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MOS Observing Procedures


This chapter gives practical guide-lines for conducting observations with MOS. Observing recipes summarize the recommended procedures for imaging, long-slit and multi-slit observations. The different steps are then separately presented and discussed.


MOS is easy to use and reconfigure. However, this flexibility necessitates vigilance on the part of the observer to assure correctly set observing parameters and to properly conduct an observing sequence. Recommended observing steps are given below. These have been developed from extensive experience, and have been proven to be the most efficient procedures. We suggest that you do not try to take short cuts or innovate too much in the fallacious hope of saving time. The basic recipes are, in fact, not very difficult. The most complex sequence of observations with MOS (i.e. in its long slit or multi slit mode) can be divided into 10 successive steps for a given field:

Recipe 1: Summary of Steps for Direct Imaging

Below is a condensed summary of the sequence of actions recommended to take direct images with MOS.
  1. Rotate cassegrain bonnette to proper position angle (mostly useful for preparing for long-slit spectroscopy).
  2. Select Filter in the MOS form (and no grism, no aperture mask).
  3. Focus telescope with CAF or FOCUS, on a star close to your field or on the field itself.
  4. Acquire field. This can be done with binning. Centering is very accurate if the coordinates are accurate and if a reference star was pointed to first (see "Field Acquisition and Centering" below).
  5. Commence guiding with Cassegrain bonnette.
  6. Offset with OFFSET to precisely center desired field. This can also be used for shifting the field on the detector in a sequence of exposures.
  7. Exposure: select the proper image raster with RASTER, then enter the desired exposure time and object name in the EXPOSE form and click on "accept" to start the exposure.
  8. Obtain calibration exposures: you need biases (and darks), flat field images as well as images of photometric standard fields taken the same night to properly calibrate your data.

Recipe 2: Summary of Steps for Long Slit or Multi-slit Spectroscopy

Below is a condensed summary of the sequence of actions recommended to take long-slit or multi-slit spectra with MOS.
  1. Take a direct image, full frame and binned 1x1 following the previous recipe, from which the aperture mask could be defined.
  2. Design aperture mask by working on the image in a LAMA session. Do not forget to add one or two reference apertures (round or square) for centering stars.
  3. Drill and install aperture mask: drill the aperture mask with LAMA, then install the mask in the slide and the slide in MOS.
  4. Reacquire field, with the same bonnette orientation (see "Cassegrain Bonnette Rotation") and the same guide star as for the direct image in step 1.
  5. Take a field exposure without mask. Measure the position(s) of your centering star(s).
  6. Take an image of the mask illuminated with the "halogen image" lamp. Measure the center of the corresponding reference aperture(s) and compute and perform the required offset.
  7. Center targets in the slit(s) by taking a new image through the mask after an OFFSET. If the star is not perfectly centered in the aperture, use OFFSET again. N.B.: If you are confident in the reproducibility of pointing at step 4, you can skip steps 5 and 6 and immediately try to obtain an image through the mask.
  8. Select grism and filter from the MOS form. A filter may or may not be necessary.
  9. Spectroscopic exposure: enter the desired exposure time and object name in the EXPOSE form and click on "accept" to start the exposure.
  10. Obtain calibration exposures: you need biases (and darks), spectroscopic flat fields, wavelength calibration spectra, direct images of mask (eventually obtained at step 6) as well as spectra of spectrophotometric standard stars to properly calibrate your data.

Cassegrain Bonnette Rotation

The Cassegrain Environment, containing the Cassegrain bonnette (with guide probe, etc.), the entire MOS/SIS assembly, as well as auxiliary and support equipment, can be rotated to allow any position angle on the sky (the "bonnette angle"). Rotation of the Cassegrain environment is controlled by a hand paddle in the control room. Ask the support astronomer or the O.A. for assistance.

Bonnette rotation with MOS should be carefully considered in spectroscopy mode. A single step of the bonnette rotation encoder is 0.05°, hence the repositioning accuracy could, in principle, be on the order of two pixels over 2048 pixels on the CCD. In fact, because of mechanical inertia, it is quite difficult to stop at a given position angle with this degree of precision. Moreover, tests conducted in February 1995 show that, for two images taken with identical readings of the encoder, the residual rotation can amount to 0.2°. This is likely a more realistic value for the rotation accuracy and corresponds to about 7 pixels over a 2048 pixel field, or 0.6".

This is not a problem if the alignment of the aperture mask with the object field can be done within these tolerances; however, if a mask is to be used over several nights, we strongly urge that a single position angle be maintained for all fields to be studied during this time. On the other hand, it may be appropriate to use position angles chosen for each individual field, to allow selection of guide stars so as to minimize occultation by the guide probe. In this case, we recommend that the entire procedure, from direct imaging, to mask creation, to the spectrographic exposure, be completed for a given field before rotating the cassegrain environment to a new bonnette angle.

The situation is less critical for programs involving imaging only.

Field Acquisition and Centering

For field acquisition, we recommend the following steps:


With MOS the focus should be done accurately and it is wise to check it for each new field or, if the outside temperature is varying rapidly, before each exposure. The most efficient method for focusing is to use CAF (see Chapter 4 for a description of CAF).

Running CAF Efficiently

Because of vignetting in the pupil plane by the bi-prism system, the light is reduced by a factor of more than 10 when using CAF, with respect to direct imaging. We also need a sufficient S/N for an accurate estimate of the star's centroid with typical 10-20s exposures. This means that appropriate focus stars should have V magnitudes between 15 and 17.

note: CAF uses the "IQE" function for computing the centroids of the images. The next time you open the IQE window, you will have to change the file name back to "current.fits" as well as the parameters (activate fwhm and other options).

"Manual" Focusing: IQE

It is also possible to focus MOS without CAF, if needed. To do so, use a focus star of magnitude 16-18 and take successive exposures (with at least 10s exposure times), changing the telescope focus with the handpaddle by steps of ~5 telescope focus units between each exposure. Using "IQE", compute various image parameters for each exposure. The image statistics of importance are the FWHM's of the image along the X and Y axes (or along minor and major axis of the best fitting ellipse) and the maximum (peak) value. To be in perfect focus, you need (i) a perfectly symetrical image on the display monitor, (ii) X and Y FWHM's as small as possible and as identical as possible, (iii) a peak value as high as possible.


Offsetting is carried out with the OFFSET function (see Chapter 4) as follow:


The Cassegrain bonnette can search for a guide star in a large field. The O.A. is normally in charge of moving the Cassegrain probe until a suitable star is found. He usually records the XY position of the Cass bonnette, as well as the XY position of your star on the guiding TV. This will save a lot of time for centering if you plan to come back to the same field on a subsequent night, as it will ensure that the telescope is on the same location on the sky. The reproducibility of the recentering is a few pixels.

Imaging Exposures

A direct imaging sequence with MOS proceeds as follows.

Spectroscopy Exposures

The sequence is similar to direct imaging, except that you should have centered your objects in the slits with OFFSET and chosen a grism.

When taking several long exposures of the same field, we recommend that you check the position of the reference star(s) with respect to the mask between each exposure, since instrument flexures, although small, are cumulative. Such a check does not take a long time if you use a sub-raster around your reference star.

Aperture Mask Preparation

Selecting Parameters in the LAMA Session

Once an image of your field has been acquired, you can process it to prepare the mask. This is done on another HP terminal with the LAMA account (login: lama; same password as the MOS data acquisition account). The Lama session manager menubar with its accompanying icons is displayed (Figure 22).

FIGURE 22; The LAMA Menubar<\h4>

First select the "Setup" form (Figure 23). It asks for the instrument in use (MOS or SIS) and for the CCD name (the important parameter here is the pixel size; it is automatically recognized from the name of the device). This will set the scale (i.e. the correspondance between pixels and arcsec.) for the mask design.

FIGURE 23; The Lama Setup Form

Then, select the "Grism ENG" form (Figure 24) and give the grism identification and parameters of your spectra: i.e. the central wavelength and wavelength range you want to cover. This will set the limits of spectra that will be overlaid on the field image when you select your objects. This is useful for defining the area of full wavelength coverage, or when you want to cut two or three series of slits per column with low dispersion grisms and/or wavelength range limited by a pass-band filter. However, be aware in that case that zero order images of the slits corresponding to a given series could fall on the spectra of another series.

FIGURE 24; The Grism ENG Form

To design a mask, select "Lama Mask" from the LAMA menu bar. This form (Figure 25) requires the following input from you:

FIGURE 25; The Lama Mask Form

Preparing a Mask File

FIGURE 26; Selection of Objects and Slit Positioning in SAOIMAGE

Cutting the Mask with LAMA

The LAMA cutting machine is now located on the fourth floor. Here you should find the number of mask-holders and blanks that you requested. Using the machine is quite easy. Just follow the detailed check-list for starting the machine in the LAMA manual which should be found near the machine. Ask your support astronomer to be present when doing it for the first time. When everything is ready, enter the YAG file name on the terminal (299742o.l0y in our example).

Cutting time depends on the number of slits to be cut, their size, and on the number of passes made with the laser. This parameter can be adjusted in the cutting program, but the default number is four passes and normally produces very clean cuts. If, for any particular reason, you want to change this parameter, ask the support staff in advance.

After finishing the mask cutting for the night do not forget to shutdown the LAMA following the procedure in the "LAMA Manual@".

Aperture Mask Installation

Mounting the mask-holders in a mask slide is quite easy and it is not possible to mount them with the wrong orientation. It is more efficient to mount several new masks at the same time in an empty mask slide and exchange the mask slides on MOS. During this step, make sure that there is no misidentification of the masks and note the mask names for each position in the slide.

To remove the mask slide from MOS, first, in the control room, send the mask slide to position 1 (open) with the MOS procedure. In the dome, remove the octagon cover (pull radially on both handles, then rotate), and then completely unscrew the MOS slide screws, while supporting the mask slide with one hand. N.B.: it is important to unscrew totally, even if it seems that the slide can be removed before that. It is obviously also quite important to prevent the mask slide from falling on the ground! Remove the old mask slide and insert the new one, pushing it all the way up. Screw the MOS slide screws completely in again, and replace the octagon cover. (The latter might be the most difficult step if you are not used to it). Back in control room, first enter the new mask identifications in the MOS form. Then send the mask slide to the position of one of the masks and take a direct image with the halogen image lamp. This will allow you to compute the offset needed for centering the objects in the slits.


Imaging: recommended calibrations

To adequately calibrate direct imaging data you need the following auxiliary files: flat fields of the dome or sky, dark frames, and photometric standard frames. These frames should be obtained with the same raster and binning as the science frames.

Spectroscopy: recommended calibrations

Spectroscopic data are calibrated in wavelength by using the bonnette calibration lamps unit and in flux by obtaining spectra of spectrophotometric standard stars. To calibrate your spectroscopic data you need the following: biases, spectroscopic flat fields, wavelength calibration spectra and spectrophotometric standard spectra. All frames should be obtained with the same binning and raster as science frames. Do not forget to select "comp" in the exposure window when you want to use lamps in the calibration unit.


Twilight or Dome flats are recommended for those interested in accurate photometry from OSIS images. Even those who do not need photometry may find some flat-field images useful to remove the instrumental signatures to make it easier to identify objects when designing a LAMA mask. Twilight flats should be obtained a few minutes after sunset, with exposure times of a few seconds.

Photometric standards

See the documents in the control room for a list of photometric sequences and finding charts. These lists and images will hopefully also be available on-line in the near future. Typical exposure times are 3 to 15 s, depending on the field and the filter (longer exposures for bluer filters). It is best to have data on two fields at different air masses for extinction corrections.


The EEV chip exhibits significant linear dark current. It is highly recommended to take dark frames of the same exposure time as your science frames. Please coordinate these images with you OA and/or Support Astronomer, as the darkened dome may impact daytime operations.


The overscan region can be used to determine the image-by-image variations in the bias level.

Spectroscopic flat-fields

These are obtained from a spectrum of a quartz lamp (located in Gumball) with the same combination of aperture mask, filter and grism as the science frames. A typical exposure time is 2 sec with the "Halogen spectrum" lamp.

Wavelength calibration spectra

Spectral calibration lamps are part of the Gumball system. A variety of arc lamps are available for spectral calibrations. The Gumball web pages list typical exposure times and give examples of the expected spectra.

Spectrophotometric standards

Spectrophotometric standard stars can also be found in documents in the control room (and on-line in the future). A typical exposure time is 10s to 1 mn, depending on the star you choose and the grism you are using. You may want to use a wide (3 arcsec) slit for better spectrophotometric calibration or use the same slit width as for your science frames.
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