Draft for review. This page describes the current AIMS handling of SkyMapper Southern Survey DR2 detections.
Source summary
SkyMapper Southern Survey DR2 was released on 27 February 2019. The SkyMapper DR2 documentation describes this release as the beginning of SkyMapper’s exploration of the deep Southern sky, including Main Survey images and an expanded Shallow Survey dataset.
Publisher summary for DR2:
- fields observed between March 2014 and March 2018;
- over 4.7 billion detections;
- over 21,000 square degrees of sky coverage;
- over 500 million unique astrophysical objects;
- approximate magnitude range 8 to 22;
- AB magnitudes;
- reduced images, object-merged catalogs, and per-image/CCD photometry tables.
SkyMapper provides access through the web site, Cone Search, image cutout services, and TAP/ADQL queries.
How AIMS accesses SkyMapper
AIMS uses SkyMapper DR2 image metadata and photometry chunks that have been staged for AIMS processing. The current source path joins:
- image metadata (
image_id, observation date, exposure time, filter); - per-image photometry detections (
image_id, RA, Dec, PSF magnitude, magnitude uncertainty, object id); - an object-id mask derived from a Gaia crossmatch.
AIMS date range and observatory
The AIMS SkyMapper DR2 surface is treated as a frozen historical dataset:
- AIMS partition start: 2014-03-15;
- AIMS partition end: 2018-03-31 inclusive;
- AIMS observatory code:
Q55.
AIMS filtering
AIMS applies an object-id mask before normalizing SkyMapper photometry rows. The mask is built from a Gaia crossmatch:
- object ids start as keepable;
- object ids with a Gaia best-neighbor angular distance < 1.5 arcsec are marked as not keepable;
- only photometry rows with valid object ids that remain keepable are retained.
This filter is intended to reduce the large population of static stars and focus AIMS on detections more likely to be useful for moving-source searches. It is not a definitive solar-system classifier.
AIMS also validates object-id values before using them as array indices. Null, negative, and out-of-range object ids are excluded from the object-id mask lookup.
AIMS sanitizes SkyMapper magnitudes by storing null for invalid or out-of-range values:
- NaN magnitudes;
- magnitudes
< -10; - magnitudes
> 30.
AIMS-derived and normalized values
| AIMS field | SkyMapper source or derivation |
|---|---|
observation_id | Deterministic q55v2:<object_id>:<image_id>:<ra_scaled>:<dec_scaled>. |
exposure_id | image_id. |
dataset_id | Constant skymapper. |
observatory_code | Constant Q55. |
| Observation time | Image date plus half the exposure time. |
filter | Image filter. |
ra_deg, dec_deg | Photometry ra_img, decl_img. |
ra_sigma_deg, dec_sigma_deg | Not supplied by this AIMS source path; stored as null. |
mag, mag_sigma | mag_psf, e_mag_psf, after magnitude sanitization. |
healpixel | Derived by AIMS from RA/Dec at nside=128 NESTED. |
The observation id uses RA/Dec scaled by 1,000,000,000 and rounded to integers. This creates stable identifiers while avoiding raw floating-point string instability.
Known limitations and interpretation notes
- AIMS ingests a filtered subset of SkyMapper DR2 photometry, not the full SkyMapper catalog.
- The Gaia-based object-id mask removes many likely static sources but can also remove or retain rows in ways that depend on Gaia completeness and crossmatch behavior.
- AIMS does not currently populate RA/Dec uncertainty for SkyMapper rows in this source path.
- SkyMapper DR2 photometry has source-publisher caveats around flags, neighbors, bright stars, PSF behavior, and known DR2 issues; AIMS users should consult the source documentation for detailed photometric interpretation.
- AIMS treats this as a frozen DR2 source. Later SkyMapper releases should be documented separately if added.