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Routine operational procedures

The CCD detector system has simplified spectrograph operation and observations require less physical contact with the instrument. Once the CCD cryostat is mounted on the telescope, there some alignment and focusing procedures to be conducted. The grating mounting rotation and choice of optics are as per the original manuals.

Since the spectrograph should be in approximate focus after previous use, the CCD rotational alignment is probably the first task. As mentioned previously the cryostat can be rotated by first releasing the 7 x M6 nuts holding the adapter ring. It is advised that this operation should be performed with the telescope pointing so that the cryostat is held in intimate contact with the casing by gravity. Integrations on arc spectra and resultant image displays will quickly reveal the pixel alignment relative to the spectrum axis and rotation adjustments should quickly converge to satisfactory alignment.

Fine focus is achieved using the Hartmann mask. The focus adjustment is still by means of a micrometer which defines the location of the camera mirror. The clamp mentioned in relation to photographic use has been removed because it fouled the adapter ring. The focus setting used during commissioning was close to 5,mm. Shift between Hartmann exposures was detected graphically on the ARGS display, there being no software routines available.

The slit dekker is normally used so that only the central portion of the slit is exposed (2,mm or 27'' sky equivalent). For arc spectra and 22,m pixels, the width of spectra were found to be correspondingly about 20 pixels wide (1 pixel = 1.1" geometrically). On the other hand, flat field exposures were made with a long slit. Because of fringing effects with the coated GEC chip it was found necessary to obtain flat fields at each grating/dispersion setting of the instrument. A temporary arrangement for a flat field source was used in which a tungsten halogen source was used to illuminate the slit through the A and G comparison arc unit. An out-of-focus diffusing screen was directly illuminated at this location. If the screen is in sharp focus at the slit then non-uniform illumination along the slit is possible from the ``grain'' of the screen. This results in a spectrum with streaks, as if the slit jaws are dirty.

The possibility of the spectrograph fouling the telescope at declination settings of greater than +75still exists. An algorithm has been incorporated into the telescope control software certain telescope motions. The installation and successful operation of this software should be checked and particular care taken whenever declination settings greater than +75are used.

The filters for photographic mode remain available for use with CCDS. Glass Schott filters with a blue cut-off are efficient for blocking blue orders in red spectra. However, a copper sulphate filter is the most efficient filter for blocking red scattered light in blue spectra. The red sensitive CCD could detect a relatively high background of red radiation without the filter. A suitable filter was borrowed from the People's Photometer.

OBSOLETEThe exposure meter was a useful indicator of integration progress and can be used as in photographic mode.

next up previous contents
Next: Wavelength and Dispersion Tables Up: Supplement To The Technical Previous: Mechanical modifications

manuals store
Tue Aug 5 00:46:52 BST 1997