Mask description

The mask description files (mdf files) are used by SpectraPy to know the geometry of the slits mask and to locate spectra on the raw frames. The mdf file is an ASCII file, containing the following columns 1:

  1. ID the unique slit ID 2.

  2. DIMX and DIMY the dimensions in millimeters of the slit.

  3. X and Y the position (always in millimeters) of the center of the slit in the field of view. 3

  4. ROT the rotation angle of the slit. Rotation angle is clockwise and starts from cross-dispersion direction.

  5. WID and LEN the width and length of the slit in arcsec. These entries are not mandatory, some instruments don’t provides them. In case these information are not available the value must be 0. 4

  6. REF the reference slit flag. This value is 1 in case the slit is a reference. Reference slits can be ignored during data calibration and extraction

Here an example of one mdf of a MODS mask

#ID     DIMX    DIMY    X       Y       ROT     WID    LEN     REF
49      0.720   3.600   -73.62  -86.11   0.0    1.2     6.0    0
85      0.720   6.000   -40.32  -91.09   0.0    1.2    10.0    0
119     0.720   3.600   -86.62  -48.47  40.0    1.2     6.0    0
Ref1    2.400   2.400   -20.70   74.29   0.0    4.0     4.0    1

No empty entries are allowed


SpectraPy handles IDs as strings.


The library assumes the center of this coordinates system in the center of the FOV.


unlike the DIMX and DIMY parameters WID and LEN are not oriented along the X,Y axes, but they are oriented according with the grism dispersion direction: LEN is the slit length along the cross-dispersion direction, WID is the width along the dispersion direction

Mask utility scripts

SpectraPy provides a set of scripts (stored in the scripts directory) to automatically create mdf files for VIMOS, LUCI and MODS data. Available scripts are:




The and require in input the fits data file, because in the VIMOS and LUCI cases, mask geometry is described in the header of the files.

Since no mask information is available in the header of MODS files, MODS scripts takes in input the mms (the file obtained by the MODS mask preparation tool).

Finally, for LUCI and MODS spectrograph, a set of already prepared LS masks is available in the conf/masks directory.