Parameters for "standards"
Start by opening a DS9 window before
running this task and by editing the parameters for the task "standards".
The task parameters will look like this:
This task will compute the photometric transformation to transform magnitudes in the
instrumental system to those in the standard system (Johnson, SDSS, or Persson).
Start by editing a text file (in1.14dec03 in the example above) containing
the images of the standard stars. You can see the format of the file in
snphot$in1.14dec03. The first column is the field name, which must match
the field names in the catalog file (snphot$landolt.dat in this example).
The next 6 columns contain the names of the UBVRIZ images obtained for
one standard field. The order is not important. Use "INDEF" for the absent
images. Make sure that there are 6 image names per line.
The next input parameter is the configuration file (snphot$config.C40 in this
example) which defines the transformation equations for the different filters.
The "observations" parameter is an output file containing the instrumental
aperture magnitudes, and the "parameters" parameter is another output file
containing the photometric transformation, the resulting coefficients, and
the rms of the fit.
The parameter "geo_ima" specifies the name of one of the images of the standard
fields where you see various standards spread over the CCD. This image will be
used to compute the conversion between (x,y) pixels and equatorial
coordinates, which must be specified in the "catalog" file. The parameter "epoch"
is used to precess the equatorial coordinates to the epoch of observation.
The CCD parameters "scale", "datamax", "readnoise", "epadu", "exposure", "airmass",
"filter", are self explanatory. The "filterid" parameter contains the filter
IDs in the headers under the keyword specified under the "filter" parameter.
The filter order must be either "UBVRIZ", "YJHK", or "ugriBV".
The "obstime" parameter is the header keyword that specifies the UT time of
observation, the "jdate" parameter is the header keyword that specifies the
The parameters for the photometric measurements are the following:
"salgorith" which specifies the sky fitting algorithm, "cbox" which specifies
the centering box in arcsec, "annulus" which specifies the inner radius of
the sky annulus in arcsec, "dannulus" which specifies the width of the sky
annulus in arcsec, "aperture" which lists the aperture radii in arcsec for
aperture photometry (you must set it to 7 arcsec which is the aperture used
by Landolt), "zmag" which specifies the photometric zero point, and
"minmage" which is the "minimum magnitude error".
Running the task "standards"
The task "standard" will start by displaying the image selected for the
geometric transformation (ccd088 in this example), and by prompting you to
mark at least 3 standard stars. After quiting you will get on the screen
the geometric solution. Make sure that the X and Y scales have similar
values and that the X and Y axis rotation values are similar.
The task will proceed by displaying the first image of the first image
set. You will be prompted to mark one standard star and identify it with
its name, after which the task will find the remaining standards. These
stars will be shown with purple circles. The task will compute
magnitudes for all these stars through an aperture with a radius of
specified by the "aperture" parameter. The task will proceed
with the second image of the first image set. Once the whole image set has
been processed, the task will proceed with the next image set.
Once the task has gone through all of the images, it will call
the "fitparam" task. This will present you a graphics window
with the magnitude residuals for the first filter as a function
of color. The transformation equation is written at the top
of the plot, and the RMS of the fit is given in magnitudes.
You need to delete deviant points with 'd' until the RMS drops
to less than 0.025 mag. A useful plot is residuals versus
airmass which you can get by typing 'g'. This will prompt you
to specify a new graph key (say 'k'), and then the parameters
to plot in the XY axis. Here you want to type "xb,residuals".
If you type 'k' you will get the desired plot which will allow
you to judge if there are fields with systematic residuals.
If so, you can proceed to delete the entire field with 'd'.
Once you have cycled through all the filters, the task will
You will have two new text files at the end. In this example "std.14dec03"
contains the instrumental magnitudes and uncertainties. The file "fit.14dec03"
is a text file with the photometric solution. For each filter you will get an
extinction coefficient, a color term, and a zero point.