Previous: Zeroing the main telescope and the finder
Up: PREPARATION FOR OBSERVING
Previous Page: Zeroing the main telescope and the finder
Next Page: OPERATING THE TELESCOPE

Telescope and autoguider focus

1. Automatic focus exposure

   The CCD is focused by making a multiple exposure, stepping both the focus and the telescope between exposures and only reading out the chip at the end of the operation. The multiple images are examined to establish the best focus (accurate to 0.05 mm or so). The exposure is controlled by an automatic procedure on the instrumentation computer. Make sure that there are no very bright stars in the field of the chip as these saturate and so are no good for focusing: the plan is to make exposures of > 5 sec in order to smooth out seeing fluctuations. To start the procedure, type:

   Adam:> FOCUS

   and follow the prompts. The parameters are the number of exposures multiplied by +1 or -1 for positive or negative focus increments, respectively, the exposure time in seconds and the direction of motion of the image on the chip (N, S, E or W). The focus is stepped by 0.1 mm between exposures, depending on the sign of the first parameter and starting at:

   Current focus 0.05(n - 1) mm,

   so that the current focus is in the middle of the range. The telescope is offset by 20 arcsec between shots: make sure that there is enough room by looking at a GLANCE run or on the finder display before you select the direction. Check the exposure time by using PHOTOM on the results of a GLANCE. At the beginning of the night, try seven exposures ( 0.3 mm around the nominal value). Fine-tuning during the night can be done with 5 exposures: differences in focus of 0.1 mm are easily distinguishable under good conditions. The telescope will be returned to its starting position at the end of the run but the focus remains at its final value. A double step between the first and second image establishes which image corresponds to which focus position. It will normally be obvious which images are closest to focus and these should be measured using STARFIT and the seeing noted from the focused image.

2. Enabling the automatic focus adjustment.

   The filters in front of the CCD have different optical thicknesses and it is necessary to compensate for these by refocusing the telescope when a filter is changed. The thicknesses are told to the instrumentation computer by the command WHEEL. The computer then requests a focus alteration whenever necessary. After a focus exposure (see above) and before changing filter, type:

   Adam:> SETFOCUS < value>

   where value is your best estimate for the focus in mm. The focus will be driven automatically to the correct position, and adjusted as you change filters in future. (This assumes that the optical thicknesses of all of the filters have been set up using the WHEEL command - see section 3.2.4 for details of how to do this). If you do not run SETFOCUS, the default value for no filter (46.90 mm) will be assumed, and adjustments for other filters will be made appropriately. Additionally, the telescope control system will apply automatic compensation for the contraction of the telescope steelwork as the temperature drops through the night, currently -0.11028 mm per C increase. If the images are clearly resolved, it is worth checking the focus every two hours or so. Check more frequently if you suspect a problem or if there has been a large temperature change, of course. Beware of starting an observation until the focus has stopped driving and the red FOCUS STOP lamp on the control desk is lit. To disable automatic focus adjustment, type

   Adam:> SETFOCUS DISABLE

   and to re-enable it,

   Adam:> SETFOCUS < value>

3. Focusing the autoguider.

   It is most unlikely that the autoguider will be detectably out of focus provided that it is initially set up at the nominal value for the filter in use. However, if the images look very extended and in particular if they have holes in their middles, then the focus should be adjusted. To do this, find a star on the autoguider and step the focus through a range of values around the nominal one using the GDFOCPOS command on the ADAM terminal. Legal values are -3.3 to +3.3 mm. After each step, press FIELD, then WAIT on the autoguider panel and inspect the image. Do not leave the autoguider integrating otherwise the images will get mixed up. When you have discovered the correct focus, or you are happy with the nominal one, type

   Adam:> SETGDFOCUS < value>

   where value is your best focus in mm. If you do not type this at the beginning of an observing session, the software will assume that the autoguider focus in the absence of a filter is +2.7 mm. To disable the automatic focus compensation, type

   Adam:> SETGDFOCUS -999

   and to re-enable it, set a legal value. The autoguider focus drives much more quickly than the telescope focus, so you need not worry about starting an exposure with it out of position.