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Hubble Space Telescope
DrizzlePac: Photometry Demonstration

AstroDrizzle: aperture photometry accuracy

Aperture photometry on the AstroDrizzle science output

Source identification

We create a master catalog of sources using the drizzled output obtained with the original F814W FLT images. We use the IDL code FIND from the Astronomy User's IDL Library to acomplish the task. Any other software (SExtractor, daofind will do). All parameters were set as defaults except for:

hmin = 80.0   (threshold intensity)
  fwhm = 2.0  pixels  (FWHM to be used in the convolve filter)

We then proceed to clean the list from false detections, specially around bright stars and in background galaxies that fall in the field of view. We also delete the sources with cosmic ray impacts and those that fall on top of bad columns.

The final catalog (master.xy) has ~1500 stars. This open cluster lies in a rich Galactic field, so many of these stars may not belong to the cluster itself. A completeness study is beyond the goal of our experiment.

Aperture photometry using DAOPHOT

We calculated the aperture photometry (instrumental magnitudes) with the widely used task phot within PyRAF. We begin by loading the photometry package in PyRAF and resetting the parameters to default values:

--> daophot
  --> unlearn datapars
  --> unlearn fitskypars
  --> unlearn photpars
  --> unlearn centerpars
  --> unlearn phot
  --> epar phot

and we use the following non-default parameters to measure the stars in the master catalog (master.xy):

datapars.gain = "CCDGAIN"
  datapars.readnoise = 8.66
  datapars.epadu = 1.0
  datapars.exposure = "EXPTIME"

The readout noise of one single ACS/WFC is ~5 electrons. For three combined images it is sqrt(3) * 5 = 8.66 electrons. The exposure time in the final drizzled image has been updated to 90.0 seconds in the header keyword EXPTIME.

In order to include most of the light from each source, we use an aperture radius of 8.0 WFC pixels (0.4") and we measure the sky in an annulus with radii 20 and 25 WFC pixels. The zeropoint is set to zero in order to obtain instrumental magnitudes.

fitskypars.annulus = 20.0 
  fitskypars.dannulu = 5.0
  photpars.apertur = 8.0
  photpars.zmag = 0.0

We allow the code to perform a re-centering of the stars using a centroid algorithm, and we execute phot.

centerpars.calgorithm = "centroid"
  phot.image = "f814w_drz_sci.fits"
  phot.coords = "master.xy"
  phot.output = "f814w.mag"

We repeat the same procedure with the F606W drizzled output.

Figure 4 shows two color-magnitude diagrams (CMD) obtained for the stars in our catalog: [F814W, F606W-F814W](left), and [F606W, F606W-F814W] (right).

Figure 4: Colour magnitude diagrams of the stars in the master.xy list. These are instrumental magnitudes measured on the drizzled output images.