## TUTORIAL IV. USING XGUIPHOT TO MEASURE ISOLATED STANDARD STARS

Xguiphot can be used for making measurements of standard stars, since it is possible to work quickly through the images in a large data set using the image display and existing finding charts. This tutorial illustrates one way of doing this kind of operation.

To begin the tutorial type the following:

        cl> xgex4 <myoutputfile>

Note that the results file has been set to a single file, so that all the measurements will be written to one file. Make sure you have write permission in the output file directory, and that the output file does not already exist.

The first thing to do after the gui activates and the first image is displayed, is to check the image data dependent parameters as follows:

        1. go the Phot menu
        2. select the "Impars pset" item
        3. set imaxdata to 32000 to flag saturated stars and press CR
        4. set ikgain and ikread to "" and "" to switch off gain and
           readout noise keyword searching and press CR
        5. set ireadnoise and igain to 75.0 and 11, the current best estimates
           of the detector readout noise and gain, and press CR
        6. check that exptime, airmass, filters, and ut are being correctly
           read from the image header
        7. press the Apply button
        8. press the Dismiss button

Next check the centering algorithm parameters:

        1. go the Phot menu
        2. select the "Cenpars pset" item
        3. check that calgorithm is "centroid1d"
        4. check that the centering box is reasonable, around
           2-3 * fwhmpsf, but never less than 2.5 pixels
        5. press the Apply button
        6. press the Dismiss button

Next check the sky fitting parameters:

        1. go the Phot menu
        2. select the "Skypars pset" item
        3. check that smode is "concentric"
        4. check that sgeometry is "circle"
        5. set srannulus to 15 and press CR
        5. set swannulus to 5 and press CR
        6. check that salgorithm is "hcentroid"
        7. press the Apply button
        8. press the Dismiss button

Finally check the photometry parameters:

        1. go the Phot menu
        2. select the "Photpars pset" item
        3. check that pgeometry is "circle"
        4. set papertures to 10.0 and press CR
        5. press the Apply button
        6. press the Dismiss button

Make a trial measurement using the default coordinates files as a finding chart:

        1. go to the results Menu button
        2. select the "Photometry plots ..." item

        then either

        3. select the "Omarkpars pset ..." item from the Lists menu
        4. click on the onumber entry box
        5. press the Apply followed by the Dismiss button
        6. select the "Mark list ..." item from the Lists menu

        or

        3. move to the image display window
        4. type the colon command :onumber yes
        5. move to the image display window
        6. type m to mark the standard stars

        then either

        6. move to a bright isolated standard star
        7. press the left mouse button
        8. select the "Point and measure" item

        or

        6. move to a bright isolated standard star and type Spacebar

Examine the plots. Do the sky and object apertures look good ? If so proceed to the next step otherwise go back and adjust them, and make a new test measurement.

Now you are ready to make measurements. If you want to look at plots of the objects as you go along leave the plotting panel up otherwise dismiss it. If you want to look at all the measurements together in a table activate the photometry table by typing the t keystroke command in the image display window, or selecting the "`Photometry table ...`" item from the Results menu.

Now go to the Files menu, select the "`Enable logging`" item to turn on results logging, and then

        1. move to the image display window
        2. type i to redisplay the first image
        3. type m to mark and number the standards
        4. measure the standards in order by moving to each numbered
           star in turn and typing Spacebar
        5. type n to move to the next image
        6. repeat steps 3-5 for the remaining images
        7. go to the Files menu
        8. select the "Disable logging" item

Quit the tutorial by hitting the Quit button.