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MOS Control and Data Acquisition with Pegasus


This section describes briefly the main procedures of the Pegasus software that are used during observations with MOS. For more informations on Pegasus, see the Pegasus User's Manual.

Checking the Status of MOS and Starting Observations

When you arrive for the first night of observation, you should find the instrument prepared in accordance with the requests you have sent (several weeks in advance of your run) to CFHT. This means that the requested CCD, filters and grisms have been installed, the internal focus has been checked for each filter, and the direction of dispersion for each grism has been precisely aligned with the columns of the CCD. You will find a "set-up form" summarizing this information close to the control terminal.

Also, an image of a grid mask has been taken and processed during the afternoon for the computation of orientation and distortion corrections. For precise cutting of masks and for a good match between the positions of the slits and of the objects, this step is essential. If, under exceptional circumstances, the set-up team asks for a few more minutes at the beginning of the night for checking this point, it is wise to accord them this extra time.

To start a Pegasus session, type "mos" after the login prompt and then type the current password (ask the TO or your support astronomer if you do not know it). The login process creates the appropriate icons and windows, including a top menubar: the Pegasus session manager (Figure 8). The different Pegasus procedures are activated by clicking on the corresponding buttons.

FIGURE 8; The MOS Pegasus Session Menubar

When starting a run, first enter the official run number and PI identification (or ask your support astronomer to do it for you). This is important for archiving and for easy future access to your own data. The proper form is as follows:

Finally, check that the disk meter indicates enough available disk space for your night. Otherwise, delete some files (see "The FILES Window: Handling Data Files" below).

The MOS Window: Configuring the Instrument

MOS control is provided through the MOS window. To open it, click the MOS button on the Pegasus menubar. The window which appears (Figure 9) then allows the user to select new filters, grisms, or aperture masks. To do so, simply click in the appropriate box in front of the selection. Once your selection of a new MOS configuration is complete, click on "accept". Allow enough time for the devices to reach their positions (times add up since motions are sequential). Typical times for each move are as follows:

FIGURE 9; The MOS Control Form

At the beginning of your run, check that the information contained in the MOS window agree with those on the set-up form. There is also a small "status" window (Figure 10) which displays a summary of the current configuration.

FIGURE 10; The Status Window

CAF window: Computer Aided Focusing

CAF is used to focus the telescope quickly and accurately. It is an integrated focusing sequence based on double images produced by a Hartmann mask consisting of two holes located in the pupil plane, one empty, and one with a prism. The device produces two images for each single star, whose separation in Y is linearly related to the telescope focus position.

First, select suitable stars for focusing (see Chapter 5 for step-by-step instructions) from a test exposure. If you do not intend to use the entire field reduce the field of view by subrasterring the CCD with the FRASTER routine. For a "balanced" focus, select focus stars approximately half-way between the center and edge of the field.

Now, open the CAF form from the menubar (Figure 11). Enter the exposure time and check that the zero point is set to the appropriate value given to you by your support astronomer (this should not be changed during a given run). After clicking on "accept", the focusing sequence starts, which means:

FIGURE 11; The CAF Form

Now, you simply add (taking the sign into account) the suggested value to the current telescope focus by using the telescope handpaddle. Normally you need to run CAF only once. If you want to check the precision you can run CAF a second time and you should not find a focus difference larger than ±5 units (the accuracy depends, of course, of the actual seeing). Note: if, for any reason, you intend to run CAF several times, you will save time by setting the grism wheel to position 1 before starting the sequence.

FIGURE 12. The IQE Form

OFFSET: Accurate Offsetting

The OFFSET procedure is used to make motions of up to a few arcmin for centering the field or for centering objects in slits while guiding with the Cassegrain bonnette. To carry out an offset follow the steps below:


The RASTER and EXPOSE Windows: Gathering Photons

The RASTER window (Figure 14) enables the observer to define sub-raster and binning parameters. Three configurations can be defined in advance and selected at will. To select one of these three configurations, first click in the box in front of the selection, then click on "accept". The selected raster is displayed in the feedback window. Do not change the raster while the EXPOSE window (see below) is open, as any changes in the raster will not take place until the EXPOSE window is closed and then opened again.


The EXPOSE window (Figure 15) is used to define and start an exposure and is self explanatory. Do not forget to select the correct exposure type. It should be "c" for calibration frames. Since the default option is "o" (object), if "c" is not explicitely selected, the central mirror will not be in place and you will not get a calibration exposure!


Note that for sequences of identical images like biases, darks, dome flats, the "combine" option yields a combined frame from the individual exposures, using the IRAF/COMBINE task (see the Pegasus User's Manual). This process combines the images by using the average sigma clipping algorithm, and has been proven to provide slightly better results than a plain median algorithm (same mean value, but better r.m.s.). To select this option, click on the "Combine data using sigma clipping" button in the EXPOSE form before starting the sequence. A combined image will be computed as soon as the image acquisition sequence is completed, and written to the disk as a FITS file with B, F or D suffixes (capital letters) for biases, flats and darks respectively.

Before starting an exposure, it makes sense to double check the parameters of the exposure: configuration (mask, filter, grism), raster (frame size, binning), exposure type, combine option, etc.

The LAMPS window: Calibration Unit Control

The selection of calibration lamps is made before starting an exposure from the LAMPS window (Figure 16). Simply click on the appropriate box(es) to select the desired lamp(s). Note that there are pairs of identical lamps: two symetrically located lamps are used to provide uniform illumination. You can select different lamps at once if needed. The lamps will be fired when you start an exposure with EXPOSE (do not forget to select "c" as the exposure type). The Gumball web pages provide useful information on exposure times and lamp spectral lines.

FIGURE 16; The LAMPS Control Form

The CAL-SEQ Window: Automatic Calibration Sequence

CAL-SEQ is used to define the parameters for an automated sequence of calibration for the aperture masks. Open the CAL-SEQ form from the menubar (Figure 17) and enter the selections for each mask. For the spectroscopic Flat-Fields, it is possible to program several iterations and record only the average. After activating the needed calibrations with the corresponding "enable" buttons, the whole sequence is started by clicking on "accept".

CAL-SEQ is usually used when you leave at the end of the night.


The IMAGE and GRAPH Windows: Examining the Data

Normally, after each readout of the CCD, the new image is displayed as a grey scale in an SAOIMAGE window (Figure 18). If you want to look at a previous image, click on the icon IMAGE and enter the file name: for example, 299742o.fits. Then, in the SAOIMAGE window, you can interactively adjust the grey scale levels, the scale ("histeq" is fine for low contrast spectra, "sqrt" is better for direct images), zoom (option "pan") and so on. Do not forget to return to "current.fits" in IMAGE when you want an automatic display again.


With a bonnette rotation of 0°, north is normally at the top and east to the left. The image rotates clockwise with increasing bonnette angle until north is to the right for a rotation of +90°.

The GRAPH window (Figure 19) enables displays of cuts along rows or columns for up to 3 files simultaneously. You can enter the file numbers themselves or the aliases current.fits and previous.fits. The full capabilities of GRAPH are described in more detail in the Pegasus User's Manual.

It is possible to make hard-copies of the images displayed with IMAGE and GRAPH.


The FILES Window: Handling Data Files

The FILES window (Figure 20) is used to remove files from the disk when space is needed and to transfer files to DAT or Exabyte tapes. Normally, the observer should transfer all of the files they will need for their subsequent data processing to tapes at the end of each night or at the end of the run.

The FILES window also allows the user to read the file headers which contain a great deal of information about the data such as object identification, coordinates, time, air mass, MOS status, CCD status, and so on.


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