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Locating slitlets and flat-field correction

The very first thing you should do now is to locate the limits of your slitlets because this information is needed for all further commands. Therefore you type

Midas ...> LOCATE/MOS

This should produce the table \fbox{{\small \tt MOS}}.tbl (mos) with the columns

:SLIT sequential number of slitlet
:YSTART first row of slitlet (world coordinates)
:YEND last row of slitlet (world coordinates)
:XOFFSET offset of slitlet from center of CCD

and write the total number of slitlets to \fbox{{\small \tt NSLIT}} (0). It may be that the threshold defined by \fbox{{\small \tt FLATLIM)(1}} (0) is either too low (e.g. below bias value) or too high for your data. Also, the width ( \fbox{{\small \tt FLATLIM(2)}}) (0) may be chosen to high or too small. If you detect too many slitlets, where only noise is visible, you should increase ( \fbox{{\small \tt FLATLIM(3)}} (0)). You also can change the scan position and width ( \fbox{{\small \tt SCAN\_POS}}).

Then you should have a look at your flat-field and try again with

Midas ...> SET/MOS flatlim=threshold,width,limit

Midas ...> SET/MOS scan_pos=xpos xbin=width

Midas ...> LOCATE/MOS

You may also try to identify the slitlets interactively with

Midas ...> LOAD {flat}

Midas ...> DEFINE/SLIT init

Midas ...> DEFINE/SLIT add ##

where ## stands for the number of slitlets you want to identify. You will have to determine the offsets between the slitlets with OFFSET/MOS (see below).

If you do not have FORS data the column :xoffset will be set to zero. This is due to the fact that for FORS data the slitlet positions given in the header of the frame are transformed to offsets from the center of the CCD. This transformations is obviously not valid for other instruments. As you will need the offsets for the wavelength calibration frame you can determine the offsets relative to the first one (which is not necessarily identical with the center of the CCD) with

Midas ...> OFFSET/MOS

As this command does a line search in the wavelength calibration frame \fbox{{\small \tt WLC}}.bdf (wlc) and correlates only the detected arc lines the resulting offsets may be wrong if there are not enough lines to get unambiguous correlation results. Setting the parameter \fbox{{\small \tt SEAPAR}} (200,5) to the values successfully used for SEARCH/MOS will help to yield reasonable results.

Normally spectroscopic flat fields show the spectral signature of the lamp with which they were taken. You can take out this spectral intensity distribution with


By default this command will fit a polynomial of \fbox{{\small \tt FFORD}}$^{\rm th}$ (3$^{\rm rd}$) order to the averaged (along the slitlet) spectral intensity of the flat field ( \fbox{{\small \tt FLAT}} (flat)) (separately for each slitlet) and divide it by these fits. The results are stored in the frame \fbox{{\small \tt NORMFLAT}} (normflat). Alternatively, you can normalize the flat field by dividing through an average smoothed with a median filter. To perform the actual flatfield correction together with the normalization type

Midas ...> FLAT/MOS

This command will do the normalization and divide the frame \fbox{{\small \tt OBJ}}.bdf by \fbox{{\small \tt NORMFLAT}}.bdf. If you have not given any name for the result frame it will derive the name of the flat field corrected object frame by adding an `F' to the name of the input frame (e.g. Ffors0001).

next up previous contents
Next: Wavelength calibration Up: MOS Cookbook - A Previous: Starting the whole thing
Petra Nass