Subject: Re: Summer plans for visit to Garching Date: Tue, 02 Feb 1999 09:54:50 -0500 From: Peter Stockman To: rfosbury@eso.org Bob Seeing the solar eclipse from Munich sounds cool. In regards to the integral field versus MOS trades, I've just revisited the issue of sky coverage -- apparently prompted by your study!. By looking at a spreadsheet that I made from the HDF statistics (in the I band), I get about the same answer as you do in that ~10% of the sky is covered at 28mag/arcsecsec squared (Iab I think). Ferguson gets about 10% using sextractor on the H band image and thinks that much of that is due to the NICMOS PSF (e.g. 2.4 m diameter) My spreadsheet model suggests that much/most of that coverage is due to ~100 relatively bright galaxies with sizes ranging from 1-5 arcsec radius at that isophote. As a result, I'm not so sure that it logically calls for a integral field spectrograph since most of this coverage may be handled from the VLT (and other facilities). My next step -- still needing some planning work -- is to look at the following. 1/ Assume a pixel scale, 4-6 pixels per resolution element. 2/ Assume a noise characteristic 3/ For various spectral resolutions, calculate the visibility and useful aperture size for various galaxies using HDF number counts and a max integration time of ~ 10^5 s and 10:1 signal to noise. This is a multi-dimensional study but I think the results will be something like: o There will be an optimum pixel scale to observe the faintest galaxies. This would set the scale of both the MOS apertures and slicer widths. How does the European team come down on resolving the sources? Two slicer widths per object or one? Since they land randomly on the sky, 2 sounds more realistic for both the MOS and the IFS. (Remember, the WFC scale basically sucks). o As one increases the spectral resolution, fainter objects will drop out and the optimum pixel sizes will INCREASE (maybe just slightly). o There may not be many faint galaxies with surface brightness features bright enough for R ~ 3000 and resolutions ~ 40 mas. o The overlap with the ground will be hard to estimate since there is not a strong redshift vs brightness relationship in the number counts. But th ground MAY be very competitive until Halpha moves beyond the K band (say z < 3 ! ) for high resolution spectroscopy and excellent, natural seeing. If the seeing is "natural", the ground can have a larger field of view than NGST. Thus one is tempted to think that at "high" resolution, NGST should move to high angular resolution, integral field science. My concern is whether there is enough science there with the detectors that we are likely to have. (On the other hand, the high res, MOS spectrograph in the NIR will be quite a challenge for the ground.) Sooo. I don't know what the answer is! Pete H.S. (Peter) Stockman NGST Study Scientist stockman@stsci.edu Space Telescope Science Institute 410-338-5007 3700 San Martin Drive 410-338-1592(fax) Baltimore, MD 21218