• recipe name: FORS_IMG_SKY_FLAT
• input frames: SKY_FLAT_IMG
• associated templates: FORS2_img_cal_skyflat

purpose: Creates a master twilight flat from a set of twilightflat images.

warning: The twilight images of FORS2, which are used to create master sky flats, show evidence for a feature, whose position depends on the rotator angle (see overview). This limits the achievable photometric accuracy to about 5%. For more information please look at the FORS Secondary Standard and Absolute Photometry Project.

description: In order to eliminate the contributions of field stars on the jittered sequence of flat fields, the frames are combined using a median rather than on a simple average. Before combining the individual frames, each is normalized with the median of its flux. The overscan regions, if present, are removed from the result. The master frame has 2 extensions, with the actual product being in the first extension and its errors being in the second extension.

At the end, the new master frame is compared with the corresponding certified master frame from the calibration database.

products:

Master	PS	GIF	Header
F2_MKFI_080410A_RSPECIAL_C1_lo_2x2s
F2_MKFI_080410A_RSPECIAL_C2_lo_2x2s