Procedure to generate a single CCD frame containing a number of equal length exposures on the sky. Between each exposure the telescope position and telescope focus are stepped. The resulting image is the equivalent of a ``focus plate'' and from the image sizes the correct telescope focus can be determined.
2=to: 2!:}Format: FOCUSIMAGE
2=to:
2!:}Prompts: FOCUSIMAGE prompts for:-
Channel name (PFIP or AUX);
Start Focus Position (mm, ask TO
for previous night's value);
Focus Step (mm, recommended values 0.1 to 0.2);
Number of steps (typically 5 to 10);
Telescope step (arcsec, typically 10-20);
Exposure time (seconds, typically 5-10)
2=to: 2!:}Comments: FOCUSIMAGE is an ICL procedure which co-ordinates telescope focus and position changes with the CCD shutter, then reads out the resulting image into the DMS, and into a scratch file on the VAX. The procedure puts in a double length position step between the first and second focus positions, to aid identification of the images. The position offsets are in the X direction as understood by the TCS, and both position and focus offsets can be positive or negative. At the end of the procedure the telescope is moved to the original position, and the focus is returned to the start focus position. The focusimage can be transferred to the SPARCstation, and IRAF IMEXAMINE used to determine the best focus (ie. smallest FWHM).