Parameters for "sne"
Start by opening a DS9 window before
running this task and by editing the parameters for the task "sne".
The task parameters will look like this:
This task will calculate psf magnitudes for the supernova and the local
standards, and convert these magnitudes to the standard photometric system
(Johnson, SDSS, or Persson).
The "snname" parameter defines the name of the supernova. This name must
be identical to that in the file specified in the "coordinates" parameter.
In this case this file is called snphot$objects.coo and in this example
the supernova name is "SN03T".
The snphot$objects.coo file 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.
The "catalog" parameter is the name of a file that contains the magnitudes
of the local standards. This file can be created with the makeseq task
and must be placed in the current directory. In this example this file is called
"standards". The format of this file can be checked in snphot$standards. The first
column has the star name, and the next pair of columns has the magnitude and the
associated error. In the UBVRI Johnson system the column pairs are in the order
BVRIU; in the SDSS system the order is ugriBV; in the Persson system the order is YJHK.
Edit a text file (imsets.dat in the example above) specifying the names of the
images for a given epoch of observation. You can see the format of the file
in snphot$imsets.dat. The task expects a file with 6 columns containing the
names of the UBVRIZ, ugriBV, or YJHK images. 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 ("config.C40" in this
example) which defines the transformation equations for the different filters.
Note that this is not exactly identical to the file used with the task "standards".
The form of the equations is identical, but here you do not fit the color terms,
and the extinction coefficients. The extinction coefficients can have any value
since the task will compute differential photometry. The color terms, on the
other hand, are critical. These coefficients are expected to change from instrument to
instrument. For every instrument that you used you must use average values computed
from many photometric nights. With this configuration file the only fitting
parameter is a photometric zero-point for each filter.
The "observations" parameter is an output file containing the instrumental
magnitudes for the stars measured (obs.25apr03a, in this example), and the
"magnitudes" parameter is another output file containing the magnitudes of the
objects in the standard system (eval.25apr03a, in this example).
The "opt_ir" parameter specifies whether you are working with CCD or infrared images.
The parameter "epoch" is used to precess the equatorial coordinates specified
in the snphot$objects.coo file to the epoch of observation.
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", "ugriBV", or "YJHKLM".
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 specifies the radius in arcsec that
will be employed to compute psf magnitudes (only pixels within the fitting
radius of the center of a star will contribute to the psf fits), "zmag" which
specifies the photometric zero point (which is arbitrary), and "minmage" which
is the "minimum magnitude error" for the psf magnitudes.
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. These stars
will be used to compute the geometric conversion from pixel coordinates to
equatorial coordinates. 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 "nfstar" sets the minimum number of stars to find
in each image. It is possible to restrict the search to a section of the image
with the parameter "sec".
Running the task "sne"
The task will start by displaying the first image of the set that
you specified in the "imsets" file (imsets.dat, 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 "nfstar"
stars, it will divide the threshold by two, and continue with the search
until it finds "nfstar" stars. The selected stars will be shown with purple
circles. Hopefully enough of these stars will be present in the "coordinates"
file (snphot$objects.coo in this example). If so, the task will compute the
geometric transformation from pixel coordinates to equatorial coordinates.
You will get on the screen some information regarding the transformation
such as the scale factors (xmag and ymag), the rotation angle in degrees
(xrotation and yrotation) and the rms of the fit (xrms and yrms).
Make sure that the X and Y scales have similar values, that the X and Y
axis rotation values are similar, and that the X and Y residual rms are
less than 0.2 pixels.
The task will proceed to prompt you to mark one of the local standards 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
previously determined geometric solution to find the remaining local standards.
If everything goes well you will get all of the sequence stars
marked with purple circles and identified with their ID numbers.
The task will prompt you to mark one or more PSF stars. If the task finds
an acceptable star (without bad pixels) it will show you a surface plot
with the star. The graphics cursor will be enabled and you have the
option of deleting (with "d") the star in case that the PSF is not clean
or accepting it (with "a"). After this you will have the image cursor
enabled and you can proceed to select more PSF stars. Use only the
brightest ones, as close as possible to the supernova. Once you have
finished with the selection of PSF stars type "w" in the DS9 in order
to write the PSF to a file.
PSF fits will be attempted for the supernova and all of the sequence stars.
The PSF fits will be performed with the fitting radius specified with the "aperture"
parameter. The green circles will show the stars for which it was possible
to perform the PSF fits. The image with the subtracted PSFs will be displayed on
frame 2. You can use the blink option in DS9 to check the residuals.
Hit RETURN to continue. The task will continue with the second image
of the 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 magnitude. 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. Once you have cycled through all the filters,
the task will use the photometric solution just derived to convert
the PSF magnitudes of the supernova and the local sequence to the
You will have two new text files at the end. In this example "obs.25apr03a"
contains the instrumental magnitudes and uncertainties. The file "eval.25apr03a"
is a text file with the magnitudes of the supernova and local
standards in the standard system.