Monday, September 29, 2008

DES weak lensing pipeline

I'm working on the lensing pipeline for estimating the shear from galaxy shapes in DES. We've already got a piece in place for measuring shapelet expansions of PSF stars. Now we need to finish a few other things. It breaks down like this currently:

  • Mike has a code working for measuring the pre-PSF shear from a stack of images. The inputs are the positions of objects in the images and shapelets representations of the PSF in each image.

  • I've got the following data-management related pieces in place

    • Can automatically locate all coadd and single epoch (SE) images on the local machine given only a tilename. This involves interaction with the database and all the pieces of metadata must be in place, so this was actually a non-trivial test of the system.
    • Can do the forward and reverse coordinate transformations image <-> sky based on the WCS info in the header including the distortion model. This is key considering our plan of action: for the shear measurement we will take the list of coadd objects and go back to the original SE images that were used in the coadd creation. So we need to take the position and bounding box from the coadd, convert to sky coordinates, and then convert back to image coordinates in the individual images. Due to the polynomial distortion model of scamp I implemented a simple root solver for this. Slow but accurate.

We still need to implement the following:

  • PSF interpolation scheme. An optimal way to interpolate the PSF info. This will be a Jarvis&Jain style scheme that will improve over time.
  • Figure out how to extract the proper PSF information and package it up in a way that is usable.
  • Connect my data management tools, WCS transformation tools, and Mike's shear code into something that can automatically process a DES coadd tile, extract shear info, and get that into the database. At first this will do something kind of fake for the PSF, like taking that of the closest star but eventually will use the full PSF interpolation scheme.


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