Summary of V3 Modificationts to GALWORKS

• See the original goals here:Galworks V3 Mods)
  
• Details given here for the Abell 3558 repeatibility tests

Summer of 2000 through the Spring of 2001

• Compiled large data set for RTB testing, including repeat scans of Abell 3558, Orion & Chameleon, and scans of large galaxies (e.g., M31, M33, M51, M66, M101, Maffei 2, Sextet group).

• Carried out baseline or "nominal" (Galworks V2) tests for comparison with modified processsor tests.

• Carried out 15 significant modifications (V3) and performed RTB tests to check progress.
  
  1. Inject Calmon Zero points (note: these zero points come from the V2 pipeline, and hence are subject to change during the cat generation process; the changes, however, are likely to be insignificantly small)

  2. Improved Large Galaxy Processing. Modification allowed processing of all large galaxies, including M31. More modifications/details given below. The down side to this improvement" are the inevitable addition of bogus sources induced by these bright extended objects. This chaff will need to be carefully removed during the cat generation process.

  3. Coverage masks and off-scan JCAT improvement. Note: although the coverage maps (image cubes) compress efficiently, their cumulative total is significant and the additional need for disk storage should not be ignored.

  4. Coversion to SNR thresholds.

  5. Remove large galaxy bias. That is, we process large galaxies without any bias as to its nature or even apriori existence (i.e., process blind).

  6. Improve background determination for scans with large galaxies.

  7. Enlarge the GALWORKS working "area" for large galaxies. That is, process galaxies with larger working areas, thus capturing more of their flux. The postage stamps are now allowed to grow as large as 5' (and hence, more disk storage is needed).

  8. Improved Star Subtraction. Modifications included improvements to both stellar "subtraction" and 2-D ellipse fitting. The basic algorithmic improvements are described here. The primary driver is to improve photometry for galaxies seen through the Galactic plane (and hence confused by foreground stars). Highlights of the new method:

  For the first "guess" at the 2-D elliptical shape of the galaxy, use the rotating 2nd moment algorithm (described in the SIS /2massc/doc/design/sis/see03b).

  Before star subtraction, use the first guess shape of the galaxy and subtract this from the image (i.e, remove the galaxy). The galaxy removal is carried out by using the firs-guess axis ratio and position angle. For each elliptical annulus, compute the lower quartile of the pixel distribution. The lower quartile is robust to star contamination. Then subtract the LQ value from each annulus pixel. The G-subtraction is carried out until the point at which the LQ is deep in the noise.

  With the galaxy removed, examine the neighboring stars. Subtract the star using the PSF "shape" ("sh") to determine its size or extent and the local background median. Subtract by using the local annulus median for each star: (a) determine size of star, (b) determine annulus size, (c) compute median, (4) using median, determine how much of the star should be removed from the non-galaxy subtracted image. In this way the star is removed without losing any of the galaxy. The danger is removing stars close to the galaxy nucleus. Here the galaxy removal step is not as accurate (due to our sampling and pixelization) and the local annulus median is not well determined. Hence, for sources near the nucleus, do not subtract the star, instead mask it. The masked pixels are later recovered with isophotal substitution.

  With stars removed, determine the true isophotal shape (2-D ellipse) of the galaxy.

  This process is iterated twice to robustly determine the ellipse shape.

  The modfication includes adding three new "planes" to the postage stamp images.
  Current planes: 1 = J, 2 = H, 3 = K.
  New Planes: 4,5 6 = J, H, K with stars subtracted or removed

  
  9. Re-built OBDT using updated training sets.

  10. Injected the OBDT classifier into GALWORKS V3. GALWORKS now computes the E and G-scores on the fly. It rejects sources whose E and G scores exceed a value of 1.75 (i.e., rather generous, erring on the side of completeness, but still eliminating unwanted stars).

  11. Added Huchra star-galaxy classifier to the OBDT; see Getting Rid of Stars

  12. Constructed a better surface brightness metric (half-light measures via the total mag extrapolation)

  13. Improved the Kron and Total photometry; see the memo 2MASS Photometric Repeatibility

  14. Improved star subtraction for edge-on galaxies. The improvement is to minimize subtraction of "sources" on the major-axis of inclined galaxies (i.e., avoid subtracting pieces of the galaxy from itself -- this is a major headache for V2).

  15. Improved the postage stamp FITS cube images. They are now *six* planes deep, with the first three planes the nominal set (J, H, Ks), and the additional three planes the same except with stars subtracted. These allow the user to perform direct photometric measurements on the postage stamps without the deleterious stellar contamination. The FITS header has also been update (and improved). Here is an example of a new postage stamp FITS header.

Useful Tests

• Run Jarrett tool "qa_general" on the processed data to see what new sources pop up. Check the photometric apertures and star subtraction.
  
• Compare galworks output files (e.g., g*.gals, bright*stars, bright*gals) to Jarrett RTB processing runs. Do a direct comparison between the V2 mags and the V3 mags. This test may uncover non-uniformities.
  
• Do repeatibility statistics.
  
• Identify artifacts (particularly edge flares) and trace their origin (note: Cong Xu has been working on this task)
  
• Measure C&R, photometry and positions -- compare with level-1 requirements (again, we need repeat scans to do this).
  
• Study output from large galaxy scans (i.e., compare good detections to garbage and flak)
  
• Runtime study (particularly for high density fields)
  
• Disk storage usage (particularly postage stamps). Compute average disk usage per scan per density range.
  
• General integrity: look at "outlier" sources, seek out potential problems (i.e, code bugs)