Flat Fielding and Illumination Correction
Flat Fields are extremely stable in SOFI, therefore taking flat-fields every night of observation is not necessary. We have monitored the variation of the flat-fields over almost one year of use. Large field images in the broad bands J, H and Ks flated with a 9-month old flat-field show little degradation of the stellar photometry with respect to the same images flated with a new flat-field. Photometry better than 3% can be obtained with an old set of on-off flat-fields frames, this is shown in this plot for the Ks band. While it is not necessary to take flat-fields every night IT IS NECESSARY to take them for each instrumetal configuration that will be used.
Scriptis to properly reduce the Special Flats is available from here.
A database of reduced special imaging dome flats and their corresponding illumination correction frames is available here.
A final step in the quality of photometric data can be achived using the socalled Illumination-Correction. The Illumination Correction Surfaces have shown an ever higher level of stability than the flat-fields, variation of less than 0.2 % have been observed on the month scale. Our last version of the Illumination Correction Surfaces for the Large Field Broad Bands can be downloaded from this page. They can be used to correct the flat-fields by simple multiplication.
A database of 4x4 illumination grids taken in the past can be found here. Use the grid taken closest in time to your observations (remember - the illumination correction is derived for a specific flat field!) and the IRAF script illumination.cl available from here.
We monitor the evolution of flat-fields and illumination correction surfaces on 1/2-1 yr timescale die to the lack of permanent astronomical support on-site. You can help us with your own data, if you have results relevant to this issue please e-mail them to us.
DISCLAMER: Don't use the illumination correction files taken on 2010-04-27! They are corrupted by detector noise in the lower right quardant!