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Дата изменения: Fri Feb 23 12:59:04 1996 Дата индексирования: Tue Oct 2 17:17:09 2012 Кодировка: Поисковые слова: ngc 6559 |
For creation of one or more sky illumination correction frames the command ( SKYCOR/CCD) is available. Similar to other corrections, sky frames will first be processed according to the processing keywords ( SC_SCAN, SC_TRIM, SC_PXFIX, SC_BSCOR, SC_DKCOR, SC_FFCOR). After this calibration step, the command will smooth the reduced sky frame(s) to create the final sky illumination frame(s).
Input for the command can be either a single sky frame, or the Association
Table containing a column with the names of the master sky frames.
In the first case the output is, obviously, a single illumination
frame. In case the Association Table is given as input the name of
the output frame names will be the names of the input frames
extended with ` _ill'. In addition, the illumination frames
will be stored in an illumination column in the Association Table. This
column can be used in the pipe line processing of the science frame.
Default input is taken for the keyword CCD_IN. Smoothing
parameters are taken from the the IL_... keywords, which
are listed in Table .
After the illumination frame has been created, one can multiply the original flat field by the illumination frame, resulting in an adjusted flat field. This approach clearly has the advantage of speeding up the calibration process since it requires one calibration frame and two computations (scaling and dividing the illumination correction) less. The output frame is called sky flat. It is the flat field that has been corrected to yield a flat sky when applied to the observations.
Having done this, this sky flat can be used as the final one. How good this new flat field is can be checked by correcting the blank sky once more, using this sky flat. If the result isn't satisfactory one can try to play with the smoothing parameters, or else ask for help of an experienced observer.
To create the sky flat fields the command SKYFLAT/CCD is available.
As input it takes the blank master sky frame, processes it if needed, and
creates the output sky flat from the smoothed processed sky and the
appropriate flat field. The smoothing parameters are stored in the
keywords displayed in Table . The way
the command works is identical to the command SKYCOR/CCD. The
default names of output sky flats are the same of the original master
flat field frames. Hence, the input master flat fields, use to flat
the master sky frame, will be overwritten.