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.