To define your object's and sky regions use
Midas
...> DEFINE/MOS
By default it automatically averages
(20) columns around the
position
(0=center of frame) of the frame
and searches for objects in this averaged frame that have an
intensity above
(-0.04)
when compared to a median over
(5) pixels.
can be absolute (> 0) or relative to the median intensity
(< 0). It searches the slitlets that are defined by
.tbl
(mos) and stores
the results in
.tbl (window).
The plot option (0)
defines whether you get a two-dimensional display of the
result, a graphical plot, both, or nothing (default).
If an object is detected a gaussian is fitted to its spatial
profile and the limits
of the object's region are defined at those pixels where the gaussian fit
has reached
(0.001) of the central intensity.
On both sides of each object a safety margin of 3 pixels is established
(can be overridden manually later);
the remaining slitlet is defined as sky region.
It may be advisable to perform a very crude sky subtraction first to get
rid of the sky continuum intensity. This can be done with SKYFIT/MOS
with skymet=nowindows. This command determines the median value
along the columns over the slitlets. If you choose this way you should use
an absolute threshold for object detection. If you want to search for
all objects at the same wavelength region you have to rebin your frame
first to constant wavelength steps with REBIN/MOS.
After the automatic definition of objects' and sky regions you are asked if you are satisfied with the results. If you are not (or are not yet sure), you answer 'no' and can now inspect the results more closely in the graphical plot and also change the results manually. You may also start with the interactive definition using DEFWIN/MOS.
The results are stored in table
(windows):
| :Obj_Slit | number of slitlet for object |
| :Obj_Strt | first row of object |
| :Obj_End | last row of object |
| :Sky_Slit | number of slitlet for sky |
| :Sky_Strt | first row of sky |
| :Sky_End | last row of sky |
The command SKYFIT/MOS is used to fit the sky. Normally, the
sky regions are taken from the table
.tbl. Several sky windows
may be defined in each slitlets. To fit the sky background you may use
the median along the sky regions (skymet=median) or a polynomial fit
(skymet=polynomial). Method polynomial requires rejection of cosmic rays
and bad pixels. These pixels are rejected by SKYFIT/MOS before fitting
the data. Read out noise (in electrons), gain (electrons/adu) and the rejection
criterion in units of 
must be specified in keywords
and
(3).
Midas
...> SKYFIT/MOS ? ? ? ? 3 poly 8,2.3,3
does a polynomial fit of 3
order
in the frame
.bdf, in the regions defined
by
.tbl (within the slitlet limits listed in
)
and stores the result in
(sky).bdf.