IF-MOS trade issues

A list of issues to be addressed and questions to be asked. Arising at a meeting between SA, RF, SC and WF at the ST-ECF on 6 July 1999.

0. Need to refine the specifications of the MOS
   • How many exposures (with order-sorting filters) are needed to cover 1-5 µm?
   • Are there separate short and long-wavelength detectors?
   • What is the spectral resolution range and how can it be selected? Is there an R~150 mode?
   • How will the angular scale of the mirrors/shutters be chosen?
   • What kinds of observing strategy will be employed?
   • RF to discuss with Stockman
1. What are the science drivers for the R~150 IFS-LR mode? Where is R=1000? SC to address
2. How do we define useful figures of merit for the spectrographs?
   • time to measure n objects
   • s/n per pixel
   • s/n per object
   • Efficiency of use of detector area
   • This is really a question of how we characterise the DRM for this study
3. Galaxy number counts at the magnitudes of interest for NGST spectroscopy
   • The Arribas study has been carried out to K_AB = 27.5
   • There is significant uncertainty in the counts at fainter magnitudes
   • Clustering, however, should only produce variations of ~ ±10% so the biggest uncertainty is in the counts themselves
   • How much fainter can we go anyway
   • WF to address
4. Need a schematic diagram to understand the mapping from the sky to the detector for the IFS-LR. SC to provide.
5. What length of slit is needed for sky subtraction in the MOS? How much does this reduce the number of objects per exposure? What are the real light profiles of objects at the faint limit: the NICMOS observations are limited by the Cam 3 resolution? SA to investigate; WF to look at NICMOS data
6. The IFS will always observe 'cumulatively' whereas the MOS will probably be pointed at galaxies of similar magnitude. How does this influence the figures of merit?
7. Need to check if the new NGST ETC changes any of the conclusions
8. How do we quantify the differences on operational complexity? Include a discussion of sky determination
9. Under what circumstances is the detector noise likely to be significant?