PSF Photometry

The PSF photometry routine requires a source list to be generated with either the detection routine or loaded with -d sourcelist.fits. Additionally, an optional background estimation image can be generated with the background estimation routine or loaded with -b image-bgd.fits, this will be subtracted from the raw image before the photometry is conducted.

Tip

You can automatically load the corresponding source list and background file for a given image by adding the -f or --find flag to the command. This will look for files with the same base filename but search for -ap and -bgd suffixed. For example $~ starbug2 -f image.fits will locate “image-ap.fits” and “image-bgd.fits” if they exist in the working directory.

Note, you can also still load a global sourcelist which will take priority over the automatic one. For example $~ starbug2 -d sourcelist.fits -f image.fits will load “sourcelist.fits” and “image-bgd.fits”. This is useful in the situation where you are running PSF photometry on a large list of images simultaneously and don’t want to manually include the appropriate background file every time but still want to use a carefully constructed sourcelist.

The PSF fitting routine is run with the -P or --psf flags. A simple run may look like either of the commands below. The first detects on the image and estimates the background, then conducts the PSF routine with these product. The second loads an existing source list and background image:

$~ starbug2 -vDBP image.fits

$~ starbug2 -vd sourcelist.fits -b background.fits -P image.fits

The routine will produce a binary fits table with the following columns:

Name	Description
Catalogue_Number	Source index in table
x/y_init	Initial centroid position of each source
x/y_fit	Fitted PSF centre for each source
RA/DEC	WCS measured from x/y_fit
xydev	Distance between x/y_init and x/y_fit
ap_FILTER	Initial guess for photometry
flux/eflux	Fitted flux and photometric error
FILTER/eFILTER	Magnitude and photometric error of each source
qfit	Measure of PSF fit quality (in prep.)
flag	Source quality flags

Relevant Parameters

CRIT_SEP

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FORCE_POS

The PSF routine can be run in two different modes, “Forced” or “Unforced”. In Forced photometry, the central point of the point source is fixed at the input position and the photometry only fits for flux. This is much faster but can result in poorer flux measurements. Unforced photometry allows the central position to be fit as well as the flux, this is much more accurate but there are three times as many free parameters to fit. Set whether to conduct the routine in forced mode FORCE_POS = 0 or 1 (false or true).

MAX_XYDEV

In “Unforced” mode, the central position of the point source can wander far from the initial position, normally in the case of a non point-like object at the centre. Setting MAX_XYDEV will set the threshold maximum deviation from the initial position allowed. Source that move further will be have their fluxes refit at the original position in forced mode.

MAX_XYDEV is in pixel units by default, but can be set in other units by append ‘p’: pixel, ‘s’:arcsecond, ‘m’:arcminute, ‘d’:degree to the number. For example MAX_XYDEV =0.3s for 0.3 arcseconds.

PSF_SIZE

Set the size of the PSF to fit, in pixel units. By default it will use the whole PSF_FILE image, but if you want to reduce the size of the PSF to speed up the process, set it here.

GEN_RESIDUAL

Generate a residual image, this is the raw image with a background and source PSFs subtracted. It will be exported to “filename-res.fits”. This is useful to inspect the quality of the PSF fits and may reveal hidden faint sources which weren’t detected. You may wish to run a second round of photometry on this residual.

PSF_FILE

Load a specific fits image to be used as a PSF.

AP_FILE and BGD_FILE

Load a source list and or a background image.

Instrumental Zero Points

PSF photometry is a very accurate form of photometry but it does not get calculated in physical flux units. After the routine has been run, we can calculate the photometric zero point shift by using the aperture photometry catalogue (here on referred to as the input catalogue). It is thoroughly recommended to prune the input catalogue so that only the “cleanest” point sources over a range of magnitude values remain. Using the command --calc-instr-zp we can measure the systematic photometric offset between the input and PSF catalogues and correct the latter accordingly. The command is run as follows, and returns a copy of the PSF table with corrected magnitude values and appends the suffix “-zp”:

$~ starbug2 -d input_catalogue.fits --calc-instr-zp image-psf.fits

Alternatively, if the offset is already known, either set it with the parameter ZP_MAG or run:

$~ starbug2 -s ZP_MAG=1.0 --apply-zeropoint image-psf.fits