their maximum tilt (8.5 degrees from the nominal 37 degrees tilt angle of SALT). A similar situation exists for object setting in the west. In principle it is therefore possible at certain times of the year for an object to be observable twice a night, before it culminates and after. The following figure demonstrates how the entrance pupil of the telescope "migrates" over the mirror array, leading to a varying collecting area, which is at its maximum value (~65 square metres) when the tracker is centred (time is arbitrary).
Because the time it takes to cross the viewing annulus is Declination dependent, the amount of time available to observe a target varies considerably. This is illustrated in the following diagram which should be compared carefully with the first figure on this page. The First figure illustrates the range of hour angles over which a source can be acquired. The figure below represents the amount of track time available if the source was acquired at the earliest opportunity on a favourable night:
The key to understanding this is to realize that the telescope structure moves to an optimal azimuth that maximizes the track length during target acqusition, but is actually fixed for the duration of the track. Therefore the track length is a function both of Declination and the Hour Angle at which the track began. This is illustated in the figure below. Tracks at intermediate declinations from -63 to -12 deg can cover the entire poterntial viewing zone of 1-2 hours. However tracks at low or high declinations, < -63 deg or > -12 deg, are much shorter than the potential viewing zone. The source can continue to be observed at the end of a track by moving the telescope structure to a new optimized azimuth and reaquiring the target. This operation will result in a few minutes of down-time for your experiment for each structure rotation but can provide almost-continuous source coverage of up to ~ 7 hours in the south and 4 hours in the north.
The SALT Visibility Calculator
Calculating the track lengths for SALT targets is a non-trivial exercise, therefore we supprt a Java tool for determining track lengths as a function of date, Declination and acquisition time using the current telecope pointing model. It also provides viewing zone limits for specific dates and target coordinates and sun, moon and twilight data. This tool will eventually be called directly from the PIPT software, but it can also be run as a standalone application. The use of the SALT Visibility Calculator is a pre-requisite for all observing proposals in order for you to determine whether your observing sequence can be contained within one or more tracks and what calendar dates fulfill your science requirements. The tool can be downloaded from the Observation and Planning Tools page.