This form can be used to estimate exposure times needed
to achieve a specified S/N ratio for observations with NSFCam of objects
with a given magnitude. The user should specify the source magnitude in
any band, the desired S/N, 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
The program will calculate the necessary exposure time per coadd
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 the 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 exposure times calculated here are on-source integration times and do not
include any overhead incurred for beam switching or dithering, which will
add to the total time required for the observations.
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).