This form can be used to estimate the S/N for objects with
a given magnitude and a specified exposure time. The user should specify
the source mag, 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 S/N 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 the source (alone) in the aperture, the counts
expected in the peak pixel from source 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
recently measured with NSFCam. It does not (yet) take into account
differences in the zero points for the different plate scales.
Please note that the
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
or Bobby Bus
If you have problems with this form, please contact Bobby Bus (firstname.lastname@example.org).