Pagina interna - LIRIS
Long-slit Intermediate Resolution Infrared Spectrograph for the WHT

Multi-object spectroscopy

Science target Acquisition

The 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.
2. 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
to 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:
If 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
lslit maskname

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 in the following way:
./ maskname.mask

Star the observation:

Now Switch to spectroscopy mode, using
lspec <grism> <maskname>
then start your observing script.
To obtain dithered (nodded) spectra using the autoguider, do
 ag spec nod <int time> ["title"] [-ncyc=int] [-nruns=int] [-offset=float] [-mndr=int] [-jitter=float]
If you required three nod points use instead
with the same parameters.
Take into account that the offset parameter must be less than half or one third for three nod points, of the minimum  slit-let lengths.

Observing Calibration standards:
Apart from arcs and spectroscopic flats (see Sect. 5.3) you might want to observe a standard star in all slitlets or at least in few of them. This observation serves also to determine the trace (spectra at the bottom and top of the detector suffer geometrical distortion by about 10 pixels) along the  spectral direction for targets located within each slitlet.  To this end, put the standard source in the first reference hole (check with the mask overlaid in the DS9 display, use LIRIS MOS button). That is achieved by the command:
lacq mask <mask name>
This puts the standard star next to the first reference slit. Subsequently, call 
lobject inslit <mask nameA>
BUT do NOT allow the command to move the telescope (answer no when required) [for yet unknown reasons this does not work properly]. Instead ask the TO to offset the telescope by the computed values.  At this step it is important to append an A at the end of the mask name, which tells the telescope control system to move the source into the first reference hole. An entry in the file /wht/var/liris_slitdb_pos.dat has to be created in advance, with the format:
masknameA 1 xref1 yref1
Then run
lspec <grism> <mask name>
lpos mult <mask name> <int time> [-offset=float] [-nruns=int]
which moves the standard star into each slitlet, performing a 2-point nodding. The offset
parameter must be less than half the length of the minimum slitlet length. At each nodding position, nruns
exposures are taken. Due to the large offsets, no autoguiding is possible. If you have troubles because the star is not being offset into the slitlets you must run the script lpos_mult  once while being in imaging configuration, to make sure that the reference star gets properly positioned in all slitlets.