acquisition of a MOS mask is similar to that for a long slit. The only
major difference is that you have to specify three reference sources
instead of one.
1. Ask the TO to rotate the instrument to the required sky position angle. Switch the autoguider OFF.
Take two dithered images with an offset of about 5" for sky subtraction
(J band is preferred due to the lower sky brightness). Subtract them
from each other in the RTD display.
3. Overplot the mask from the
DS9 display, using the LIRIS MOS button in the DS9 display.
Alternatively it can be load directly in DS9 via Region -> Load
-> /wht/var/liris <maskname>.reg
4. Measure the three x- and y-coordinates using the Star profile task in the RTD control
(see Sect. 4.2.4). Take note of the x,y centers.
5. Acquire the mask in a first approximation, calling
lobject inslit <mask name> <x1> <y1> <x2> <y2> <x3> <y3>
This routine will calculate shifts and rotations. The rotations are expected to be
small (but can still lead to large movements of the guide star, hence the guider has
be switched off for this step). The reference stars must be given in
the right order (check in the overlay on the DS9 display).
The order of the reference stars can also be checked by having a look at
Therein, you will find an entry for your MOS mask in the form
maskname 3 x1 y1 x2 y2 x3 y3
with xi and yi being the center pixel coordinates of the reference sources.
6. The mask is now roughly acquired. Switch the autoguider ON.
7. Repeat steps 4 and 5 for fine-tuning the mask position.
If you are still not happy with the mask positioning, you can ask the TO for small manual
shifts for fine-tuning.
Check using through mask exposures:
one wants to be sure that the targets are actually in the slits the
best option is to take an image through the mask using the command
the centers of the reference stars and the holes must match at this
point. It is possible to check that using an IRAF script called offset_mos (find it here)
. In order to run it one needs to define the
task within IRAF by typing the follow:
cl> task offset_mos = directoriotarea
This routine has three arguments
image = 'new image to measure offset from'
reference = 'reference image for sky subtraction'
slitposition 'file with slit position (the file /wht/var/liris_maskname.mask)'
It will produce an output with the required offset to be applied by the TO. It does not compute the rotation.
There is another possibility to compute the offset when using images through the mask. You must call the python routine called recenter_mos.py
in the following way:
./recenter_mos.py imagen.fit maskname.mask