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5.4 Focusing the Spectrograph and Telescope

Focusing the spectrograph is done by moving the collimator. Auto-collimation is at 500, and offsets from the nominal focus up to 100 units have been noticed for different gratings. Steps of 20 units starting at higher numbers are recommended for focusing. By moving the collimator, however, one will be operating slightly out of the auto-collimation position. The grating will then be illuminated in a slightly converging or diverging beam, rather than by parallel light. In this mode, the resolution will be degraded slightly, but the effect is probably not noticeable in this instrument where resolution is low to start with. (If focus offsets become large, say several hundred units, then refocusing the CCD behind the camera will be necessary.) With a 50 - 80 wide slit, one should be able to achieve a FWHM of pixels when in focus.

When using the higher dispersion gratings, say 831 l/mm, the line profiles may exhibit some asymmetry with FWHM being slightly greater than expected. This is normal for gratings used at high inclinations. See Section 5.5 for information on using gratings at higher dispersions.

The following steps can be used to focus:

1. Select the quartz lamp with the ND at 100
2. At the spectrograph, select the ``holes" position with the decker control.
3. While looking into the post slit viewer, adjust the slit until the edges of the slit clear the round apertures.
4. Take an observation of the quartz at the highest collimator setting. Always adjust the collimator by increasing numbers 50 above the selected value then decreasing numbers to the value. This takes out the backlash.
5. Use SPLOT (ONEDSPEC) to plot several columns along the spectra, starting at about 500 and ending at 2500. At each plot use the command a to get the FWHM of one of the apertures.
6. Move the collimator down in increments of 20, until the FWHM of the apertures have minimized and begin increasing.
7. Select the collimator setting with the best FWHM readings across the chip from 500 to 2500.
8. Put in the HENEAR source, move the decker back to a normal decker setting, adjust the slit to 50, and check the spectral focus.
9. Adjust the decker to the observers position.

The telescope can be focused in this way also. Use the continuum of the star in place of the apertures. Try to duplicate the FWHM of the quartz.