[Top] [Prev] [Next] [Bottom]

Instrument Set-Up (Staff)

Spectrograph Alignment

Principles

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 XY coordinate frame is aligned with the CCD XY 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 CCD Y. We can note that since MOS and SIS share the same set of grism wheels, once the grisms are aligned for SIS, the grism wheel can be installed on MOS. The MOS alignment will only consist of a CCD rotation until spectra run along the CCD columns. There is then no need to align grisms on MOS, if they were properly aligned on SIS.
Of course, this is not valid for grisms installed in the MOS/ARGUS wheel, since the orientation here is at to that expected in MOS or SIS.

Alignment Procedure:

To properly align MOS, proceed as follows:

Insert a mask with the reference cross in the entrance focal plane.
Select the proper raster to isolate only one hole of the cross, with a full size raster in Y.
Adjust the MOS camera focus to get an approximate focus (accurate focussing is done later, see "Spectrograph Focussing" below).
Select a raster (vertical strip) to isolate one arm of the reference cross.
Take an image of the cross (V filter + Ar lamp = 1s). 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 10s).
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 octagon.
Align all grisms. Once MOS is aligned, and if you need to align SIS:
Remove the grism wheel from MOS and install on SIS.
Drive a grism into position. Since the alignment was done on MOS, 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 SIS 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 MOS.

Spectrograph Focussing

The internal focussing of the spectrograph is done by moving the MOS 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 perfectly planar.

To focus the camera, proceed as follows for each filter:

Insert a mask with a reference pinhole in the entrance focal plane.
Select a subraster centered on the pinhole and take a sequence of images by moving the camera focus in the MOS form (starting with 50 unit steps, then finalize with smaller steps). Evaluate the image quality with IQE. The best focus image should be round and with X and Y image quality close to
For reference, the spectrograph intrinsic image quality is around 22µm.
When the internal focus has been determined for all filters, save the values entered in the MOS form. Note that the values are significantly different for different CCDs (for instance, close to 1000 for Loral3 and close to 3000 for Lick2).

CAF Set-Up:

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 (1s 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 units).

OFFSET Set-Up:

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 (10s) 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 3mm. 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 XY TV coordinates).
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" program maps telescope coordinates versus CCD coordinates and allows very accurate offsetting.

Checking the LAser MAchine:

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 1-2µm over a full MOS mask.
slit edge roughness should be on the order of 2-3µm

[Top] [Prev] [Next] [Bottom]

Send comments to: veillet@cfht.hawaii.edu