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Zeroing the Telescope

It is not critical whether this is done before or after focussing. It does assume the normal state of affairs i.e. that the CCD is not seriously out of focus. See Appendix 1 for the current telescope focus position and the variation with temperature and Appendix 9 for the required focus offsets for each filter.

Zeroing the telescope (alias SNAFU) is done with the CCD long axis at a cardinal point (usually E-W, with the rotator position reading 359.2 degrees).

Select SNAFU on the User Interface and move the telescope so that the star is in the centre of the CCD chip (see Part II for details of how to do a GLANCE and to determine the position of the star in pixel co-ordinates using the LIST option of PHOTOM). The centre of the RCA2 chip is at (160,256) and the GEC2 chip at (192,289) respectively. You must also check that the drift scan table is centred (See III.7.3) The pixels are 0.41 and 0.30 arc seconds across, respectively. With the telescope mounted East of the pier and the CCD at turntable 359.2 then on the Lexidata display N is to the top and E is to the left. Convert the displacement of the star image from the centre of the display (in pixels) to arcsec by multiplying by the pixel size and use the increment mode on the T/S to locate the star in the middle of the chip. When satisfied, to exit from SNAFU mode, press the ACK button on the control desk TWICE.

Fri Jun 10 17:31:56 BST 1994