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Whenever observations which do not need external photons are carried out (for e.g., bias, darks, He-Ar arc spectra, internal flats) a calibration screen is moved into the optical path. This blocks light reflected into the instrument by the main mirror reducing stray light. Also the internal lamp exposures are carried out by illuminating this screen by the lamp and collecting the photons scattered into the optical path.

Consequently, the guide star is lost when the screen is moved in and so no guiding is possible. This point should be noted because some observers may need to obtain He-Ar arc lamps and internal flats on the target to eliminate the effect of CCD flexure. So all such calibration frames should be taken after all the science spectra have been taken. If not, the guide star will be lost and the target will have to be reacquired into the spectroscopic slit.

The instrument only needs to be focussed once when it is first mounted on the telescope. This task is taken care of by the support team and the observer does not have to do anything. Since all the optical elements are in a parallel beam no refocussing is necessary when optical elements are changed during the night. The observer only needs to worry about the telescope focus. The only elements which "may" necessitate refocussing are Stromgen and interference filters.

There are currently 4 available internal lamps.

• Helium and Argon Lamps   These are the arc lamps supported for wavelength calibrations. These lamps are switched on one after the other to obtain a composite He-Ar spectrum. The observing templates as defined now do not allow exposures with just a single lamp.
• Neon Lamp   This is also available but is seldom used
• Quartz Lamp   This emits a featureless continuum and is used to take internal spectroscopic flats and internal mask images. This is however not a good lamp for imaging flats since the lamp illumination (on the calibration screen) is quite different from the dome illumination on the flat field screen

Send comments to : ls-spectro Last modified: Sat Nov 17 22:57:51 CLST 2001