Nearby_Search.Survey_Times.ps shows Greg Aldering's calculation of the expected time needed for a search able to find 15 nearby Type Ia supernovae using various limiting magnitudes and telescope/detector combinations. As a function of peak R-band magnitude, the time needed to detect a SN Ia 1 magnitude below peak in a subtracted image is shown. Basically this is a plot of the relatvie time needed to survey a given volume to a given absolute magnitude (M = -19.45 - 1.0 if this were B-band). It can be used to determine how much time is needed on a given telescope, or to compare the efficiencies of different telescopes. Each plot lists the image FWHM, in arcsecs, assumed in the calculations. Another important parameter listed on the plots is the deadtime, which is the time between exposures on separate fields. For some telescope/detector combinations this is dominanted by CCD readout time; for others it is dominated by the time needed to point the telescope on a new field. For longer exposures and/or telescopes where images are not well sample, the readout time is counted twice since two exposures are needed to reject cosmic rays. All plots show the time to acheive subtracted images with S/N = 5 for a point source of the given FWHM as solid curves. Dashed curves showing the time needed to reach S/N = 7 are given for the larger telescopes. If comparable reference images are not available, a full search would require twice the indicated time.
Note that for the Curtis Schmidt FWHM = 3 arcsec was assumed even though this width is considerably less than 2 pixels. Also note that the range on the time axis is twice as large for the Schmidt surveys as for the other surveys. Finally, note that the calculations for MOSAIC assume thinned science-grade CCD's; the current CCD's are so poor in terms of QE and cosmetics that using them for a nearby search would not be fruitful.