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Acquiring guide stars

Guide Star acquisition is done almost automatically with ICL software that uses the on-line Guide Star Server to predict guide probe coordinates for a star from the HST Guide Star Catalogue; the guide probe is then moved to the predicted star location. It may be necessary to first load the required ICL procedure; check with your Support Astronomer or local staff. You should also make sure there is a filter (e.g. V) in the autoguider filter wheel; change autoguider filters by typing PAUTOFILT <filter> at the ICL interface.

To find a guide star, type:

GSSEARCH at the ICL interface.

GSSEARCH will then find the brightest star in the autoguider field, and move the guide probe to that location; you should see the guide probe x,y values change on the MIMIC display. If GSSEARCH fails, this may be due to the Guide Star Catalogue disk not being mounted, and an error message to this effect should appear. If this is the case, a Duty Technician will likely need to be called to remount the disk. If all goes well, and GSSEARCH returns without any errors, type:

FIELD at the ICL interface,

which will take an image of the autoguider field and display it on the autoguider monitor.

Hopefully the star will be in the guiding box, but it may be necessary to center the star in the box by moving the guide probe in x and/or y:

PGDX <X position> , to move probe to given <X position>

PGDY <Y position> , to move probe to given <Y position>

PGDXY <X Y position> , to move probe to given <X Y position>

Then type FIELD again to redisplay the autoguider field. After some experimentation, you will know which directions on the autoguider field correspond to increasing/decreasing X,Y. ??THESE DIRECTIONS HAVE TO BE DETERMINED!! THEY WILL NOT CHANGE ...??

Once the guide star is centered in the guiding box, start the autoguider guiding by typing:

FON at the ICL interface.

Finally, to close the guide loop and start sending autoguider errors to the TCS, type:

AUTOGUIDE ON at the ICL or TCS interface; there is a programmed function key on the TCS terminal to do this.

You are now ready to begin an exposure. If the autoguider is not guiding well, you can try increasing the autoguider exposure time by typing:

GUIINT <time> at the autoguider interface, or
SETINT <time> at the ICL interface

where the time is in milliseconds. Guiding integration times can be as short as 50 milliseconds, but it is better to integrate for longer times if possible (1-2 seconds is typical); a sensible maximum is 10 seconds. It may also be necessary to increase the size of the guiding box by typing:

GUISIZE <n> at the ICL interface,

where <n> is the size of the guiding box in pixels. Finally, it is possible that the autoguider is not in good focus. Ask the TO for the previous night's value of the autoguider focus; the focus may be changed by typing

PGDFOC <value> at the ICL interface

To stop guiding at the end of an exposure, type:

AUTOGUIDE OFF at the TCS or ICL terminal, and:
FOFF at the ICL interface, or GUIDE OFF at the Autoguider interface.

next up previous contents
Next: Taking Data Frames Up: Setup and Observing Procedures Previous: Focal Station Setup

manuals store
Wed Sep 17 12:36:20 BST 1997