Thursday, September 1, 2011

I think it's finally done.

My contrast curve plotter/point source detector finally works as intended!

To detect a point source companion or plot a contrast curve requires the program to first determine the background noise in the image. The program does so by first determining the aperture of the star (radius of one FWHM) and then determining the pixels that make up annuli that are 2 FWHM wide around the star. The radius of each annulus is defined as the midpoint between the inner and outer radii that define the annulus. Each annulus increases in radius by FWHM/2. This overlap of annuli allow us to find point sources that might have been on the edge of an annulus and otherwise missed.

For the point source detector, we further break up each annulus into spheres arranged in a ring in the annulus. Each sphere has a radius of 1 FWHM and the spheres overlap each other by half a FWHM. We determine the background of the spherical regions and then find point sources according to the darkpix, thresh, and adj values I've mentioned in previous posts. By breaking up the annuli into regions, we can more accurately determine the local background noise in each region and see if any region contains a point source.

(To refresh: darkpix is a decimal that determines the percent of darkest pixels used to find the mean noise of the background. thresh is the threshhold, or the multiplier we use with sigma, the standard deviation of the flux. Only pixels with intensity values greater than thresh*sigma+background mean will be labeled as potential point sources. adj determines the number of adjacent pixels greater than thresh*sigma+background mean a potential point source has to have in order for a pixel to be labeled as the position of the point source. )

For the contrast curve graph, we measure the difference in magnitude in each annulus with the peak flux of the central star. We call this difference contrast, or delta M. The graph plots contrast with respect to the radius of each annulus (the midpoint between the inner and outer radii). The magnitude value in each annulus is calculated in this way:

After getting the mean background of the regions in each annulus, we average the values of each region to get the average intensity of the annulus itself. We then convert this to a delta mag number by using the equation:

Contrast = -2.5*log10(flux1/flux2)

Flux 1 is the average flux of the annulus; flux 2 is the peak flux of the star. Finally, we plot the contrast with respect to magnitude on our graph.

If there is no companion point source, the program will plot a contrast curve; otherwise, the program will label the location of the main point source and its companion(s). The user can opt to save either of these figures by pressing "w". A text file with some important information will be saved along with the figure.
This serves as a general description of what my program does. There are some specific details that I also need to mention:

- The darkpix value in my program, when used to detect extra point sources, has been set as 0.8 to increase robustness of the point source detector. The darkpix value is generally set to 0.95 when determining which pixels constitute the "background" of the FITS file.
- The ratio of arcseconds to pixels changes for each fits file. However, due to the variety of FITS files I am working with (some of them without headers as well), I cannot create subclasses for each FITS file (as I originally intended) and automatically make my program look for the arcseconds to pixels ratio. Thus the user needs to manually type it in into the module. Hopefully this process will become more user-friendly as I clean up the program a bit.
-When the user enters multiple fits files into the command line when working with IPhot, the user needs to make sure that all files are working correctly. If not, the program will exit as it reaches that file. I have decided as of now to not let my program simply "pass" over these files as it would let a silent error through.
-I will post more as I find more errors.

1 comment:

  1. Excellent write-up and nice work with the code! Looks like we're nearly ready to start using your powerful toolset for science. Exciting!