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FOC(US) - Make a focus exposure

This command is used to make a multiple exposure, stepping the telescope focus and offsetting on the sky between exposures. Its purpose is to provide data suitable for evaluating the best focus by fitting profiles through the images (using the CCD command STARFIT). Its inputs (always prompted for) are:

Number of exposures (multiplied by +1 or -1 for positive or negative focus increments, respectively)

Exposure time in seconds

Direction of motion of the image on the chip (N,S,E or W)

The initial focus will be used as the mid-point of the range. Thus, the focus is stepped in units of 0.1mm, starting at Initial focus (n1)/2 x 0.1mm, where n is the number of exposures.

The telescope is offset by 20 arcsec between exposures, so that the image of a star moves N, S, E or W (as specified) on the chip. The first offset is twice 20 arcsec so you know it is the first when you see the row of iimages. At the end of the run, the chip is read out, the image is displayed and the telescope returns to its starting position. The focus remains at its last value.

Example

Adam:> FOCUS

How many exposures (if > 0, +ve focus step if < 0 -ve focus step)? > 5

Exposure time? > 5

Direction of step on chip (N(orth), S(outh), E(ast), W(est))? > N

If the initial focus is 47mm this does 5 exposures each of 5 seconds at foci of 46.8, 46.9, 47.0, 47.1 and 47.2mm, moving the star northwards on the chip by 20 arcsec between exposures. (40 arcsec between the first two.)

Notes

The resulting multiple image is displayed on the Lexidata, but has the status of a GLANCE file, i.e. it is not kept unless the KEEP command is used (section 2.5.4).

It is easiest to do a focus run with the rotator close to a cardinal point. The image will then be displayed with a standard orientation (N up, W to the right for rotator position 180) and it will be obvious which focus corresponds to which image.

The current telescope focus will be used as the mid-point of the FOCUS run. Therefore, if you wish to change this, you should use SETFOCUS to set the telescope focus to the required value first. If the number of exposures is negative, the focus will be decremented instead of incremented.

You can change both the telescope focus increment and the telescope offset by using FOCINC and OFFSET.