Reading about photometry

I am going to read through this site to increase my photometry know-how(http://photutils.readthedocs.io/en/stable/) I will take some notes here:

Background Estimation

This attempts to figure out how much non-source signal there is in the frame.

The median method works ok, but images with a few sources have overestimated MAD values.

sigma clipping: takes away pixels that fall above a certain threshold.

masking sources: you can ID the sources and remove them from the image, then everything else is background.

if the background noise varies like if there is a big source right outside the frame and there is a lot of light coming into one side of the frame, then we need a 2d image of the background.

Masking: necessary if the image is rotated and there is blank space in the image (pixels with values of 0 or NA)

Meshes have to do more with the 2d background.

Source Detection:

We need to know what is a star/source in our image and what is just something else.

DAOStarfinder does this in photutils, it uses the DAOfind algorithim to search for dense spots with high maxima above a certain threshold. Input to the algorithim is the desired threshold (in sigma) and the desired FWHM (effectively measuring the spread of the source on the image) of the sources. It will give pixel coordinates of the points at the center of groups of pixels which match the input specifications.

you can plot this data and it will put little circles on your original image showing you where it found sources.

Local Peak detection: findpeaks will tell you where exactly on the image peak values are occuring, it also tells you precisely what these values are. you input what sigma above the background these peaks are and how much they need be separated by.

Grouping Algorithms

DAOPHOT GROUP is based on Stetson’s algorithm for figuring out whether sources have enough critical separation to be considered part of different groups. photutils uses the DAOGroup class to execute this algorithm.

you must define a crit_separation for DAOGroup to look for in terms of a FWHM, then DAOGroup will give you numbers for each group it finds.

Photutils also has a algorithm called DBSCAN that uses another algorithm. it got the same results as DAOGroup in their test.

Aperture Photometry!

This is our stuff! I will get to this later when I have a chance.