Next: Making the instruction file Up: ALFOSC Multi Object Spectroscopy Previous: Introduction Contents
The template exposure is used for defining the size and the positions of the slits. The template exposure should be obtained with ALFOSC with no filter in the ALFOSC beam (but FASU is OK), approximately centered on the wavelength region corresponding to the grism which is going to be used for the MOS exposure. As a rule of thumb the exposure time should be of the order of 1-5% of the exposure time required to obtain the spectra, unless emission lines are studied. To avoid large slit losses due to differential atmospheric refraction it is desirable to obtain the template image at the parallactic angle. This requires careful planning of the MOS observations.
The template images should be obtained prior to the run. As the observing time required for doing this is limited, they may either be taken during a previous MOS observers observing run or by staff during technical/service time.
The mosplate program allows for (X,Y) pixel coordinate input from a file. For very faint objects or emission line objects, it is therefore possible to use a short template exposure to transform (Ra,Dec) or other coordinates to ALFOSC CCD coordinates. It is the task of the observer to provide this transformation, once the template image has been acquired.