EFOSC2 Miscellaneous Issues


Calibration Screen

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.

Instrumental Focus

The instrument only needs to be focussed occasionally (once a week or less often). 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, which will be adjusted by the telescope operator (so the observer need not worry much). The only elements which "may" necessitate refocussing are Stromgen and interference filters.

Internal Lamps

There are currently 4 available internal lamps.