Photometric stability

Some work has been put into characterizing the photometric precision on the shifted, aligned and combined (median or average combine) images obtained with the Iraf scripts mentioned above. For these tests we used the science array data in order to demonstrate what we also expect for the new science array. The Engineering Grade Array has a somewhat lower quality and basically the flat-field correction is the limiting factor. The results of this characterization is currently being documented.

JHKs photometry of an open cluster has been used and the positional accuracy with the Wide Field camera was found to be $\pm$0.15" in both RA and DEC on the average, while it amounts to max 0.4" in the corners due to distortions. The measured magnitudes compare well with 2MASS with median differences of $\Delta$J = -0.004, $\Delta$H = 0.04, and $\Delta$Ks = 0.006 mag. The scatter around these values is 0.06 mag in J and H and somewhat larger in Ks. No distortion correction had been performed on these images before combining them.

Improper alignment of individual images may lead to substantial flux losses in the combined image. In the corners of the FOV the alignment will always be poor due to the distortions by the camera. The distortion must be well corrected for in order to have good photometric accuracy all over the FOV in the combined image. Strong seeing variations will have the same effect, but these are less of a problem in general.

The photometric scatter across the field-of-view during a dither observation is of the order of 0.01 - 0.02 mag for good S/N ratios and small dithering steps (10-20") for the Science Array.