Files needed for target acquisition

The target aquisition is done using two IRAF tasks. The first task uses two images as input - one of the MOS mask illuminated by a calibration lamp and one of the astronomical object field. The image of the MOS mask should be made with the halogen calibration lamp, the calibration lens and a B filter. A one second exposure is sufficient.

The aperture files (.ap) has to contain the dimentions 4.0 as WIDTH and LENGTH. This is the only way for the task to identify a fiducial hole. And example of a correct ".ap" file:

Aperture   XCEN      YCEN     WIDTH    LENGTH
   #       [pix]     [pix]    [ " ]     [ " ]
   1      100.97    886.25     4.0       4.0
   2      570.46   1880.12     4.0       4.0
   3     1867.09   1811.33     4.0       4.0
   4      961.70    354.91     4.0       4.0
   5      292.55   1211.12     2.0      10.0
   6      786.51   1343.49     2.0      10.0

An example of a wrong ".ap" file:

Aperture   XCEN      YCEN     WIDTH    LENGTH
   #       [pix]     [pix]    [ " ]     [ " ]
   1      162.52    715.39     3.0       0.1
   2      332.91   1270.09     3.0       0.1
   3     1928.18   1701.38     3.0       0.1
   4      893.50    488.20     3.0       0.1
   5     1490.34    855.28     1.8      12.0
   6     1363.45    740.50     1.8      12.0

Edit the ".ap" file be hand to comply to this restriction.

The ".ap" files also needs to have 8 header lines. They will have this by default when produced by the mosplate program.

The task then calculates the amount to offset and rotate the telescope in order to make the MOS mask coincide with the objects in the field. These two images and the aperture file must exist in the directory florence:/var/guest/mosimages in order to be found by the IRAF tasks described below (it has been tested that images also can be loaded directly from the mounted directory, which is /data/alfosc. If you do this there is no need to copy the images).

The second IRAF task moves the telescope based on the results found by the first IRAF task.

As with all IRAF tasks, the parameters can be given in different ways. They can all be written on the IRAF command line after the command. The command can be typed and the parameters can be given when IRAF prompts for them. Finally, the parameters can be edited using the epar IRAF command. This manual assumes a basic knowledge of IRAF.

The following is a complete description on how to do the target aquisition.

Log in on the computer florence as user guest.
Type 'cd iraf'.
Type 'ds9 &' to start your favorite image display program.
Type 'xgterm &' to start a graphics capable xterm.
In the xgterm type 'cl' in order to start IRAF.
Type 'cd ../mosimages'.
Make sure that the aperture definition file for the MOS mask in use is in this directory (/var/guest/mosimages). The name will be MOS00??.nc.ap, where ?? is a sequence number.
Type alfosc to load the IRAF alfosc package.
Type
```
	    mostweak <mask-image> <objectfield-image> <aperturefile> 4.0
```
,where <aperturefile> is the file MOS0020.nc.ap if MOS mask 20 is in use. Please see the IRAF manual page of the mostweak task for a complete description.
The mask image will be displayed and the slits marked with green circles.
Position the cursor over the slit that is marked with a yellow circle. Press 'a'. When the statistics appear in the xgterm window press 'q' to move on to the next slit that will be marked with a yellow circle. Be aware that the circle and the slit in question will not always coincide completely, but it should be obvious which slit it is. The position found by the program will be marked by a small red circle.
When all usefull slits are marked, the image of the object field is displayed and the first alignment star will be marked with a yellow circle. Position the cursor on that star and press 'a'. Press 'q' after the statistics appear in order to move on to the next usefull star. The position of the star found by the program will be marked with a small red circle. Also in this case the circles will not coincide complely with the stars so take care when identifying them.
After the last star has been marked, the program displays the amount to offset the telescope in the pixel X and Y direction and how much to rotate the field.
Apply these changes using the IRAF task mosmove in the following way. Type
```
           mosmove
```
and the task will prompt you for the current TCS access code. The value of the access code has the default value from the last time the task was run, so if the access code has not changed you can just press enter. Please see the IRAF manual page of the mosmove task for a complete description.
Turn on autoguiding as fast as possible using relaxed autoguiding (TCS command: ag-re-on).
Make an image of the field through the MOS mask and check the alignment of the slits and corresponding objects.

This procedure will probably have to be iterated two times for the objects to be correctly aligned with the mask.

John Telting 2009-01-22