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Tilt

If the detector surface is tilted with respect to the focal surface of the camera, images will not be in focus over the full field of view. The tilt of the detector can be changed by adjusting three capstans labelled A, B and C on the dewar. To check for tilt you need to carry out a Hartmann test with the HL and HR Hartmann masks using the DMS routines FOCUS-LEFT and FOCUS-RIGHT after the appropriate exposure. Make sure you run FOCUS first. The slight curvature of the camera focal plane will mean that there will be a small difference in focus between the centre of the field and the outer positions. However the pattern of Hartmann shifts should be symmetrical about the on-axis position. It will not be exact because the detector may not be precisely centred on the optical axis.

Some typical results for the case where both the focus and tilt are wrong are shown in Table 9(a). Table 9(b) shows the situation after correction for both focus and tilt. You should aim to achieve similar figures after correcting for tilt although a constant will be added to the values shown if you have not yet corrected the focus.

The amount by which you need to move the capstans is indicated by an empirical relationship between Hartmann shift and the physical displacement of the camera. As an example consider the values in Table 10. These show a differential shift from left to right of 0.5 pixel, in the sense that the right hand values are negative. Assuming the exposures taken with the grism wheel in position LH were analysed with FOCUS-LEFT, and those taken with the grism wheel in position LR by FOCUS-RIGHT; then this shift can be corrected by rotating capstan A 0.5 turn clockwise, and capstan C 0.5 turn anticlockwise. To correct a differential shift of 0.5 pixels from top to bottom, in the sense that the lower values are positive, requires that capstan B is rotated 0.6 turn anticlockwise, and A and C each 0.3 turn clockwise. To lower the correct LFOCUS position by 300 units requires that all three capstans are rotated 0.5 turns clockwise.