The alignment of MOS or SIS involves three separate adjustments:
- The mask slide, and the LAMA machine.
- The grisms in the grism wheel.
- The CCD.
A properly aligned MOS or SIS means that:
- The LAMA machine X, Y coordinate frame is aligned with the CCD X,Y coordinate frame. This ensures that slits run parallel to CCD rows.
- The spectra dispersion for each grism is running along CCD columns, the blue part of the spectra being usually at small CCD Y, the red part at large
We can note that since MOS and SIS share the same set of grism wheels and, once the grisms are aligned for MOS, the grism wheel can be installed on SIS. The SIS alignment will only consist of a CCD rotation until spectra run along the CCD columns. There is then no need to align grisms on SIS, if they were properly aligned on MOS.
Of course, this is not valid for grisms installed in the MOS/ARGUS wheel, since the orientation here is at 90 degrees to that expected in MOS or SIS.
To properly align SIS, proceed as follows:
- Insert a mask with the reference cross in the entrance focal
- Select the proper raster to isolate only one hole of the cross, with a full size raster in Y.
- Adjust the SIS camera focus to get an approximate focus (accurate
focussing is done later, see section 9.2).
- Select a raster (vertical strip) to isolate one arm of the
- take an image of the cross (V filter + Ar lamp = 1 sec). Use
``align'' or ``graph'' to cut the image on the two extreme hole images.
Measure the angle of rotation that needs to be applied to the CCD.
- The rotation of the CCD is done in two steps: coarse rotation by
unlocking the ``dogs'' if a rotation of more than is needed, or
fine rotation by using the black rotation knobs on the side of the CCD
mounting plate after releasing the locking screws around the plate. One turn
of the black knobs will rotate the CCD by If the CCD is
rotated clockwise, the image of the cross rotates clockwise on SAOIMAGE with Loral3. The alignment should be done to better than 1 pixel over 2000.
- Insert a grism.
- Select a raster (vertical strip) on one of the holes of the cross
(off the vertical arm).
- Take an image (comparison) with the halogen image lamp (approx 10 s).
- If the grism needs to be aligned, two setting screws are
available on each grism cell for fine rotation. To access these screws,
the grism wheel needs to be driven to the position exactly opposite to
the grism being adjusted. One turn of the set screws will rotate a grism
by . An accuracy of alignment better than 1 pixel over 2000 is
required. Spectra rotate couterclockwise on SAOIMAGE (with Loral3) if the
grism is rotated clockwise as seen from the CCD toward the
- Align all grisms.
Once SIS is aligned, and if you need to align MOS:
- Remove the grism wheel from SIS and install on MOS.
- Drive a grism into position. Since the alignment was done
on SIS, all grisms are aligned with respect to each other. Any grism will
therefore provide a reference to the dispersion direction which can be
used to accurately rotate the CCD for MOS alignment.
- Take a spectrum and use ``align'' to cut the spectrum at two extreme positions. Compute the rotation angle that needs to be applied to the CCD.
- Rotate the CCD as described for SIS.
The internal focussing of the spectrograph is done by moving the SIS camera focus. Each filter in the filter wheel will have
a specific camera focus; this is due to the fact that the filters are not accurately plane.
To focus the camera, proceed as follows for each filter:
Note that the values are significantly different for different CCDs (for instance, close to 1000 for Loral 3 and close to 3000 for Lick2).
Once satisfied with the internal focussing, the zero
point for the Computer Aided Focus is ready to be computed. CAF is normally
set with a zero point which indicates the difference Y between the
two images produced by the Hartmann focussing device to be taken out of the
CAF computation when the telescope is exactly in focus. Inversely, when
you know that the spectrograph is in focus, (such as after focussing the
camera) setting zero point = 0 will force CAF to compute Y, hence giving the zero point. This comes from the linear formula which transforms Y into telescope encoder motion:
Proceed as follows:
- Select the grid or pinhole as an entrance mask.
- Select CAF, enter the proper exposure time (1 sec with one Argon
lamp on) and enter ``0'' for the CAF zero point. Select ``accept'' to start a CAF sequence. The CAF program outputs a telescope focus motion T in telescope encoder units.
- Enter the output value from the CAF progam into CAF as
, this value being the separation Y (in microns)
between images when the telescope is exactly in focus.
Select ``save values'' to retain the zero point value for later use of CAF.
- Verification: running CAF again with this setup should give a suggested value for telescope encoder motion very close to 0 (no more than 2-3 units).
The fine offset calibration is done on the sky with the
``autocal'' tool. Over several runs we found that a new calibration was not needed for each new run, provided that the alignment procedure as defined above is thoroughly executed.
However, if you need to calibrate the fine offset, proceed as follows:
- Select a field with a bright enough star for short (10 s)
exposures. Ask the TO to find a guide star and guide with the star close
to center of the guide TV.
- Define the exposure time in ``expose''.
- Start the ``autocal'' sequence, with deplacement steps of 3 mm. You need to take 3 exposures according to the following pattern:
- The first exposure is taken and the image displayed. Select the chosen star with the cursor in SAOIMAGE and record its position.
- The bonnette is moved one step in X. You then have to drive the guide star
back to the guide box by moving the telescope (it is imperative that
during the whole sequence the guide box is left at the same X,Y TV
- A second exposure is taken. Select the same star as previously with the cursor.
- The bonnette is moved one step in Y. You then have to drive the guide star
back to the guide box by moving the telescope.
- A last exposure is taken. You have to select the same star as
previously on SAOIMAGE.
The calibration values are then entered into the respective ``offset'' fields. The ``autocal'' programme maps telescope coordinates versus CCD coordinates and allows very accurate offsetting.
The set-up procedures should include the following:
- cutting of a test mask.
- measurement of cutting accuracy:
- relative positions should be accurate to better than 1 - 2 m over a
full SIS mask.
- slit edge roughness should be on the order of 2 - 3 m.
Please send comments and suggestions regarding the HTML version of this manual to Christian Veillet.