AstroDrizzle: Aperture Photometry Accuracy
We begin by performing an image alignment. This task is crucial to obtain accurate photometry. We create a text file with the names of the FLT images, one for each filter. We call them list_f606w and list_f814w. In what follows we explain the registration method with filter F814W only. We work within the Pyraf environment:
The task AstroDrizzle (AD) (the main task within the software package DrizzlePac)
differs from MultiDrizzle in the
handling of the distortion model. AD handles different components as separate inputs:
polynomial coefficients, column corrections for the CCDs, and non-polynomial offsets.
The new task
updates the headers with new keywords and also
appends several arrays as new extensions that contain the full distortion model.
--> import drizzlepac --> unlearn updatenpol --> epar updatenpol
This will open a new window called TEASL (Task Editor And Launcher) which is a user interface.
This GUI allows the user to change parameters:
For the task
updatenpol we write:
input = @list_f814w refdir = /your/path/to/reference/files/
This should run without any problems and the images are now ready for registration.
We now use the new task
which computes residual offsets for a group of flat field-calibrated images.
These offsets are incorporated into the images by updating their WCS information to match that of a reference image.
--> unlearn tweakreg --> epar tweakreg
tweakreg TEAL interface, we modify the following parameters:
input = @list_f814w conv_width = 3.5 threshold = 200.
The value conv_with = 3.5 is approximately twice the FWHM measured directly on several bright unsaturated stars
using the IRAF task
The value threshold = 200.0 is much higher than the default, simply to choose the brightest objects for catalog matching.
tweakreg finds sources in each image and outputs catalogs.
This should take a few seconds
if you had set a high threshold. The code then matches the catalogs to a reference file (usually the first
in the list: in our case j8ny02yuq_flt.fits)
When the matching is finished between the first two files, a window will pop up and show the residuals
for the matched stars, and
tweakreg will also output the residual shifts
in X (XSH) and Y (YSH), the rotation angle (ROT) and the scale (SCALE).
Also, the RMS values for the fitting are presented. One would expect to obtain an RMS lower than 0.05
in both X and Y for good alignment.
This process might take several iterations and it is recommended to spend some time with this until
a good registration is achieved.
tweakregprompts the user to accept or reject the solution. If you are not happy with the solution, you may want to select different threshold values or even input your own list of sources for the catalog matching. In our case, using the above non-default parameters values we get very good RMS values and we accept the solution for both images. Figure 1 shows the residuals for image j8ny02z9q and Table 2 presents the catalog matching solutions for all F814W images.
Figure 1: Output plot from the task
showing the residuals in X and Y. These residuals are calculated
by matching source catalogs between the reference image (j8ny02yuq) and the
second image in our list (j8ny02z9q).
Table 2: Output values of the catalog matching process for the F814W images. Estimated residual shifts in X (XSH) and Y (YSH), the rotation angle (ROT) and the scale (SCALE) are shown. The rotation angle is given in degrees; the residual shifts (XSH, YSH) are in units of pixels.
Once we are satisfied with the results, we re-run
tweakreg with the
input = @list_f814w updatehdr = yes conv_eidth = 3.5 threshold = 200.
This will update the headers' WCS in all images to have exactly the same orientation and the images are registered and ready for AstroDrizzle. Expect to run AstroDrizzle several times on your data, so it would be important to save copies of these files in a separate folder.