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% @(#)skyflaccd.hlq 19.1 (ESO-IPG) 02/25/03 14:15:05
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%++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
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%.COPYRIGHT (c) 1993 European Southern Observatory
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%.KEYWORDS MIDAS, ccd package, SKYFLAT/CCD
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%.PURPOSE On-line help file for the command: SKYFLAT/CCD
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%.VERSION 930827 RHW Documentation created
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%---------------------------------------------------------------------
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SKYFLAT/CCD 25-Mar-1993 RHW
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SKYCOR/CCD [in_spec] [out_frm] [xboxmn,xboxmx] [yboxmn,yboxmx] [clip]
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Apply sky observation to flat field to remove illumination pattern
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Apply sky observation to flat field to remove illumination pattern
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SKYFLAT/CCD [in_spec] [out_frm] [xboxmn,xboxmx] [yboxmn,yboxmx]
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[clip] [lowsig,higsig]
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in_spec = single sky frame or MIDAS CCD reduction table (with the
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extension .tbl) containing the names of the sky frame(s)
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in column :SKY. Default input is taken from the keyword
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out_frm = name of the illumination corrected sky flat field. Default
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the output file the same as the input sky flat frame. Hence,
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by default the input sky flat frame will be OVERWRITTEN.\\
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This parameter is only used is case of single frame input
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for the first parameter. In case the output is taken a
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reduction table the output frame(s) will also OVERWRITE the
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xboxmn,xboxmx = minimum and maximum smoothing box size along the x
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axes. The minimum box size is used at the edges and grows
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to the maximum size in the middle of the image. This
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allows the smoothed image to better represent gradients
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at the edge of the image. If a size is less then 1 it is
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interpreted as a fraction of the image size. If a size is
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greater than or equal to 1 then it is the box size in
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pixels. A size greater than the size of image selects a
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box equal to the size of the image. Default values are taken
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from the keyword IL_XBOX.
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yboxmn,yboxmx = see above. Default values are taken from IL_YBOX.
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clip = Clean the input frame(s) of objects? If yes then a clipping
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algorithm is used to detect and exclude objects from the
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smoothing. Default taken from IL_CLIP.
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lowsig,higgsig = sigma clipping thresholds above and below the
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smoothed illumination. Default values are taken from
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the keyword IL_SIGMA. The keyword is only read in case
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The command operates very similar to the command REDUCE/CCD, which
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the exception that the command not check for the exposure type of
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In case the input is an association table the command will check for
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the existence of the flat frames in the column :SKY in the association
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table. Flat frames that do not exit will be created first, provided
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the association table contain the :SKY column. Other relevant
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calibration frames for processing the flat frames, will be taken
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from the corresponding calibration columns.\\
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After the reduced sky frame is obtained, the sky illumination frame
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is produced by heavily smoothing the calibrated sky frames using a
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moving "boxcar" average. The effects of objects in the frames may be
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minimized by using a sigma clipping algorithm to detect and exclude
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the objects from the average. This illumination frame will be used to
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make an illumination corrected flat field (sky flat) according to the
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f_new = f_old*ill_sky/scale, where\\
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f_new is the corrected flat field, f_old is the original input flat,
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used to reduce the blank sky frame, ill_sky is the illumination
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correction determined from the sky, and scale is the nomalisation
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The original flat frame is multiplied by the scaled sky illumination
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correction frame to produce the illumination corrected flat. The
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output will OVERWRITE the original (reduced) flat frames. Hence, the
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output sky flat will automatically be used as flat in subsequent
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calibration procedure(s).\\
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NOTE: THE MASTER FLAT FIELDS WILL BE OVERWRITTEN BY THE CORRECTED ONES.
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In the case of single frame input and all calibration frames input
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will be taken from the SC_ keywords and must exist. Also here, by
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default in the output illumination corrected flat will replace the
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The IM_SEC keyword may be used to assign a region of interest for which
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the correction has to be done.\\
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For more information see the CCD chapter in Volume B of the User's
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SET/CCD, SHOW/CCD, HELP/CCD, REDUCE/CCD, SKYCOR/CCD,
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SKYFLAT/CCD redtbl.tbl
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Reduce all sky frames in the the ccd reduction table redtbl.tbl. All
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master calibration frames will be obtained from that reduction table.
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Do the calibration according to the keyword settings, including the
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flat fielding. From the calibrated sky produce the illumination frame,
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using the `IL_' keyword settings. Hereafter compute the output sky
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flat by multiplying the illumination frame with the original flat
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field. The output sky flats will have overwrite the original flats.
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SKYFLAT/CCD m100rraw_sk m100rraw_sf
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Start the ccd reduction sequence for the frame m100rraw. The output
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frame will be m100rraw_sf. In case only dark and flat field correction
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is to be applied the names of these two calibration frames will be
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obtained from the keyword SC_DKFRM and SC_FFFRM, respectively.