WESIX -- 20008 Summer School

Overview

The purpose of this talk is to show how to use both the Browser interface and web service interface to the WESIX tool. We will use WESIX and VOPlot to obtain an approximate photometric calibration for an image.

Image Calibration in 3 Easy Steps

Step 1: Obtain image catalog and cross-match

For this exercise, we will use an R-band image from the Fundamental Plane survey (P.I. Mike Hudson) of cluster 2006 from the C4 catalog (Miller et al. 2004). In order to calibrate the photometry from the image, we must first decide which filter to calibrate to. Below is an image of the SDSS filters with the Cousins R filter.

As seen above, the R-band overlaps with both the r' and i' filters. Since the peak in the R-band filter curve corresponds more closely with the peak of the r' response curve, we choose to calibrate against stars observed in the r' filter.

In order to extract the sources and cross match with SDSS, we will use WESIX. You may point your browser to:
http://nvogre.phyast.pitt.edu:8080/wesix

If you do not wish to use WESIX, you may download the cross matched catalog here.

In the URL text box, enter the url of the fits file: http://burt.astro.washington.edu/simon/nvoss2008/simonscience/2006.fits. From the cross match catalog box, choose SDSSDR4. Make sure you have chosen to use the advanced page before clicking next. On the next page, you can choose the fields to return from SExtractor and SDSS. Make sure, at the minimum, you choose:

SExtractor:
MAG_ISO
FWHM_IMAGE

SDSSDR3:
psfmag_r
type

SExtractor params:
Detection threshold = 15
Check the box so that no convolution filter is used

Now click "Go do it." Once the server returns, you may download the cross matched catalog, or simply click the plotter option to do the analysis in your browser.

Step 2: Get the calibration parameters

Load your cross matched catalog into VOPlot. We will need to plot the SDSS magnitudes with the SExtractor magnitudes. To insure that we only are looking at stars we will set a filter. Under "Functions" choose "Create Filters." I call my filter star. We want to make sure that the sdssdr4_type value is 6. In my VOTable, sdssdr4_type is in column 4. Thus, my filter statement will read:
$4 == 6.

Now, we will find the offset value. Add a new column by choosing "Create New Columns" from under the "Functions" tab. Add a new column which is the difference between the SDSS r magnitude and the SExtractor iso magnitude. For my table, I create a column using the expression "$3 - $6."

Now plot the new column on the Y axis and the SDSS r magnitude on the X axis. Make sure that you have used the filter to select only stars. We can now read off the approximate offset value. For me, this is about 28.8.

We now need to read off the seeing. Plot the fwhm output by SExtractor as a histogram. The peak of this distribution is a good approximation of the seeing. For my file, it is about 4.0 pixels. You may want to adjust the number of bins by editing the plot properties under "View."

Step 3: Use SExtractor to Extract the Catalog

We will use the WESIX web service client program to execute SExtractor with the appropriate input parameters.

$> source $NVOSS_HOME/bin/setup.csh
$> cd $NVOSS_HOME/python/src/wesixclient/

$> sh wsdl2pyWESIX.sh
Listing 1

In order for SExtractor to return the data we want, we need to set some of the input and output parameters. The listing below shows the code as it should appear before editing and then again after. The changes are in bold.

wesix_test.py  Before:
63:  we = WesixExample()
.
.
.
69:  sp.MAG_ZEROPOINT = 28.2
70:  sp.DETECT_THRESH = 30

wesix_test.py  After:
63:  we.WesixExample(u = "http://burt.astro.washington.edu/simon/nvoss2008/simonscience/2006.fits")
.
.
.

69:  sp.MAG_ZEROPOINT = 28.8		     
70:  sp.DETECT_THRESH = 1.0		     
Add the following lines and comments to wesix_test.py:

  sp.FILTER = "Y"    
  sp.FILTER_NAME = "gauss_4.0_7x7.conv"    
  sop.FLUX_MAX = True
  sop.THRESHOLD = True
  #vodata = we.callFILE(sp, sop)
  #we.printVOTable(vodata)
  #we.printVOTableTXT(vodata, "out2.dat")
Listing 2

Now we can run the file through SExtractor with the correct convolution filter and magnitude offset. Make sure to use the output from out1.dat.

$> python wesix_test.py
Listing 3

Step 4: Plot Your Results in VOPlot

When you ran the client program, you should have created an ascii catalog file. You can now plot this with your favorite program. I converted this into a VOTable which is linked here. I will be using VOPlot to plot the catalog.

We are interested in the detection threshold at a particular confidence. I wish to find the threshold at the 5σ level. Since we have returned the THRESHOLD and FLUX_MAX values to be returned, we can create a filter which mandates that the maximum flux for the object is at least five times greater than the threshold value. The following figure shows the window in which I have defined my filter.

Now that we have a filter all we have to do to find the magnitude threshold of the image is to histogram the magnitude values from the catalog. I use the 'MAG_BEST' values and hit the histogram button after making sure that the 5sig filter is selected. I find that the tail of the histogram is about 22.5. Thus, I have discovered that the detection threshold on this particular image is about 22.5 in SDSS r.

The NVO Summer School is made possible through the support of the National Science Foundation and the National Aeronautics and Space Administration.