Imaging

A model of the distortion of the Wide Field camera was made for the JHKs bands. The model is based on high quality data from a stellar rich field in Baade's window from June 2009, using 2MASS positions as reference. The data, composed of 9 offset images per filter, gives a grid of about 2300 accurate X-Y positions per band. In January 2012 the model, presented as IRAF/geomap database files, was made publicly available for download. In the NOTCam quicklook reduction package v2.4, distortion correction is added as an option in the tasks, and if selected, the individual images are corrected before aligning and combining. Only specific models exist for the J, H, and Ks bands, and for narrow-band filters it is suggested to use the nearest (in wavelength) model, while for Z and Y primarily the J-band model is used. The correction always improves the images, but for not fully understood reasons for some data it is better than other. We have found no dependence on rotator position or altitude.

The plan is to obtain high quality data to make distortion model for Z and Y filter.

The scatter in the photometric measurements of a standard star measured at difference positions on the array is believed to be limited by the flat fielding errors. For high signal-to-noise data the scatter is below 1-2 percent after correction for non-linearity and optical distortion. Data has now been taken in photometric conditions to map the illumination correction for the wide-field camera with the JHKs filters