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%.
NSFCam 1 platescales: 0.055, 0.148, 0.30 NSFCam 2 platescale : 0.40 Guidedog platescale : 0.1162 Kyle platescale : 0.10Contact John Rayner for further information or assistance.
Inputs/Command: SN Ratio: 10.0 Itime_sec: 10 Coadds: 1 Exposures: 1 Plate Scale: 0.148 Seeing: 1.0 Aperture R: 1.5 Command: none
Output: none