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Last modified: 27 March 2024

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11: stack astrometry correction with Gaia/WISE

  1. Observation Selection
  2. Pre-Calibrate/Pre-Detect Pipeline
  3. Fine Astrometry Pipeline
  4. Calibrate Pipeline
  5. ComboDet Pipeline
  6. Source Validation Pipeline
  7. MLE Pipeline
  8. Rebundle
  9. MLE Pipeline run 2
  10. Stacker Pipeline
  11. Stack Fine Astrometry
  12. Master Match Pipeline
  13. Source Properties Pipeline
  14. Convex Hull Source Properties Pipeline
  15. Limiting Sensitivity Pipeline

[New] This pipeline stage is new in CSC 2.1.

In CSC 2.1, the astrometric reference frame for the majority of stacks is tied to the Gaia DR3 celestial reference frame (Gaia-CRF3). The astrometry update is based on a translational transformation of the mapping between SKY (X,Y) coordinates and WCS RA and Dec coordinates that was constructed by matching CSC sources to Gaia counterparts, and then performing a weighted least square transformation of the coordinates. For stacks with insufficient CSC/Gaia matches, we match the CSC sources to ALLWISE catalog sources, and then use the Gaia archive-provided best neighbor pairings between Gaia and ALLWISE to perform a two-step transformation. For a given stack, the transformation is applied to all stack detections, and to the per-observation detections of the contribution observations. This reflects in the final coordinates of the master sources. The transformation also applies to the associated full-field and region data products both at the stack and per-observation levels, such as background maps, sensitivity maps, event files, etc. The resulting coordinate shift for each detection is recorded in the deltax and deltay columns in both the stacked observation detections table and per-observation detections table. The man_astrom_flags column is set if the stack to which the detection belongs has seen its astrometry updated. A detailed description of the algorithm used can be found in the Stack Astrometry Correction Specification Document.

Stack astrometry correction and absolute position error

Anchoring the CSC 2.1 astrometry to Gaia-CRF3 has resulted in a significant improvement in the absolute position error in the new version.

[Thumbnail image: A histogram showing the number of sources (Y axis) and separation (X axis). The separation is drawn using a log scale and goes from 0.01 to 1 arcseconts. The histogram has a peak around 0.2 arcseconds.]

[Version: full-size]

[Print media version: A histogram showing the number of sources (Y axis) and separation (X axis). The separation is drawn using a log scale and goes from 0.01 to 1 arcseconts. The histogram has a peak around 0.2 arcseconds.]

The distribution of separations between CSC 2.1 and CSC 2.0 coordinates.

The vast majority of sources (~90%) have a coordinate shift of 0.4 arcseconds or less in the new version of the catalog, with the distribution peaking at 0.2 arcseconds.

[Thumbnail image: The X axis shows separation, drawn using a log scale from 0.01 to 1 arcsecods, and the Y axis is the cumulative fraction. There are two plots: CSC 2.0 in blue and CSC 2.1 in orange. The CSC 2.0 data really only starts at 0.1 arcsecond whereas the CSC 2.1 data already has 30% of values at 0.1 arcseonds, and is always higher than the CSC 2.0 data until the last bin around 1 arcseconds.]

[Version: full-size]

[Print media version: The X axis shows separation, drawn using a log scale from 0.01 to 1 arcsecods, and the Y axis is the cumulative fraction. There are two plots: CSC 2.0 in blue and CSC 2.1 in orange. The CSC 2.0 data really only starts at 0.1 arcsecond whereas the CSC 2.1 data already has 30% of values at 0.1 arcseonds, and is always higher than the CSC 2.0 data until the last bin around 1 arcseconds.]

The cumulative distribution of separations for Westerhout 40, a star forming region with a significant number of CSC/Gaia matches. The stack astrometry correction has resulted in a much larger fraction of CSC sources having small (<0.1 arcseconds) separations with respect to Gaia sources. Specifically, in CSC 2.1 about 40% of the sources in this region have Gaia separations of 0.1 arcseconds or smaller, versus about 15% in CSC 2.0

[Thumbnail image: Both plots show RA (X axis) and Declination (right axis) and contain about 13 sources from CSC (red) and GAIA (blue). Not all sources are expected to match, but for those that do the difference between the two is much larger for the CSC 2.0 sources (left) than the CSC 2.1 sources (right).]

[Version: full-size]

[Print media version: Both plots show RA (X axis) and Declination (right axis) and contain about 13 sources from CSC (red) and GAIA (blue). Not all sources are expected to match, but for those that do the difference between the two is much larger for the CSC 2.0 sources (left) than the CSC 2.1 sources (right).]

The plots show the systematic shift of coordinates with respect to GAIA-CRF3 for a set of sources in Westerhout 40. The left plot shows the CSC 2.0 values and the right the improvement seen in CSC 2.1 because of the the stack astrometry correction.

The stack astrometry correction has reduced the systematic component of source position uncertainties in CSC 2.1. This systematic error term in the position error has gone from 0.7 arcseconds in CSC 2.0 to 0.29 arcseconds in CSC 21, per axis, at the 95% confidence level. The determination of the absolute astrometric error was done using the same procedure as in Rots & Budavari 2011 (ApJS 192, 8), based on the cross-match between CSC and Gaia sources.

Note on Quality Assurance

To ensure that the automatic stack astrometry correction does not introduce incorrect shifts due to low number statistics or confusion in very crowded regions, we perform a manual QA step for those stacks where the density of sources is above a certain threshold, when there are too few matches, or when the resulting automatic shift is larger than 1 arcsecond. In that case we manually select the matches, or opt for the identity transformation shift if that is more appropriate.

  1. Observation Selection
  2. Pre-Calibrate/Pre-Detect Pipeline
  3. Fine Astrometry Pipeline
  4. Calibrate Pipeline
  5. ComboDet Pipeline
  6. Source Validation Pipeline
  7. MLE Pipeline
  8. Rebundle
  9. MLE Pipeline run 2
  10. Stacker Pipeline
  11. Stack Fine Astrometry
  12. Master Match Pipeline
  13. Source Properties Pipeline
  14. Convex Hull Source Properties Pipeline
  15. Limiting Sensitivity Pipeline
/ csc / proc / stackfineast.html