NGVS point spread functions
This page describes the NGVS point spread function (PSF) service, how
to access the service and how the PSFs were generated.
The PSFs are only available for the MegaPipe images.
Two sets of PSFs were generated, one set with
the other with
The PSF service is intended to deliver PSF postage stamps relevant to
a single location on a particular NGVS image. So far the primary usage
has been to model galaxy surface brightness profiles in batch
mode. The URLs have the format:
| How to access the PSF service|
For example, to get the PSF for the g-band, long exposure time, MegaPipe local background subtracted
image for the field NGVS-1-1, at position x=12000, y=8000 the URL would be:
Remember that most browsers convert the plus character (+) into a space ( ).
Encode the plus character as "%2B". For example, for the NGVS+1+1 field:
Typical usage is not via the browser, but via wget or curl. For example:
wget -O NGVS-1-1.l.g.Ml128.psf.12000.8000.fits 'https://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/cadcbin/community/ngvs/NGVSpsf.pl?image=NGVS-1-1.l.g.Ml128.fits&x=12000&y=8000'
By default, the PSF service delivers a PSFs generated with DAOphot. To obtain PSFs generated
with psfex, add "&psfex=true" to the URLs above. For example:
The PSFs are delivered as 31x31 pixel FITS images with the PSF centred on pixel (16,16).
Alternatively you can download the .psf file produced by DAOphot directly from URLs like:
or you can download the .psf file produced by PSFex with URLs like:
| How the PSFs were generated|
The PSFs are generated with the standalone version of DAOphot. SExtractor and DAOphot are used
to select PSF stars. SExtractor is run on the image. Stars are identified by comparing the 2-arcsecond aperture magnitude to the 3-arcsecond aperture magnitude.
Stars form a well defined locus in this parameter. Stars near other bright objects are discarded. Here, near means within 40 pixels and
bright means brighter than 4 magnitudes fainter than the star in question. A second list of PSF stars is compiled using the routines
find and pick. Only stars in both the SExtractor and DAOphot lists are used.
The DAOphot routine psf is then used to generate the PSF.
The gain and readout noise are set using the effective gain and readout noise as described
here. The fitting radius is set to 1.5
times the average FWHM of the image. The PSF radius is set to 15. A
bivariate Gaussian model with a correction look-up table is used. The
PSF is allowed to vary across the field with second order variations.
Two passes are made. After the first pass, the images are
searched for second sources lying in the wings of the PSF stars. These
PSF stars are removed from the list and the PSF is recomputed in the
Diagnostic images comprised entirely of postage stamps of the PSF star
are generated an examined by eye. The images show 3 panels: the
original PSF stars and the residuals after the first and second passes.
For each NGVS image, two diagnostic images are generated. One shows
the PSF star postage stamps arranged by magnitude, brightest to
faintest, the other shows the PSF stars arranged by approximate
location on the original NGVS image. The diagnostic images are
visually examined for systematic variations of the PSF residuals as
function of magnitude and position.
The residuals show no systematics, and the DAOphot PSFs described above
have been successfully used to determine galaxy morphologies.
Compared to DAOphot, PSFex has far fewer parameters to be adjusted. SExtractor version
2.5.0 was run on the images to extract vignettes around each
source. PSFex version 3.9.1 was run on the resulting LDAC catalog.
The PSF varies with x and y as a third order polynomial.
The PSF is not over-sampled.
The PSFex configuration file is available
FWHM and ellipticity diagnostic plots and the residual image were examined for
In general, it would appear that the PSFex residuals are smaller
than the DAOphot residuals. DAOphot is of course designed to
work on individual images where the noise characteristics
are well non a priori. Because of the way the NGVS images have been
scaled and stacked, DAOphot might not be expected to perform
Users who require the highest level of PSF
precision are cautioned not to use these PSFs or indeed any
PSF generated from stacked images. High precision PSF work
should only be done with the original, unstacked images.