Acquisition modes: Probe or Telescope?

 

When the `acquire' button on the autoguider control panel is pressed there are two ways of moving the guide star into the centre of the autoguider field so that the guiding loop may be locked: by offsetting the telescope, leaving the probe stationary, or by moving the probe whilst the telescope tracks undisturbed. Their advantages and disadvantages are:

• Move the probe (after AC/PR at the User Interface): Location of a guide star is normally much less accurate than the positioning of the main telescope, because of the finder flop error. Therefore, this is the default mode, because by moving the probe any residual error in the mapping will be compensated for. (Of course, when the guide loop is closed, the probe will be kept stationary.) Since the probe is moved from the Instrument Computer, but the position errors are sampled by the Telescope Computer, this mode requires data to be transfered via the interprocessor link. It will not work if the link is broken.
• Move telescope (after AC/TE at the User Interface): If the guide star position is known precisely, either in RA and Dec by accurate measurement or in probe x,y coordinates from a previous observation, then this mode can be used to make an accurate blind offset. This is not very important with an imaging detector, i.e. the Prime focus CCD, but does help with the IDS and FOS; acquisition is also significantly faster than when the probes are moved and the process is independent of the interprocessor link.

Thus, in general it is best to move the probe. If, however, you have an accurate probe position for the guide star (from a previous run), or if the interprocesor link is broken, then move the telescope.

In order to switch modes, type at the User Interface terminal:

AC/PR to move the probe (with fixed telescope).

AC/TEL to move the telescope (with fixed probe).