Return to: UVOT Digest
In order to apply the UVOT photometry calibrations the source count rate must be extracted in an aperture that matches the aperture used to calibrate the photometry. As of 2007-05-22 this is a circular aperture with a radius of 5 arcseconds. There are various situations where this aperture may not be appropriate. Small apertures make it easier to exclude contamination from nearby sources, an aperture with a radius that matches the full-width at half-maximum (FWHM) of the point-spread function (PSF) is optimal for detecting sources, and a small aperture results in a higher signal-to-noise (and thus a lower statistal error in the derived magnitude) for a source. If a smaller source aperture is used then an aperture correction must be applied to the count rates in order to correct for the missing counts. This process is discussed in detail in the UVOT Aperture Correction analysis thread. Users interested in the general details of aperture correction should consult Howell (1989, PASP, 101, 616) and Stetson (1990, PASP, 102, 932).
Rigourous aperture corrections require a good deal of effort to compute. They depend on the shape of the PSF, and thus can vary with filter and count rate. In addition the FWHM of the UVOT PSF is dependent on the temperature of the UVOT focusing rods. Their temperature varies slightly with time because the voltage on the heaters changes through the spacecraft orbit. This can cause variations in the shape of the PSF. Currently this variation has not been well-characterized. As a result aperture corrections ideally need to be computed for each UVOT exposure as discussed in the UVOT Aperture Correction analysis thread. However this is time consuming, and not easy to automate. Because of this the UVOT analysis software implements an automated method of computing approximate aperture corrections.
The Swift software includes a task called uvotapercorr that takes a count rate, an aperture radius, and a file describing the PSF, and returns an estimated aperture correction. This correction is computed by comparing the volume under the PSF out to the standard photometry aperture radius to the volume under the PSF out to the user-specified aperture radius. The user must provide the appropriate PSF file, but mean PSFs for each UVOT filter are included in the Swift/UVOT CalDB distribution. Please see the fhelp file for uvotapercorr for details on using uvotapercorr. Details of this method of aperture correction are given in Poole et al. (2008, MNRAS, 383,627).
The aperture corrections computed by uvotapercorr are approximate at the level of a few percent. The aperture corrections for several apertures are given in the following table. These numbers are intended for making approximate aperture corrections in cases where high-precision photometry is not necessary. Users who wish to do high-precision photometry need to consult the UVOT Aperture Correction analysis thread.
Uvotapercorr characterizes the systematic uncertainty that is introduced into the aperture-corrected count rate by variations in the PSF in this way. The user provides an argument, "fwhmsig", which estimates the fractional rms variation of the FWHM of point sources, expressed as a percentage. The appropriate value for "fwhmsig" is under investigation. Preliminary recommendations are given in the fhelp file for uvotapercorr. Set "fwhmsig" to -1 (minus one) to use the most recent UVOT Team recommendation. Users may wish to set "fwhmsig" to zero and add their own estimate of the uncertainty in the shape of the PSF to the uncertainty in the aperture-corrected count rate. Users should be aware that the shape of the PSF may vary with time and filter, so using a single value "fwhmsig" may not be appropriate for all exposures.
The aperture corrections computed by uvotapercorr are only valid for point sources and this algorithm can not be used to correct magnitudes derived in an aperture with a radius larger than 5 arcseconds.
Uvotsource is the standard software tool for doing
photometry on a single source in a UVOT SKY image. This software has
an option to use the uvotapercorr algorithm to do approximate
aperture corrections. This can be used by setting
apercorr=CURVEOFGROWTH when running the program. Please
see the fhelp for uvotsource for details on doing this.
Uvotmaghist is a tool that calls uvotsource for every image extension in a FITS file. It performs aperture correction in exactly the same way that uvotsource does.
Uvotdetect calls SExtractor to perform source detection and extract counts for each source. SExtractor uses the ISOCOR method to determine the counts for each source. This method does not require aperture correction.
Uvotevtlc performs aperture corrections in the same way that uvotsource does.
Uvot2pha calls uvotsource to do photometry. Aperture corrections are done by default.
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