Point Spread Functions and Computing Fluxes

Each image acquired as part of the OSSOS-CFHT observations has a PSF created for it. This PSF is created automatically and NOT vetted by a visual inspection.

How the PSF was Built

The flowing scripts are followed in building the PSF:

• mkpsf.py -> jmpmakepsf.csh -> kinky.pipeline.jmpprepimage -> kinky.pipeline.jmpmakepsf

Using this PSF requires knowing how the PSF was built. The most straight-forward approach is to use the PSF within the IRAF/daophot package.

Using the PSF

IRAF/daophot PSFs have a few critical parameters.

• PSFRAD - spatial size of the PSF
• FITRAD - the radius of pixels that should be used to match the PSF to a stellar profile
• APERTURE - the size of the flux aperture that the PSF magnitude is scaled to
• ZEROPOINT - the magnitude zero point used for this PSF.

PSFRAD and FITRAD can both be retrieved from the fits header of the PSF itself.

APERTURE and ZEROPOINT are stored in vospace.

APERTURE

The size of the apertures are stored in a file in the same vospace location as the PSF file, just change the extension to .apcor e.g.: vos:OSSOS/dbimages/1607618/ccd25/1607618p25.apcor This file contains a single line (no header) with the values of being: apmin apmax apcor uncertainty. The PSF is built using fluxes that are scaled from apmin values i.e. without the apcor values. Fluxes that IRAF returns will be on the apmin scale and you must subtract the apcor to get a calibrated flux value.

ZEROPOINT

The zeropoint of each image is provided in a similar way to the aperture correction, change to the extension .zeropoint.used, e.g. vos:OSSOS/dbimages/1607618/ccd25/1607618p25.zeropoint.used This file contains a single value which is the zeropoint of the exposure in the filter reference frame of the image (so far this is always r) the zeropoint is in the AB system. This zeropoint already includes the exposure time and airmass corrections. To convert a measured flux into calibrated flux the following equation is used:

mag_AB = -2.5 log10(flux(apmin)) - apcor + zeropoint

where flux(apmin) is either the flux determined from the a PSF fit or from a circular aperture of size apmin.

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Example

get a postage stamp to work on

vcp vos:OSSOS/postage_stamps/13AE/FITS/o3e01/o3e01_56331.58356_214.15121_-13.85378.fits ./

look in the header of the pages stamp to see that this original exposure was 1607618p25

EXPNUM = 1607618 / CFHT odometer number
EXTNAME = 'ccd25 ' / CCD number in the mosaic
EXTVER = 25 / Now matches the CCD number in the mosaic

exposure 1607618 and extension ccd25 (25) thus the calibration parts are stored at:

vos:OSSOS/dbimages/${EXPNUM}/${EXTNAME}/${EXPNUM}p${EXTVER}.{EXT}

vcp vos:OSSOS/dbimages/1607618/ccd25/1607618p25.psf.fits ./ vcat vos:OSSOS/dbimages/1607618/ccd25/1607618p25.apcor vcat vos:OSSOS/dbimages/1607618/ccd25/1607618p25.zeropoint.used

Start IRAF:

cl

In IRAF load the daophot package

noao digiphot daophot

Display the postage stamp an locate the KBO, normally the centre of the image. RA/DEC in image name.

display o3e01_56331.58356_214.15121_-13.85378.fits

Set the zeropoint/apertures based on the values from VOSpace

datapars.itime = 1 photpars.zmag = 32.01 photpars.apertures = 5

phot wants a coo file, but can be garbage position

print "0 0" > junk.coo
phot o3e01_56331.58356_214.15121_-13.85378.fits inter+

put your cursor on the source and press the space bar (tells phot to measure at that location).`

now run all-star to subtract the source (look for residuals and get a PSF flux). 15 dir coo

allstar o3e01_56331.58356_214.15121_-13.85378.fits o3e01_56331.58356_214.15121_-13.85378.fits.mag.1

respond to IRAF's questions

You should now have a .sub image that shows how good the subtraction was and a .als file that gives the source flux on the scale of the small aperture: Don't forget to subtract the apcor value now.