• recipe name: FORS_S_IMG_MASTER_SKYFLAT
• input frames: SKY_FLAT_IMG
• associated templates: FORS1_img_cal_skyflat

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

warning: The twilight images of FORS1, 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: Before the master is generated each twilight flat is checked for overexposure. If more than MAXOV percent of the pixels of a frame are overexposed it is not used for the master process.
Then the individual input frames are corrected for bias, dark and bad pixels, after that the frames are combined to create the (preliminary) master. Both processes are controlled by the keyword AVMODE, which is a two--digit integer. If the first digit is 0, the bias correction is performed by just subtracting a master bias image. If the first digit is 1, the bias correction is performed using the scaled master bias. The scaling is done as follows: Each of the sections of the master bias is divided by its median and multiplied by the median of the corresponding pre-/overscan region of the input frame. If the first digit is 2, the master bias is also scaled, but each of the sections of the master bias is divided by the median of its corresponding pre-/overscan region and multiplied by the median of the pre-/overscan region of the input frame.
Iteratively each input frame is compared with the master frame for large deviations. If one frame differs too much it is removed from the set of input frames and a new master is produced. As soon as no frame has to be rejected the final master frame has been found. The rejection limits are given by the THRESH parameter.
The median process is modified in that way that instead of the median the mean of all values in an interval centered on the median is taken. Each input frame is bad pixel corrected in advance.
A master frame is generated only if MIN_COUNTraw frames are remaining for the averaging process. This ensures a minimum quality of the product.
At the end, the new master frame is compared with the corresponding certified master frame from the calibration database.

products:

Master	PS	GIF	Header
F1_MKFI_020901A_BBE_fhi_1x1s
F1_MKFI_020901A_IBE_fhi_1x1s
F1_MKFI_020901A_UBE_fhi_1x1s
F1_MKFI_020901A_VBE_fhi_1x1s
F1_MKFI_020902A_RBE_fhi_1x1s
F1_MKFI_070714A_UBESS_C1_200Klo_2x2s
F1_MKFI_070714A_UBESS_C2_200Klo_2x2s
F1_MKFI_070714A_BBESS_C1_200Klo_2x2s
F1_MKFI_070714A_BBESS_C2_200Klo_2x2s
F1_MKFI_070714A_VBESS_C1_200Klo_2x2s
F1_MKFI_070714A_VBESS_C2_200Klo_2x2s
F1_MKFI_070714A_RBESS_C1_200Klo_2x2s
F1_MKFI_070714A_RBESS_C2_200Klo_2x2s
F1_MKFI_070714A_IBESS_C1_200Klo_2x2s
F1_MKFI_070714A_IBESS_C2_200Klo_2x2s
F1_MKFI_070711A_uHIGH_C1_200Klo_2x2s
F1_MKFI_070711A_uHIGH_C2_200Klo_2x2s
F1_MKFI_070711A_bHIGH_C1_200Klo_2x2s
F1_MKFI_070711A_bHIGH_C2_200Klo_2x2s
F1_MKFI_070711A_vHIGH_C1_200Klo_2x2s
F1_MKFI_070711A_vHIGH_C2_200Klo_2x2s
F1_MKFI_070711A_gHIGH_C1_200Klo_2x2s
F1_MKFI_070711A_gHIGH_C2_200Klo_2x2s