IVOA Simple Image Access Extractor

Point at ANY IVOA Simple Image Access (SIA) service and list every astronomical image over a patch of sky — one row per image, with its access URL. Give a sky position (RA, Dec) and a region size, and this actor returns the metadata for every image whose footprint overlaps that region, from any compliant Virtual Observatory image archive.

One actor speaks to the entire Virtual Observatory. There is no per-archive scraper and no host list to maintain: IRSA / IPAC, NASA SkyView, GAVO, HEASARC, ESO, CADC, NOIRLab, STScI and thousands of national and observatory image archives all expose the identical IVOA Simple Image Access query.

What it does

The Simple Image Access protocol is one HTTP GET:

GET <service>?POS=<ra>,<dec>&SIZE=<deg>

and the reply is a VOTable listing every image covering that region. This actor:

• runs that query against one or many SIA services at once;
• parses the VOTable — TABLEDATA, BINARY and BINARY2 serializations (GAVO / DaCHS services default to BINARY2, which many naive parsers silently drop) — into flat rows;
• surfaces the image access URL as a stable _accessUrl field on every row, no matter what the archive calls its own column (IRSA calls it download, SkyView URL, GAVO accref …), by locating it via the VOTable UCD VOX:Image_AccessReference;
• keeps the archive's own native columns (WCS, bounding box, pixel scale, band, exposure, date …) alongside a lossless _raw copy.

This is a metadata runner. It returns one row per image with the URL to fetch it — it does not download the FITS / JPEG pixels. Point your own downloader at the _accessUrl values when you want the actual images.

An in-band QUERY_STATUS=ERROR in the VOTable is surfaced as a real error (never a fabricated empty result). A region that genuinely contains no images returns zero rows.

Input

Field	Required	Description
service	one of service/services	Base URL of a single SIA service. Keep any archive-selecting query parameter (e.g. SkyView's ?SURVEY=DSS) — the actor adds POS/SIZE itself.
services	one of service/services	List of SIA service URLs to query with the same region at once. Each row is tagged with its _service.
ra	yes	Right ascension of the region centre, decimal degrees J2000 (0–360).
dec	yes	Declination of the region centre, decimal degrees J2000 (−90 to +90).
size	yes	Region size in degrees. A single number is a square (0.1 = 6 arcmin square); width,height gives a rectangle.
format	no	SIA FORMAT filter, e.g. image/fits, image/jpeg, GRAPHIC, ALL, METADATA. Default: the service's own.
maxRecords	no	Global cap on rows emitted across all services (each row is one billable event).
timeoutSecs	no	Per-request read timeout (default 120).
userAgent	no	Override the request User-Agent.
bearer	no	Bearer token for an auth-gated deployment (never logged).
extraHeaders	no	Extra HTTP headers as a JSON object (values never logged).

Example

{
  "services": [
    "https://skyview.gsfc.nasa.gov/cgi-bin/vo/sia.pl?SURVEY=DSS",
    "https://irsa.ipac.caltech.edu/cgi-bin/2MASS/IM/nph-im_sia"
  ],
  "ra": "10.6847",
  "dec": "41.269",
  "size": "0.1",
  "maxRecords": 1000
}

Output

One record per image. Each record contains the archive's own columns plus these stable fields:

Field	Description
_accessUrl	The image access URL (the FIELD with UCD VOX:Image_AccessReference).
_format	The image MIME type (UCD VOX:Image_Format), e.g. image/fits.
_service	The SIA service the row came from.
_mode	Always sia.
_ra, _dec, _size	The region you queried.
_rowIndex	Global 0-based index across the run.
_raw	A lossless copy of the archive's native columns for this image.

{
  "_accessUrl": "https://irsa.ipac.caltech.edu/cgi-bin/2MASS/IM/nph-im_sia?...",
  "_format": "image/fits",
  "_service": "https://irsa.ipac.caltech.edu/cgi-bin/2MASS/IM/nph-im_sia",
  "_mode": "sia",
  "_ra": 10.6847,
  "_dec": 41.269,
  "_size": "0.1",
  "_rowIndex": 12,
  "_raw": { "...": "archive's native columns" }
}

Pricing

Pay-per-event: you are charged per image record returned. No subscription, no rental — you pay only for the rows you receive.

Notes

• The size=0 convention (images covering the exact point) and rectangle sizes (width,height) follow the SIA spec.
• Services enforce their own maximum result size and server-side timeouts; a very large region may be truncated by the server (reported as QUERY_STATUS=OVERFLOW).
• Not every service honours the FORMAT filter; leave it empty for the service default.