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Hubble Space Telescope
DrizzlePac: Photometry Demonstration

AstroDrizzle: Aperture Photometry Accuracy

Aperture photometry on the individual FLT images

In order to ascertain the quality of the photometry obtained using the AD output, we compare it with aperture photometry obtained with the input FLT images which are pipeline output that include corrections for bias and flat field.

Source identification

We use the same catalog (master.xy) as in the previous section. The star's coordinates are given in the drizzled reference system. We calculate the coordinates in the FLT reference system with the task pixtopix

--> unlearn pixtopix
--> epar pixtopix

For example, to obtain the coordinates in the WFC2 chip we use the following parameters:

inimage = "j8ny02yuq_flt.fits[1]" 
outimge = "f814w_drz_sci.fits"
direction = backward
coords = "master.xy"

This method allows us to create two stellar catalogs (one for each chip) that we name: master_wfc1.xy and master_wfc2.xy.

Aperture photometry using DAOPHOT

We calculate the aperture photometry (instrumental magnitudes) with the phot task for consistency.

According to the ACS Instrument Handbook, if one wants to perform photometry directly on the distorted FLT files, it is necessary to perform a field-dependent correction which is achieved by multiplying the measured flux on the FLT image by the pixel area at the corresponding position using a pixel area map (PAM).

Once each input FLT has been corrected with the PAM, we are able to proceed with the aperture photometry.

As with the AD images before we set:
datapars.gain = "CCDGAIN"
datapars.readnoise = 5.0
datapars.epadu = 1.0
datapars.exposure = "EXPTIME"

The readout noise of one single ACS/WFC is 5 electrons. The exposure time of each FLT image is 30.0 seconds. Same header keyword (EXPTIME) as with the AD images.

The pixel scale in the FLT images is 0.05". As with the AD images, we use an aperture radius of 8.0 WFC pixels and we measure the sky in an annulus with radii 20 and 25 WFC pixels. Also, 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 as in the following example:

centerpars.calgorithm = "centroid"
phot.image = "j8ny02yuq_flt.fits[1]"
phot.coords = "master_wfc2.xy"
phot.output = "j8ny02yuq_flt.mag"

We repeat the same procedure for the six FLT images (three F814W images and three F606W images), both chips in each case.

Figure 5 shows two color-magnitude diagrams (CMD) obtained for the stars in our catalog: [F814W, F606W-F814W] (left), and [F606W, F606W-F814W] (right).

Figure 5: Colour magnitude diagrams of the stars in the master_wfc1.xy and master_wfc2.xy lists. These are instrumental magnitudes measured on the original FLT images after PAM correction.