XVista manual page

NAME

phot - aperture photometry of stars in a CCD frame whose positions have already been measured by the stars command

SYNOPSIS

phot image [infile= ] [outfile= ] [binsize= ] [mconst= ] [adu= ] [readnoise= ] [aper= ] [skyinner= ] [skyouter= ] [verbose] [mean] [sky= ] [skysig= ] [saturate= ]

DESCRIPTION

phot uses positions of star centroids which have already been found, either by the stars routine or by hand (but which must then be put in the same format as the output of the stars command), and adds up all the counts within some specified radius (or several) of the centroid. Pixels lying partially inside a circular aperture of the specfied radius are included with a weight equal to that fraction of their area inside the aperture. A value for the sky background at that position is found by taking the modal (mostfrequently-occurring) value in an annulus centered on the star, and the sky is subtracted from the (star+sky) counts, leaving only the star. The brightness of the star is then converted into magnitudes - with an arbitrary zero scale above the local sky background and printed out.

OPTIONS

infile= takes input from the given file instead of the standard input.

outfile= sends output to the given file instead of printing to the screen.

binsize= use the specified binsize to find the modal value of sky background. There are only 10000 bins, so any binsize less than 4 risks not handling pixel values greater than this correctly. A warning message is printed when VERBOSE is given if the binsize can't cover the whole range of valid pixel values. The default is 1, so if sky is > 10000, the mode will not be calculated correctly. But larger values lead to shorter execution times.

mconst use this as the zero point on the magnitude scale (the sky brightness). Default is 15.

adu= is the number of electrons per analog-to-digital unit on the CCD chip used to make the image. Affects error calculations only. Default is 1.

readnoise= The readout noise of the chip used to make the image (in units of electrons). Affects error calculations only. Default is 1.

sky= overrides the sky value.

skysig= overrides the sky sigma value.

skyinner= the inner and outer radii (in pixels) of the skyouter around a star used to find the local sky value. Changing the size does not affect the program run-time very much. Default values are 10 and 20.

aper= specifies the radius of a star aperture in pixels. Up to 100 may be specified, and their respective estimates of stellar magnitude are reported in the order in which they appear in the command line. The default value is 3.

verbose prints (many) additional messages about each star candidate as it is found, and some reason for rejecting or accepting it

mean uses the mean of pixel values in the sky annulus as the sky background, not the mode. Somewhat faster.

saturate= any star with one or more pixel values greater than or equal to the given value has its formal photometric uncertainty given as a negative number. This may be useful for quickly pruning an output file of all saturated or nonlinear stars.

OUTPUT

The program prints a list, either to the standard output, or a file, if one is specified, consisting of lines of the following form:

23 50.01 103.43 340 34.1 4.56 0.234 4.40 0.120

Where the fields have the following meanings:

first integer is the number of the star in this run of stars
second is the row centroid
third is column centroid
fourth is the value of sky background used for the star
fifth the standard deviation of pixels used to find the sky
sixth and seventh pairs the magnitude value and rms uncertainty for the first aperture
eighth and ninth are the mag, and uncertainty of aperture 2 (if specified
etc. up to 10 apertures

If the peak value of the star indicates that it may have been saturated (as defined by the saturate command-line parameter), the uncertainty will be multiplied by negative one - that is, instead of "0.234" in the above example, "0.234" would be printed. This is done purely as a signal to the observer, who ought to remove the minus sign (and probably flag the star as worthless) before performing any calculations.

EXAMPLE


phot m35 infile=stars.m35 aper=3.5 aper=4 aper=4.5

Uses the file `stars.m35' for positions of star in the CCD frame `m35.fts.' Three apertures, of radii 3.5, 4 and 4.5 pixels, are used to measure the magnitudes of the stars listed in the input file. Output is sent to the screen.

phot m45 outfile=phot.m45 mconst=20 skyinner=7 skyouter=10

Looks in the standard input for a list (in stars output format) of star positions which it will use to measure the brightnesses of stars in CCD frame `m45.fts.' The sky annulus has an inner radius of 7 and outer radius of 10; only one star aperture will be used, with the default radius of 3 pixels. The sky is given a magnitude value of 20, so all stars detected will have magnitudes <= 20.

NOTE

Stars near the edge of the frame will have only have the pixels which are inside the union of the frame and the annulus used for their sky backgrounds.

A value of 99 for the magnitude or uncertainty of a star indicates that there was some problem encountered in the calculations; it is often seen when a non-existent star position is placed in the input.

The uncertainties are based upon errors in the sky value, which must be subtracted from the total number of counts to get a star's contribution alone. It is necessary to specify accurate values for the ADU and READNOISE parameters in order to get a `true' error value, but it may serve as a relative measure in other cases.