Parameters for "objects"
Start by opening a DS9 window before
running this task and by editing the parameters for the task "objects".
The task parameters will look like this:
This task will use the photometric solution previously obtained with
the task "standards" to compute magnitudes in the standard system
(Johnson, SDSS, or Persson) for stars in the neighborhood of each
Start by editing the snphot$objects.coo file. This is a text file
with a list of stars around each supernova. The first column specifies
the supernova name; the second column is a correlative number identifying
the star in the vicinity of the supernova; the next two columns have
precise equatorial coordinates (2000.0) for each star. Enter the name
of this file in the "coordinates" parameter. The photometry will be computed
only for the stars in this file.
Edit a text file (in2.14dec03 in the example above) specifying
the UBVRIZ images of the first supernova of the night. You can see the format
of the file in snphot$in2.14dec03. The first column is the supernova name
(e.g. SN03ib), which must be identical to the name in the file snphot$objects.coo.
The next 6 columns contain the names of the UBVRIZ images obtained for
the first supernova of the night. The order is not important. Use "INDEF"
for the absent images. Make sure that there are 6 image names per line and
that you do not repeat images taken with the same filter.
The name of this file must be entered in the "imsets" parameter.
The next input parameter is the configuration file (snphot$config.C40 in this
example) which defines the transformation equations for the different filters.
Make sure that this is the same file used with the task "standards".
The parameter called "parameters" is the file created by "standards" (fit.14dec03
in this example) containing the photometric transformation from the instrumental
to the standard system, derived from the Landolt standards.
The "observations" parameter is an output file containing the instrumental
magnitudes for the program objects (obj.14dec03, in this example), and the
"magnitudes" parameter is another output file containing the magnitudes of the
program objects in the standard system (eval.14dec03, in this example).
The parameter "database" is just the name of a text file that was created by
the task "standard" in the current directory, containing the geometric transformation
between (x,y) pixels and equatorial coordinates.
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 in the snphot$objects.coo file
to the epoch of observation. If the database already exists in the current
directory you can leave "geo_image" unspecified.
The parameter "opt_ir" can take 3 possible values: OPT, SDSS, or IR
depending on whether you are working with Johnson, SDSS, or IR images.
The parameter "aut_man" is generally set to "aut", in which case the
field stars will be automatically found for you. Sometimes this mode
doesn't work, in which case you must set it to "man".
The CCD parameters "scale", "datamin", "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 that
will be employed for the calculation of the curve of growth, "apnum" which
defines the aperture that will be used to calculate instrumental magnitudes
for the selected stars, "zmag" which specifies the photometric zero point
(which must be identical to the parameter used with the task "standards", and
"minmage" which is the "minimum magnitude error". It is necessary that the
largest radius is 7 arcsec, which is that used for the Landolt standard.
The "fwhmpsf" is a parameter that specifies the FWHM of the PSF in arcsec
and is a parameter that is used to automatically find field stars for the
calculation of the curve of growth. The parameter "threshold" is in units
of the FWHM of the sky brightness distribution, and defines the threshold
to begin star detection in the image, and "nfind" sets the minimum number of
stars to find in each image.
Running the task "objects"
The task will start by displaying the first image of the set that
you specified in the "imsets" file (in2.14dec03, in this example),
and searching for the brightest field stars. The "threshold" parameter will
be used to identify the brightest stars. If the task doesn't find "nfind"
stars, it will divide the threshold by two, and continue with the search
until it finds "nfind" stars. Then it will compute aperture photometry
for all the apertures specified in the "aperture" parameter, and will
present you with a graph showing the curve of growth. Often times
a non stellar object will be included in this plot, which you can
identify because it will have very different growth curves. Make sure that
you delete those points, and any other deviant point with the "d" key.
Refit the data with the "f" key. Once you have an RMS dispersion (which
you can read off the graphics window) lower than 0.01 mag, use "q" to quit.
The task will use this fit to compute the magnitude difference between
the largest aperture and that specified with the "apnum" parameter.
The derived magnitude will be printed on your screen and should be a
negative number amounting to a few hundredths of a magnitude.
The task will proceed to prompt you to mark one of the selected field stars
in the DS9 window, and identify it with its ID correlative number. If you decide
to use the supernova you need to use number 0 (zero). The task will use the
geometric solution (under the "database" parameter) to find the remaining standards
of the field. If everything goes well you will get all of the sequence stars
marked with purple circles and identified with their ID numbers.
Instrumental magnitudes will be computed for all of the sequence stars
through the small aperture specified with the "apnum" parameter, and
the magnitude correction derived from the curve of growth will be added
to these magnitudes to bring them to the same photometric system employed
for the Landolt standards. Hit RETURN to continue. The task will continue
with the second image of the image set.
Once the whole image set has been processed, the task will prompt you to
type a simple comment regarding the photometric quality of the night
(e.g. Phot, or Phot?) which will be saved for future use.
The task will then use the photometric solution (specified in the
"parameters" parameter) previously derived with the task "standard"
to convert the instrumental magnitudes of the local sequence to the
You will have two new text files at the end. In this example "obj.14dec03"
contains the instrumental magnitudes and uncertainties. The file "eval.14dec03"
is a text file with the magnitudes in the standard system.
The radial profiles of the measured stars have been saved in
the metacode file "obj_plot". I suggest that you inspect the
individual profiles with the command
gkimos obj_plot nx=2 ny=2
to check for stars with cosmic rays or bad pixels within the
aperture employed for the photometry. If so, flag these objects
in the "eval.14dec03" file.