The MOS Environment
The MOS environment is composed of the following main elements:
Changing mask slides is the only operation that the observers (or the
OA) may have to do at night in the dome. The mask slides are
introduced through a port in the bottom cover of the central octagon
(see Chapter 5 for practical
procedures). Presently, MOS slides can accomodate three 78 x 78mm
mask-holders in addition to the open position (Figure 5). The mask
slide is locked into position at each location to ensure that there
is no lost motion or flexure.
FIGURE 5; Front View of the MOS Mask Slide
The tolerances for filters designed to fit in the MOS/SIS filter wheels are quite tight. If you intend to bring your own filters or have them fabricated for your observations, they should have the following specifications (CFHT standard):
Filters currently available at CFHT for use with MOS may be found on
our filter web pages.
- Size: 76.2mm (3") diameter. 75mm round filters can also be
accomodated if necessary.
- Thickness: MUST not exceed 7mm.
- Imaging quality: 1/4 wave or better, peak to valley over the
- Coating: Hard, anti-reflection coating on both surfaces preferred.
- Blocking: 10-4 or better between the low cut-off of the filter and
3000Å and above the high cut-off of the filter and
- Band width and central wavelength: specified for 0°C and a
The grisms mounted in the MOS/SIS cassettes have circular cross
sections and 65mm diameters. The maximum practical thickness is 60mm
(?). If you think that a new grism should be purchased for MOS, first
check that it can be designed within the above specifications. The
CFHT grisms available for MOS are listed on our Grism web page, along with the relevant
parameters. Figure 6 shows the grism efficiencies
FIGURE 6; MOS Grism Efficiencies
Currently, two CCD detectors are available for use with MOS: Loral 3
and EEV 1. Loral 3 has several drawbacks, including larger pixels,
reduced sensitivity, and slower readout. The only significant
drawback of EEV 1 is the relatively large amount of fringing beyond
7000Å. In practice, EEV 1 is recommended for most applications,
unless weak line fluxes beyond 7000Å are desired. A more
complete description of these CCDs is given in our CCD web pages.
The system for introducing uniform illumination of wavelength and
flat-field sources was designed by Y.P. Georgelin and G. Monnet. The
calibration unit fits into one of the ports of the cassegrain bonnette
and uses the rear surface of the existing central 45° mirror to
introduce light onto the optical axis of the telescope. Either one of
two spectral lamps or a flat field halogen lamp can be selected to
illuminate a transmissive diffusing screen. The optical scheme uses
two commercial 9.87 grooves/mm fresnel lenses as field lenses and one
biconvex lens for a relay. Two symmetrically-mounted lamps are used in
tandem for each set to ensure better than 5% uniformity in the blue
and better than 3% in the red. The calibration system is controlled by
the data acquisition and instrument control computer, independently
from the MOS/SIS control system.
LAser MAchine (LAMA)
Once an image of a field has been acquired, the mask preparation is
carried out with another HP terminal by starting a LAMA session (login
as lama; ask your support astronomer for the current
To design a mask, you will display the field image and interactively
superimpose the aperture contours on the objects of your choice. The
details of the procedure are given in Chapter
5; we just note here that you need to select the size of your
slitlets (for objects) and round apertures (for centering stars),
then use the MOS and PAN icons to move the apertures from one object
to the next, and to center them precisely.
The useful area for slitlets is less than the whole area of the
CCD. A first limitation comes from the mask holder geometry: on a
2048 x 2048, 15µm CCD, the slit coordinates should lie in the
range 105 < X < 1898 and 185 < Y < 1793. Then, unlike for
long slit observations, multi-slit spectroscopy implies that each
slit has a unique spectral domain. This domain is directly related to
the slit position on the aperture mask. For a slit located at
Yccd, and a CCD with Ymax pixels,
(e.g. Ymax = 2048 for LORAL3), the spectral domain is
where Lambda 0 is the zero deviation wavelength in Å and
D is the dispersion in Å/pixel. One important
consequence is that if your program requires a given wavelength
range, you will be restricted to a fixed area in which to position
slits. The LAMA mask-design tool assists you in this respect by
representing the full spectral range for the grism of interest,
making it clear if the spectrum of a particular object will fall
within or off of the detector.
A few weeks before your observing run, CFHT will ask you how many
blanks you will need for the entire run, in order to have them ready
in advance. Try to estimate how many fields you could observe if
everything goes well, then multiply by 1.5 for safety. For instance,
if you plan to use 10 masks ask for 15 blanks.
Send comments to: email@example.com
Copyright © 1997, CFHT Corporation. All rights reserved.