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Aperture Photometry

Aperture photometry determines sum of counts from a star in a set of predefined apertures.

Synopsis

munipack aphot [.. parameters ..] file(s)[,result(s)]

Running of photometry adds a HDU extensions to the original frame as a table described in Processing Format.

Algorithm

Aperture photometry is performed by the way:

The pixels near the border are approximated by only a fraction covering the pixel inside.

By defaults, twelve apertures is used. Aperture radii are computed as points of logarithmic spiral r = r0exp a φ with the initial radius r0=2 and pitch angle 8.9073° (to be compatible with DAOPHOT II).

For a correct determination of photometry errors, the camera-specific parameters must by provided.

Strictly speaking, for further processing just one aperture is required. The prepare of several apertures has profit in fast switching to another aperture without needs to do photometry again. More apertures can be also used in simultaneous use of ones for PSF of grow-curve photometry.

The results of aperture photometry are stored in columns of a newly created APERPHOT extension.

The photometry is performed for all detected stars (all stars in FIND extension) in all specified apertures. Sometimes it is impossible compute the sum, the case is indicated by a negative value (-1). Common situation are when the sum of counts with subtracted background is negative (sky is polluted by a bright source, background has a fast gradient) or the star is too near of border and summing pixels are out of area) or a star is saturated.

The results of aperture photometry are stored to next PHOTOMETRY extension. In the extension, the results in a selected aperture (by -s) are selected. Just stars with valid (non-negative) values are presented here.

Input And Output

On input, list of frames containing the table with already detected stars is expected.

On output, tables with the aperture photometry and results from the selected column, represented by FITS extension, is added to all input images.

Parameters

--apertures r1, r2, .. rmax
define apertures radii (in pixels), count and sizes are arbitrary, must be specified in increasing order and separated by commas
--ring ri,ro
inner and outer sky ring radius in pixels, the outer radius must be equal or bigger than the greater aperture

See Common options for input/output filenames.

History

The routine is on base of original code of DAOPHOT II by P.B.Stetson (has been forked from Midas's implementation at 1996). Some additional enhancements has been done:

Examples

Run aperture photometry:

$ munipack aphot pleiades.fits

Select apertures and sky estimation by hand (as powers of 2):

$ munipack aphot --apertures 1,2,4,8,16,32 --ring 16,32 pleiades.fits

See Also

Growth Curve Overview, Detect Stars, Common options, Photometry Format.