• NOTCam Zeropoints
• Background levels
• Limiting magnitudes
• Photometric scatter across the array

Each standard star observation is made as a 5-point dither. The images are skysubtracted and flatfielded before they are median combined (to minimize bad pixels in the apertures). Each standard is usually measured at several air masses. Exposure times are chosen to keep the peak pixel below the number of ADUs where non-linearity reaches 1%. Both the reset-read-read and ramp-sampling readout modes are used.

Note that the zeropoint magnitudes above are given for 1 e^- per second (not 1 ADU/s). The zeropoint magnitudes have been obtained by the best fit to the equations:

J = j_obs + J_zp - J_ext * X 
H = h_obs + H_zp - H_ext * X 
K = k_obs + K_zp - K_ext * X 

The IR sky background is strongly variable. For the J and H bands it is dominated by airglow emission (OH lines, which are numerous and highly variable). These lines are also present in the K-band, but here the thermal emission starts to become more important.

* Thermal leak. Currently under investigation.

Note that the measurements for Z and Y are based on one night only.

The mean SWIR3 WF-camera background values on 12-Feb-2011 were:
Y = 17.3, J = 16.1, H = 14.3, Ks = 13.3 magnitudes per square arc sec.

The mean WF camera backgrounds with the Engingeering grade array were:
Yn = 15.0, J= 15.5, H = 13.8, Ks = 12.9, and K = 12.8 magnitudes per square arc sec.

The mean WF camera background values on 14-Jan-2006 with the First Science Grade Array were:
J = 15.3, H = 13.6, Ks = 13.2, and K = 13.0 magnitudes per square arc sec.

In the case of background limited observations, the limiting magnitude can be predicted from the derived zeropoints according to the formula (see McLean 1997; relation is modified to be valid for zeropoints for 1 e/s instead of 1 ADU/s):

where zp is the zeropoint magnitude (for 1 electron/sec), S/N is the requested signal-to-noise ratio, n is the number of pixels over which a point source is distributed, B is the background level (in electrons per second per pixel), and t is the total on-source integration time. For point sources the seeing is measured as the FWHM (in arcsec) of the point spread function, and the pixel size is 0.234 arcsec for the WF camera and 0.078 arcsec for the HR camera.

1. Point source: n = 3.1415 * (FWHM/pixel_size)²
   
2. Extended source: n = (1/pixel_size)² and unit is magnitude per square arc seconds..

The photometric scatter across the FOV is defined as the standard deviation around the mean of N individual aperture photometry measurements of the same standard star across the array. The number of positions used are currently N=25, although in the past we have used also N=5 and N=16.

The photometric scatter is measured to see if there is a spatial variation of the photometric response. Contributions to the photometric scatter will also be from improper flat fielding and any presence of bad pixels. No bad pixel mask was applied in this analysis.

*) The JHK flat fields were obtained three months earlier, while the Ks flatfield was obtained 19/9-02. This probably explains much of the scatter in JHK.

The 16 positions of the standard star overlaid on the flat field image of the Engingeering Grade Array, SWIR1 (left), corresponding to the first measurement, and the 25 positions of the standard star overlaid on the flat field image of the new science array, SWIR3 (right), corresponding to the 3rd and all subsequent measurements.