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 :- Obtaining a pre-image of the target field
- Identifying targets on this image such that they do not overlap along the spatial axis
- Defining the slits (location and length) for the above targets
- Punching the above slits on a MOS plate
- Calibrations (wavelength, flat field, mask image)
- Science observations
Pre-imaging
New (the 'new' pages still hosted on the 3.6m telescope pages, but good for MOS on EFOSC2 at the NTT)MOS slitlet mask
New- Defining the slitlets
- Punching the slitlets
- Loading the mask into EFOSC2
Calibrations
- Checking slitlet overlap
- Slitlet mask image
- Other calibrations (bias, flat fields, He-Ar arcs)
Science Observations
- Acquisition
- Image through the MOS mask
- Science spectra
- A sample jP2PP MOS science observing block
Pre-Imaging
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 :
- The request should be made well in advance of the run: The NTT operates in visitor mode the vast majority of the time, and technical or calibration nights are the only time that the pre-imaging can be done. The pre-imaging request should reach the team at least 2 calibration nights ahead of the scheduled observations (typically 4-6 weeks in advance).
- All the necessary information including target co-ordinates, finding chart with field centre marked, image orientation (see below), filter and exposure time are provided. Please send an email to the La Silla account lasilla@eso.org where a ticket will be made for your request. You should also cc your request to the EFOSC2 instrument scientist.
- Not more than 3 images will be provided per observer. Additional images if necessary should be obtained by the observer during their first scheduled night - it takes about an hour (with some familiarity) from pre-image to loading the masks into the instrument.
- The pre-image exposure time should not be more than 5 minutes - if an object is not visible in a 5 minute exposure one will not obtain a useful spectrum unless targets happen to be a cluster of strong emission-line objects. Observers who need more than the above should apply for the same as part of their regular proposal.
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 to be updated following move to NTT 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:
- 0.95" x 6.46" (punch #1).
- 1.12" x 9.52" (punch #5).
- 1.45" x 9.52" (punch #6).
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. This takes considerable time and will only be done during the day. MOS masks cannot be changed during the night
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.
Calibrations
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 wefosc:/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
- The acquisition image is displayed and the user (or rather the telescope operator) is asked to select the three reference stars by clicking on the screen.
- The mask image is displayed next and one has to select the corresponding 3 slits in the same order as the reference stars - this step is only done once during the procedure and the slitlet locations are stored for use during later iterations. Right at the beginning, before the Preset and Focus, the procedure asks for the name of the slitlet mask image.
- The procedure calculates the rotator offset angle required to align the slitlet mask with the 3 reference stars and then offers the following options:
- rotate and move on to the next step in the acquisition procedure (rotation < 1 degree),
- rotate but take one more acquisition image to make sure the rotation worked fine,
- do not rotate but move on to the next step in the acquisition procedure (if the offset < 0.1 degree),
- abort the entire OB.
- The acquisition image is displayed and the user (or rather the telescope operator) is asked to select the object by clicking on the screen
- The procedure calculates the telescope offset required to move the object to the x-pixel defined in the template and on the y-column where the slit is located. It then offers the following options:
- offset and exit from the acquisition procedure (when the shift is less than 1-3 arcsec depending on the size of the slit),
- offset but take one more acquisition image to make sure the shift worked fine,
- do not offset but exit from the acquisition procedure (if the offset was very small, say <0.1 arcsec, for example),
- abort the entire OB.
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 wider 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!
Spectra
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:
- An acquisition sequence for Mos#4 with a R filter, a 45 second exposure and a rotator offset angle of 40 degrees (i.e. slit position angle = 40 + 90 = 130 degrees)
- Note : the rotator offset angle has to be set to the value used for the pre-image.
- An image through the MOS mask Mos#4 with a 45 second exposure and R filter.
- 1 spectrum of 1200 seconds with Mos#4, Grism Gr#12, normal readout, no filter and 2x2 binning; followed by
- 1 spectrum of 1800 seconds with Mos#4, Grism Gr#7, normal readout, no filter and 2x2 binning.