EUROPEAN SOUTHERN OBSERVATORY La Silla Science Operations: NTT
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Multi-object spectroscopy is supported in the RILD mode, using one of the grisms. The long-slit plates mounted in the starplate wheel can be replaced with blank starplates. A punching machine is mounted inside EMMI which produced slitlets at user-defined positions. Thus, slitlets are punched inside of EMMI and are automatically positioned in the focal plane of the instrument for MOS spectroscopy. Slit masks can be prepared during the night and it is possible to continue observing with EMMI (not in RILD or REMD) while the mask is being made. However, at the present, the process requires multiple interactions with the punching machine, hence we recommend observers not experienced with MOS mode to read the EMMI manual and to contact the observatory for the necessary pre-imaging (see below). Note that the MOS mode is not available in Service Mode observations.

Up to 4 starplate blanks can be used during any one night. It is not permitted to change star plates during the night

We *strongly* recommed observers to contact in advance lasilla@eso.org to obtain pre-images for their MOS runs.Observations will be carried on based on best effort basis and available time.

Of order 25 slitlets can be punched per mask without overlapping spectra. (If a filter is used to limit the length of the spectra, it may be possible to use two slitlets on the same CCD row.) The field of view available for punching is 5 by 8 arcminutes; the orientation of the field can be adjusted to any angle on the sky by rotating EMMI. Note that the full wavelength coverage will depend on the positioning of the slitlet with respect to the center column of the CCD.

• Obtaining a pre-image of the target field
• Identifying targets on the image, taking into account the limitations imposed by the smaller FOV of MOS compared to EMMI CCD and checking that the slits do not overlap
• Defining slits for the targets, i.e. drawing the mask
• Punching the plate
• Calibrations (slitlets positions, arcs, flats....)
• Science observations

Below each of the abovementioned steps is described in more detail

• Defining the slitlets
• Punching the slitlets
• Loading the mask into EMMI

• Checking slitlet overlap
• Slitlet mask image
• Other calibrations (bias, flat fields, He-Ar arcs)

• Acquisition
• Image through the MOS mask
• Science spectra

Images that will be used for MOS must be taken with EMMI, to avoid errors due to distortions and to possible errors in astrometry.
As suggested in the introduction, observers who need pre-imaging data must contact lasilla@eso.org, cc to ls-spectro@eso.org, as soon as they can, providing, for each field:

• field name
• RA, DEC (J2000)
• Rotator angle (PA of the slitlets on the sky+90)
• Filter to be used (don't use V, which introduce an offset)
• Exposure time
• Finding chart (failure to provide it will give you point and shoot observations)

The first thing to be checked as soon as the mask is punched 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 specific template for this, but a simple way is to execute an internal flat field template with the mask in focal plane, and grism and filter in Free position and a count level of ~ 2000 ADU.
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! The same image will be used to measure the position of three reference slitlets needed to align the mask on the sky.

Mask Image

The MOS acquisition template works by matching the position on the sky of three reference stars with that of three reference slitlets included in the mask beside the science targets. The procedure calculates the rotation of the adapter and offset of the telescope required to align the objects and the slitlets.
The coordinates of the reference slitlets must be measured on a mask image in its default orientation, i.e. N on the bottom and E on the right, not on the rotated image used to draw the masks. To measure them you can use for example the MIDAS command get/cursor on the displayed mask image and writing down the world coordinates in x and y.
These coordinates must then be inserted in the acquistion template.

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.

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.
In Table 2 below an estimate of the wavelength shift in nm vs offsets of the slitlets from the central position, assumed as the slit 1.0" arcsec, is given.

Acquisition Template

The MOS acquisition procedure is as follows (after the Preset and an eventual image analysis):

Rotation to align the slitlet mask with the objects

• The acquisition image is displayed with three green diamonds marking the positions of the three reference slitlets and the user (or rather the telescope operator) is asked to select the three reference stars by clicking on the screen in the same order by which (s)he has inserted the coordinates of the reference slitlets in the acquisition template, measured previously on the on the image of the mask (MIDAS world coordinates).
• 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), to the though mask image
• abort the entire OB.

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 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. 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.
In addition to this, special care should be used in selecting the three reference stars: taking stars too close to each other or with little or no angular separation will mean a bigger uncertainties in estimating the rotation angle. As a general world of advice, there might be some residual mis-alignment left after the completion of the acquisition template, but if the three stars are well centered within their slits there is little else that can be done.

Science Template

Send comments to: ls-emmi at eso dot org
8 Apr. 2005