• recipe: FORS_S_IMG_STDSTAR
• input frames: STANDARD_IMG
• associated templates: FORS1_img_obs_crstd, FORS1_img_obs_std

purpose: Do a standard reduction of the input frame (debias, flatfielding), search for objects, determine their luminosities and align the derived photometry table with a standard star reference catalog.

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: The incoming standard star frame is searched for overexposed pixels. Some basic statistical parameters are determined. Then the standard star frame goes through a standard reduction procedure, i.e.\ bad pixel, bias and dark correction as well as flatfielding. All these operations are done in case an appropriate calibration frame is present in the set of reference frames. If SCALE=NO, the bias correction is performed by just subtracting a master bias image. If SCALE=MED, 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 SCALE=OVE, 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. Alternatively to the master bias and master dark images constant values ( BIAS_CONS , DARK_CONS ) may be subtracted.
The luminosities of the objects on the image are now determined and stored in a photometry table (see recipe FORS_IMG_OBS_PHOTOMETRYfor details). This table is aligned with a standard star reference catalog. The parameter POINTACC defines the search radius for the catalog search. Only objects within a radius POINTACC around a catalog position are potential matches. If there is more than one coincidence for a catalog object the catalog magnitude is compared with the measured values and the best match is used. Object magnitudes are estimated by means of an already existing extinction coefficients table if there is any, otherwise are computed using default extinction coefficients.
These aligned photometry tables can be used to determine zeropoints, extinction and color coefficients using the recipe FORS_IMG_CAL_CALCOEFF .