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Spectrograph focus; adjusting the collimator

This is the final `fine' focus of the spectrograph. This should be the only stage that the Observer need concern him/herself with as the previous stages will have been performed before the run starts by the Support Astronomer or Duty Technician.

Again, use a Hartmann test to focus the spectrograph. The procedure to follow is outlined above in Sec. 34.2 and briefly repeated here:

To carry out the Hartmann test, you will have to set up the calibration unit to illuminate the slit with a comparison lamp. Use a reasonably wide slit (say 2.5 pixels) so that the line profiles are adequately sampled. Move the Hartmann unit in, and take pairs of exposures with the Hartmann shutter up and down for several different collimator settings. Measure the Hartmann shift for each pair of images. There are two ways of doing this:

• The faster but less accurate approach is to use the DMS Y--FIND function. Once each image has been displayed on the DMS, type Y--FIND on the DMS keyboard, position the cursor on a bright/isolated/unsaturated line close to the centre of the detector, and hit return. The DMS will display the line profile and calculate the line centroid. The Hartmann shift is equal to the difference in the line centroid between each pair of images.

• A slower but more accurate approach is to transfer the data to the VAX, and use FIGARO to cross-correlate each image pair. Use the FIGARO EXTRACT routine to extract spectra from close to the centre of each image, and use SCROSS to cross-correlate the resulting spectra to obtain the Hartmann shift.

Plot the Hartmann shift as a function of focus, determine the value of the focus for which the shift is zero, and move the collimator to that position. Check the focus by taking a further pair of exposures with the Hartmann shutter up and down, and check that the Hartmann shift is close to zero.

It may be useful to know that a Hartmann shift of one pixel corresponds to a shift of about 4000 m on the UV collimator and about 3600 m on the wideband collimator. If cross-correlating the spectrum taken with the Hartmann shutter up with the spectrum taken with the Hartmann down ( FIGARO command SCROSS <up-file> <down-file>) produces a negative shift, then the collimator focus needs to be increased.

The spectrograph focus depends on which collimator is being used (UV or wideband) and which focal modifier lens is being used (red, blue or none). If either the collimator or the focal modifier lens is changed, then it is necessary to apply an offset to the collimator focus. Fortunately the focus is not wavelength dependent, so the offset is constant. The offsets are summarised in Table 23.

N.B. In practice UES is not used with a focal modifier lens hence the only offset of importance is when the collimator is changed from Wide to UV (see sec. ).

In Spring 1994 an automatic ICL> focus procedure will be avaliable (probably to be called UES_FINEFOCUS). This will take up and down Hartmann exposures at a range of collimator positions and determine the optimum collimator position at best focus. Please check with the Support Astronomer for availability.