XVista manual page

NAME

stars - finds stars in a CCD image

SYNOPSIS

stars image [box=] [sky=] [fwhm=] [skysig=] [minsig=] [minfwhm=] [maxfwhm=] [minround=] [maxround=] [minsharp=] [maxsharp=] [outfile=] [maxstars=] [verbose] [show] [convolve] [image=] [movemax=] [timeout=]

DESCRIPTION

This routine, based on Peter Stetson's DAOPHOT FIND command (see PASP 99, 191, 1987), goes through an image, looking for brightness peaks more than minsig standard deviations above the sky background; when it finds one, it checks to see if the FWHM of the object surrounding the peak is within the limits for an unresolved point source (i.e. star). A list of stellar centroids, together with an approximate peak brightness value, is given as stars are found. Usually, the sky command is run on an image immediately before stars, in order to give this program an idea of the sky value and the sigma in the sky value for the image.

OPTIONS convolve - causes the program to convolve the image with a lowered gaussian of the given fwhm, which serves to emphasize stellar features over extended objects or cosmic rays. It is somewhat slower, especially when a large FWHM is used, but better at finding stars inside complicated regions. The output value of FWHM is less meaningful when convolve is specified, since the convolution stamps its own width onto all features to some degree. Only if convolve is specified does the fwhm keyword have meaning; when convolving, however, the sky and skysig values are ignored.

sky - uses the supplied value as sky brightness; if none is given, looks for the SKY variable set in the XVISTA symbol table; if not defined there either, uses zero.

skysig - uses the supplied value as the standard deviation of the background sky noise; if not supplied, looks for the SKYSIG symbol; if not present, uses a value of 1.0 (not very realistic).

fwhm - specifies the expected full-width at half-maximum of stellar features in the frame. Only used when convolve is specified as well. The default value is five pixels. Since this is the value used of the convolution, it is relatively important; for best results, check the true FWHM on an image before running this command.

minsig - looks for stars with peaks >= minsig standard deviations above the background noise. The default is three. minfwhm, maxfwhm - define the range of full-width-at-halfmax which are acceptable for stellar images. The defaults are generous, but may not work for very oversampled or undersampled images.

minround, maxround - define the limits of the range of `round' values a star may legally have, where

xwidth - ywidth
round = 2 * (-----------------)
xwidth + ywidth

and `xwidth' and `ywidth' are the second moments of the star profile in x and y, respectively. The default values are -1 and +1.

minsharp, maxsharp - define the range of the `sharpness' parameter inside which stars may fall. This is similar to the `sharpness' defined in DAOPHOT, but identical only when CONVOLVE is specified. In all cases, `sharpness' is the ratio of the peak pixel to the mean of the pixels around it (with sky subtracted). The default values are (0.1, 0.6) (for no convolution) and (0.2, 1.0) (for convolution).

maxstars - specifies the maximum number of stars to print out before exiting and giving an error message. The default value is some absurdly high value.

movemax - limits the amount that a star centroid may drift from the peak value location. The default value is 1.0 pixel.

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

verbose - prints (many) additional messages about each star candidate as it is found, and some reason for rejecting or accepting it. Also, as the convolution proceeds, status messages are printed so that the user can see how much longer it will take.

show - draws a crosshair on each star centroid as it is found. If an image is not already up on the screen, does nothing. You may use the marks command if you wish to redraw the points.

image - specifies the name of a FITS file into which a convolved version of the image will be written. If not supplied, or if convolve is not specified, no file is written.

timeout - sets an optional maximum time for the star program to execute. If the programs takes longer to execute than the specified time in seconds the program prints out an error message and terminates with a non-zero exit code.

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 50.0 103.4    804    2.88 1.03    3.77

The first integer is the number of the star in this run of stars (i.e. the above is the 23rd star found so far in this run), provided for identification. The second and third numbers are the row and column centroid of the star. The fourth is an approximate peak pixel value with sky subtracted. The fifth is an approximate FWHM of the stellar image. The sixth is the `roundness' index of the star, as defined above. The last is the `sharpness' index, which is not as defined in daophot (see definition above).

EXAMPLE

stars m13 box=3 minsig=4 - looks for stars in the region covered by box 3 in file `m13.fts'. Only stars with peaks more than 4 sigma above sky noise will be considered. Note that the sky, command must have been run before this example, in order to set the sky and skysig values in the symbol table.

stars sag box=2 verbose showconvolve - Looks for stars in box 2 of file `sag.fts', using convolution of the original image (in box 2) with a lowered gaussian. Prints out lots of info on each star candidate interspersed with the star positions, and draws a cross on each star on the screen. The convolved version of the image is written to the FITS file "conv.fts".

BUGS

Close multiple stars are not handled well and cause the program to take extra time (and probably decide they're nonstellar). Stars near the image edges are also often missed.