openFits

function openFits(reader, options?): Promise<ReadHdusResult>;

Defined in: hdu/read-hdus.ts:352

Walk every Header-Data Unit through a RandomAccessReader, reading only header blocks: each data unit is located and measured from its keywords then seeked past, never fetched, so a remote file is enumerated without materializing it and a later readImage cutout fetches only its region.

Sizing, classification, random-groups rejection, and the strict/lenient contract are the same code as readHdus (the per-HDU body and the parseHeader END detection are shared, not reimplemented), so for well-formed FITS the enumerated HDUs are identical. On malformed or truncated input both fail safe (dataSizeKnown: false, nothing unsafe read), but the reported dataOffset and warnings can differ: with a missing END, openFits caps the scan at maxHeaderBlocks blocks, while the in-memory readHdus is bounded only by the buffer length. The sync readHdus is unchanged for in-memory inputs.

With reader.size known, dataSizeKnown reflects whether the declared data unit fits within it. With reader.size undefined (a source of unknown length) the header-declared length is trusted, dataSizeKnown is true on that basis alone, and an actually short source is caught by readImage’s own truncation check, not here.

Parameters

Parameter	Type
reader	RandomAccessReader
options	OpenFitsOptions

Returns

Promise<ReadHdusResult>

Example

const reader = new HttpRangeReader(url);
const { hdus } = await openFits(reader);
const tile = await readImage(hdus[0], reader, {
  region: { start: [4096, 4096], shape: [256, 256] },
});

See

FITS Standard v4.0 §3.3-3.4: primary HDU and extensions (random groups: §6)