NSFCam Limiting Magnitude Calculator

NSFCam Limiting Magnitude Calculator

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@ifa.hawaii.edu) or Bobby Bus (sjb@ifa.hawaii.edu).

If you have problems with this form, please contact Bobby Bus (sjb@ifa.hawaii.edu).

Input Observing Parameters

Required Signal-to-Noise Ratio:

Integration time (sec) for individual frames:

Number of coadds:

Number of separate exposures:

Choose a Plate Scale:

Seeing (FWHM in arcsec) at 2.2 microns:

Photometric Aperture Radius (in arcsec):

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