NSFCam Exposure Time Calculator

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 source photometry.

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 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 : 0.10 
   

Inputs/Command:
    Source Mag: 12.0
      SN Ratio: 10.0  
        Coadds: 1
     Exposures: 1
   Plate Scale: 0.148
        Seeing: 0.7
    Aperture R: 1.0 
       Command: none
   
  

Output:
none