ESO Faint Object Spectrograph and Camera
Multi Object Spectroscopy

MultiObject Spectroscopy is used to obtain simultaneous spectra of many objects in an EFOSC2 field. MOS is in principle similar to long slit spectroscopy but differs from it in many critical details. For this reason a separate web page has been devoted to MOS. The main stages in carrying out a MOS programme include :

Pre-imaging / new
MOS slitlet mask / new

Calibrations Science Observations

It is essential that MOS slits are defined based on a prior image taken with EFOSC2. This minimises the effect of CCD distortion. The telescope team will endeavour to provide these pre-images, as a favour to the observer, subject to the following constraints/conditions :

It is in the observer's interest to place the request well in advance of their run - to avoid weather hitches, competition with other scheduled tasks during set-up nights etc.

MOS observers are recommended to arrive in La Silla 2 days prior to the start of their run due to the need to define and punch plates.

Image Orientation   The orientation (long axis) of the slitlets on the sky, in terms of their position angle (from north through east), is given by PA = 90 + Rotator_Offset_Angle. Very Important  the rotator offset angle should be between  -100 and +80 degrees.  (See the Adaptor page for more details).

MOS Slitlet Mask
A choice of 3 punch heads are currently offered: Defining the Slitlets  / new (follow the link for details)

Punching the slitlets  / new (follow the link for details)

Loading the Mask

The MOS masks will be loaded into the instrument by either the telescope operator or the support astronomer. Make sure that the lower side of the plate is clearly marked on it.

The telescope needs to be parked in the zenith position before the instrument can be accessed and this takes a while. One should minimise the number of occasions a mask is loaded during the night to avoid loss of observing time.

Each MOS mask is loaded into a numbered slot on the slit wheel and the numbered slot is provided a name in the instrument database (the name will be of the form MOS#n, n : 1-5).  The template definition (in jP2PP) is in terms of MOS#n while the instrument recognises only slot numbers on the slit wheel. The operator will update the database mapping one to the other. Observers with multiple masks in their programme should keep track of the mapping between their target field name and MOS#n and provide clear instructions to the observer as to which mask should be associated with a particular MOS#n.


Checking Slitlet Overlap

The first thing to be checked as soon as the mask is loaded into the instrument is that there is no overlap between the spectra of adjacent slitlets along the spatial direction. The slitlet defining software checks for and eliminates overlapping slitlets but the user can override this check (user beware!). The other reason why overlaps occur is because the slitlets were defined using a smaller punch head than the one actually used for punching.

There is no template for making this check - but a simple way is to do this (or rather request the telescope operator/support astronomer) is to directly use the OS panel. Select the appropriate grism and MOS plate (filter free) and take a 10-20 second exposure with the quartz lamp on. Any overlap between adjacent slitlets will be immediately obvious as the image is displayed on the RTD. In case of overlap, the observer has to decide whether the mask is still acceptable or a new one has to be made - hence the admonition to reach La Silla with time to spare!

Mask Image

The MOS acquisition template works by matching an image of the sky with an image of the slitlet mask - using 3 reference objects (usually stars) and their corresponding slitlets the procedure calculates the rotation and translation of the telescope field required to align the objects and the slitlets. For this the procedure looks for an image of the slitlet mask (name EFOSC_ImaInt.#.fits , #:1, 2, 3, ...) in the directory w3p6ins:/vlt/insroot/SYSTEM/DETDATA.

This is actually the directory where images observed on the current day are stored. Thus all one needs to do is take an internal image of the slitlet mask on the same day (during the afternoon or the same night)  and the image file will automatically be placed in the proper directory. In case of many internal images the user will have to note down the correspondence between the target field (MOS#n) and the internal image name. This information should be provided to the acquisition procedure in a panel which pops up demanding the same.

A typical jP2PP panel for a MOS mask image

Do not change any of the parameters on the jP2PP panel except for the Slit  (i.e. choose some other MOS#n as necessary).

Other Calibrations

The rest of the calibrations including Bias and darks, Flat fields,  Wavelength calibration (He-Ar lamps) are identical to those needed for long slit spectroscopy and the observer is referred to that page. The only difference is that one has to choose the appropriate MOS#n  for the Starplate in jP2PP instead of a long slit.

Note that usually MOS slitlets are not all aligned along the central column and so different slitlets will cover different spectral ranges. One may have to take a few more arc lamp exposures to compensate for the reduction in photons for slitlets whose spectra has shifted towards the blue.

Science Observations

Acquisition Template

The MOS acquisition procedure is as follows (after the Preset and Focus described in Acquisition Images):

Rotation to align the slitlet mask with the objects

Translation to move the objects into the slit The FITS file is named  EFOSC_AcqMOS.fits

Image through the MOS mask

Usually, aligning the slitlet mask using the 3 reference star works very well. However sometimes observers tend to live on the edge by choosing stars on the CCD margins (where the distortion is greater) and/or putting targets at the edges of the slits. On such occasions some of the targets may not fall on the corresponding slitlets, especially when narrow slits have been punched (we have never had this problem for the 1.75" slitlets). In order to be sure that all or at least the crucial objects are on the slits, especially for long integration spectra we recommend that observers take an exposure of the field through the slit after the acquisition template and before the spectral templates.

This is a regular Spectrocopy template (Efosc_spec_obs_Spectrum) with the starplate set to MOS#n and the Grism set to Free.

After confirming that the objects are all located where they ought to be one can then relax with nary a worry for the next hour or two..... OR decide, if you so wish, to repeat the acquisition procedure all over again!


The MOS science spectroscopy templates are identical to the ones for long slit spectroscopy except that one uses a MOS mask for a starplate instead of a long slit.

A typical jP2PP MOS science observing block

A typical MOS observing block includes an acquisition template followed by an image of the field through the slit and finally science spectra. Note that one can combine templates with different grisms. The above example shows an OB comprising a sequence of:

back to top
Send comments to : ls-spectro

Last modified: Wed Oct  2 22:25:01 CLT 2002

[3.60 & CAT Homepage][La Silla][ESO][Observing][Search][Help][News]