Astrometry Resource Needs

Number/name of Requirement or Goal: Ast-R1, Ast-R2, Ast-R3

Ast-R1 is to place 1"-accuracy blind astrometry solutions into image headers by SISPI. This will probably be done during Commissioning.
The approach will be to do low-order polynomial solution of measured stellar positions in DECam exposures against external catalog of stellar positions. Need to do all filters (to confirm lateral color coefficients) and a range of airmasses (to confirm differential refraction corrections). Once these initial solutions are created, any further exposures during Commissioning or SV that have astrometric standards (e.g. Stripe 82 fields, or 2MASS/USNO just about anywhere) can be used to test whether requirement is being met.

Ast-R2 and R3 are to create astrometric solutions that measure relative displacements to <100 mas repeatability (R2) with <50 mas systematic residuals on any position of the array (R3). The approach will be to generate a solution in each filter by forcing internal agreement among a sequence of dithered exposures of a single field. The dither pattern is TBD, but will require dithers on scales from a few arcmin to 1 degree. The field should be selected to have moderate Galactic latitude, so that stellar density is high but crowding is not an issue for astrometry. This observation and solution will probably be done once during Commissioning and then the process repeated during SV to test for stability. The requirements are tested by using exposures taken in typical DES tiling patterns throughout the SV period.

DECam observations

For Ast-R1,

  • Field(s) to observe: A selected Stripe82 field or other field with astrometric standards across full DECam FOV
  • Number, duration of exposures per filter: 2 exposures per filter, ??? s each
  • Dithering / tiling pattern: Dither of ~1/2 CCD
  • Sky conditions needed (seeing, photometric, moon phase, etc:): Anything except thick clouds
  • Other timing constraints: Before other tests that require astrometry in headers. Will require another pair of observations at airmass >1.7 in at least one filter to confirm that differential refraction correction is correct.
For Ast-R2 and R3, observations to create the astrometric solution:
  • Field(s) to observe: A moderate-latitude field with usefully higher stellar density, and with sparse astrometric standard stars available
  • Number, duration of exposures per filter: Chosen dither sequence (TBD; 10-20 exposures???), repeated in each filter
  • Dithering / tiling pattern: Pattern TBD to maximize accuracy of internal astrometry solution.
  • Sky conditions needed (seeing, photometric, moon phase, etc:): Anything except thick clouds
  • Other timing constraints: Probably during commissioning, then again during SV for stability check
Plus during the SV period, additional data for validation:
  • Field(s) to observe: Anywhere in the DES footprint
  • Number, duration of exposures per filter: Typical full-depth DES tiling sequence
  • Dithering / tiling pattern: DES tiling
  • Sky conditions needed (seeing, photometric, moon phase, etc:): Anything except thick clouds
  • Other timing constraints: Can be spread across SV time period

Data processing needs

Which of the following products would you need to conduct the test:

  • Raw images available at some off-mountain site? No image data needed directly
  • Single-exposure object catalogs? Yes
    • with astrometric solution accurate to: 1 arcsec initial solutions as per AST-R1
    • and photometric zeropoint accurate to: N/A
    • Only need bright stars, or also want galaxy magnitudes? Only bright-star pixel centroids needed.

Telescope or other engineering data

Just standard pointing information (HA, declination)

External data sets

Dense astrometric standard star catalog for Ast-R1; sparser astrometric standards suffice for the dithered-sequence field used to build R2,R3 solution.

Codes and scripts for analysing the test data

  • Code needed that can produce few-arcsec accuracy WCS from "cold start," i.e. knowing only rough sky location of exposure. SCAMP should be capable of this.
  • Code for higher-precision refinement of WCS once first code produces enough accuracy for reliable source matching between exposures.
  • Code to measure astrometric residuals from a collection of catalogs and WCS.
  • Code to blindly adjust WCS for lateral color & differential refraction needs to be in SISPI before Commissioning.


Are there real-time decisions to be made during the observations? No

How quickly can / must the data be available and analysed? Catalog 24 hrs after observations should suffice

Who will do it, and where? Solutions can be derived off-site. People: GaryB, Joe Mohr, Andres Plazas, ???

Other important resources or things to note