Differantial or PSF Photometry
Differential photometry has a little more
complicated procedure than the aperture photometry. In the list below
you will see what will we do to obtain the photometry.
1- Find all the stars on the
frame
above a user set brightness threshold. As like as we had done for the
aperture photometry.
2- Run aperture photometry upon
all the stars.
3- Build a semi-empirical model psf.
Each of the psf stars is scaled according to its aperture magnitude
estimate, the weighted average is then fitted with a suitable analytic
function, and a look-up table computed of the residuals.
4- Using the centroid of a star as the
profile centre, and the sky level as determined for aperture
photometry, the PSF model is shifted and scaled to fit the observed
stellar image by non-linear least-squares. The scaling yields the
magnitude estimate.
Before starting any process we will
have to setup some parameters about our data, CCD, and fitting, which
will be later used by the tasks. In order to do this we will use the
following commands.
da>epar datapars
PACKAGE = daophot
TASK = datapars
(scale
=
1.) Image scale in units per pixel
(fwhmpsf=
2.5) FWHM of the PSF in scale units
(emissio=
yes) Features are positive ?
(sigma
=
20.) Standard deviation of background in counts
(datamin=
3200.) Minimum good data value
(datamax=
55000.) Maximum good data value
(noise
=
poisson) Noise model
(ccdread=
) CCD readout noise image header keyword
(gain
=
) CCD gain image header keyword
(readnoi=
3.) CCD readout noise in electrons
(epadu
=
1.5) Gain in electrons per count
(exposur=
EXPTIME) Exposure time image header keyword
(airmass=
AIRMASS) Airmass image header keyword
(filter
=
FILTER) Filter image header keyword
(obstime=
JD) Time of observation image header keyword
(itime
=
1.) Exposure time
(xairmas=
INDEF) Airmass
(ifilter=
INDEF) Filter
(otime
=
INDEF) Time of observation
(mode
=
ql)
I
guess these parameters are suitable for a typical TUG data. But
ofcourse FWHM of the PSF, datamin and datamax parameters can be changed
from image to image.
da>epar centerpars
PACKAGE = daophot
TASK = centerpars
(calgori=
centroid) Centering algorithm
(cbox
=
5.) Centering box width in scale units
(cthresh=
4.) Centering threshold in sigma above background
(minsnra=
1.) Minimum signal-to-noise ratio for centering algorit
(cmaxite=
10) Maximum iterations for centering algorithm
(maxshif=
1.) Maximum center shift in scale units
(clean
=
no) Symmetry clean before centering
(rclean
=
1.) Cleaning radius in scale units
(rclip
=
2.) Clipping radius in scale units
(kclean
=
3.) K-sigma rejection criterion in skysigma
(mkcente=
no) Mark the computed center
(mode
=
ql)
These parameters define the selection
criteria of a source as a star.
da>epar fitskypars
PACKAGE = daophot
TASK = fitskypars
(salgori=
mode) Sky fitting algorithm
(annulus=
15.) Inner radius of sky annulus in scale units
(dannulu=
10.) Width of sky annulus in scale units
(skyvalu=
0.) User sky value
(smaxite=
10) Maximum number of sky fitting iterations
(sloclip=
0.) Lower clipping factor in percent
(shiclip=
0.) Upper clipping factor in percent
(snrejec=
50) Maximum number of sky fitting rejection iterations
(sloreje=
3.) Lower K-sigma rejection limit in sky sigma
(shireje=
3.) Upper K-sigma rejection limit in sky sigma
(khist
=
3.) Half width of histogram in sky sigma
(binsize=
0.1) Binsize of histogram in sky sigma
(smooth
=
no) Boxcar smooth the histogram
(rgrow
=
0.) Region growing radius in scale units
(mksky
=
no) Mark sky annuli on the display
(mode
=
ql)
And these are for the selection of the
sky annulus.
da>epar photpars
PACKAGE = daophot
TASK = photpars
(weighti=
constant) Photometric weighting scheme
(apertur=
5) List of aperture radii in scale units
(zmag
=
25.) Zero point of magnitude scale
(mkapert=
yes) Draw apertures on the display
(mode
=
ql)
And these are for photometry.
After these parameters you are ready to
make the aperture photometry using the phot task. Since we have just
set the needed parameters we won't set any more parameters. The only
thing we need to give is now the input image list and the coordinate
file.
da> phot @cr0001_1_al.fit cr0001_1_al.fit.coo.1
After this step we will select our PSF
stars to create a PSF model. To do this we will define the parameters
of daopars.
da> epar daopars
PACKAGE = daophot
TASK = daopars
(functio=
auto) Form of analytic component of psf model
(varorde=
0) Order of empirical component of psf model
(nclean
=
0) Number of cleaning iterations for computing psf mod
(saturat=
no) Use wings of saturated stars in psf model computati
(matchra=
3.) Object matching radius in scale units
(psfrad
=
11.) Radius of psf model in scale units
(fitrad
=
4.) Fitting radius in scale units
(recente=
yes) Recenter stars during fit ?
(fitsky
=
no) Recompute group sky value during fit ?
(groupsk=
yes) Use group rather than individual sky values ?
(sannulu=
15.) Inner radius of sky fitting annulus in scale units
(wsannul=
11.) Width of sky fitting annulus in scale units
(flaterr=
0.75) Flat field error in percent
(proferr=
5.) Profile error in percent
(maxiter=
50) Maximum number of fitting iterations
(clipexp=
6) Bad data clipping exponent
(clipran=
2.5) Bad data clipping range in sigma
(mergera=
INDEF) Critical object merging radius in scale units
(critsnr=
1.) Critical S/N ratio for group membership
(maxnsta=
10000) Maximum number of stars to fit
(maxgrou=
60) Maximum number of stars to fit per group
(mode
=
ql)
The important ones here are, the
fitting function, matchra, psfrad, fitrad and the dimensions of the
Annulus for sky and the source. And one more step before selecting the
PSF stars is to set the parameters of the pstselect task.
da>epar pstselect
PACKAGE = daophot
TASK = pstselect
image = cr0001_1_al.fit
Image for which to build psf star list
photfile=
default Photometry file (default: image.mag.?)
pstfile
=
default Output psf star list file (default: image.pst.?)
maxnpsf
=
10 Maximum number of psf stars
(mkstars=
no) Mark deleted and accepted psf stars
(plotfil=
) Output plot metacode file
(datapar=
) Data dependent parameters
(daopars=
) Psf fitting parameters
(interac=
yes) Select psf stars interactively ?
(plottyp=
mesh) Default plot type (mesh|contour|radial)
(icomman=
) Image cursor: [x y wcs] key [cmd]
(gcomman=
) Graphics cursor: [x y wcs] key [cmd]
(wcsin
=
)_.wcsin) The input coordinate system (logical,tv,physical,wo
(wcsout
=
)_.wcsout) The output coordinate system (logical,tv,physical)
(cache
=
)_.cache) Cache the input image pixels in memory ?
(verify
=
)_.verify) Verify critical pstselect parameters?
(update
=
)_.update) Update critical pstselect parameters?
(verbose=
)_.verbose) Print pstselect messages?
(graphic=
)_.graphics) Graphics device
(display=
)_.display) Image display device
(mode
=
ql)
The important parameters here are
maxnpsf, and whether you want to select them interactively or not. I
guess selecting these stars interactively is much more convincing.
After running the pstselect tool you
will be able to point a star in the DS9 window, and then the surface
plot of it will be drawed in the irafterm graphic window. You will
select a star from the image with the a command and if you press the a
button again in the IRAFTERM window after seeing its surface plot that
star will be added to the PSF Star list.
Now we are ready to create the model
PSF for all images we have. but to do so we have to create some list
files.
da>ls *fit > im.lis
As our image list file.
da> ls *.mag.1 > mag.lis
As our aperture magnitude list file.
da> ls *.pst.1 > pst.lis
As our psf star file list.
da> psf @im.lis @mag.lis @pst.lis interact-
To examine the result :
da> seepsf cr0001_1_al.fit.psf.1 cr0001_1_al.fit.psf.1s
da> disp cr0001_1_al.fit.psf.1s
1
da> imexam
And now we can check the generated psf
model, we should be careful about how the profile looks. It should look
like a single star without contamination from neighbours. And the last
list file can be created as follows :
da> ls *psf.1 > psf.lis
da> allstar @im.lis @mag.lis @psf.lis
This produce the *.als.1 files
which contain differantial photometry values of the stars in the field.
And thats all we have done the differantial photometry using DAOPHOT.