This form can be used to estimate the limiting magnitude
of objects observed with NSFCam for a given S/N and a specified exposure
time. The user should specify the desired S/N, the integration time (in sec)
for an individual frame, the number of coadds, the total number of separate
exposures (i.e., dither positions), the plate scale (arcsec/pixel), the
seeing (at 2.2 microns), and the aperture size (in arcsec) used for the
source photometry.
The program will calculate the limiting magnitude for the input
parameters in each passband (J,H,K,K',L,L',M,M'), as well as the seeing in
each band, the fraction of the total source flux contained in the aperture,
the counts expected from a source (alone) with the limiting magnitude in the
aperture, the counts expected in the peak pixel from a source with the limiting
magnitude plus background, and the counts in the sky background per pixel. Note
that saturation will occur if the exposure time for a single image is
too long, and the counts exceed the well depth. Also, the minimum allowable
integration time for a full frame (256 x 256 pixels) is 0.0774 sec; shorter
exposure times can be achieved only by reading out a smaller sub-array.
The program assumes that the flat-fielding is perfect and that the background
is determined from an aperture containing three times as many pixels as the
source aperture. It uses values of the
sky background and photometric zero points measured with NSFCam. It does not
(yet) take into account differences in the zero points for the different plate
scales.
Please note that the
observing efficiency
needs to be included separately. At thermal wavelengths the
efficiency is typically ~33-50% due to the short on-chip
integrations (~0.1sec) and so the exposure time
calculated with the calculators needs to be increased by a factor of
three to two times. For on-chip integration times longer than one
second or so the observing efficiency is close to 100%.
Questions about NSFCam and its expected performance should be
directed to John Rayner
(rayner@irtf.ifa.hawaii.edu)
or Bobby Bus
(sjb@irtf.ifa.hawaii.edu).
If you have problems with this form, please contact Bobby Bus (sjb@irtf.ifa.hawaii.edu).