What a header-only FITS review can tell you

FITS headers use fixed 80-byte cards inside 2,880-byte blocks. AstroSession Sheet reads successive primary-header blocks until the END card, then stops. It does not request or decode the pixel array. These structures are documented in NASA HEASARC's FITS overview.

The supported subset

Use uncompressed files ending in .fit, .fits or .fts with a SIMPLE primary image and 2 to 9 nonempty axes. Conventional uppercase keywords up to eight characters are supported. String values can contain slashes and doubled apostrophes; numeric values can use FITS D exponents. The primary header must end within 32 blocks, or 90 KiB.

Gzip files, tile-compressed FITS, archives, XISF, extension-only images, HIERARCH keyword conventions and long-string continuation cards are unsupported. A small or empty primary HDU followed by an image extension is not treated as a supported light frame. An unsupported file is excluded with a reason, rather than silently summarized.

Map your capture software's keywords

The defaults are DATE-OBS, EXPTIME, FILTER and IMAGETYP. If your capture software stores seconds under EXPOSURE, change the exposure keyword to EXPOSURE and select Apply keywords. The mapping reads values already in the header. It does not translate units or invent defaults. Confirm the keyword's meaning in your capture software before using it.

Per-frame edits override mapped values and remain when you change the mapping. Reset session clears those corrections. The audit stores the original parsed scalar values, keyword mapping and edits separately.

Limits are part of the result

A selection supports at most 1,000 files and 32 MiB of combined header reads. A Web Worker keeps the page responsive. Cancel terminates that worker and discards the in-progress selection. If the combined limit is reached, no partial selection is retained.

The parser checks required primary-image metadata, duplicate scalar keywords, printable ASCII, END padding and whether the declared pixel size fits the file. It does not validate FITS checksums, pixel integrity, extension metadata or complete FITS conformance. Use a dedicated FITS validator for those tasks.

Continue with filter ID decisions and date and exposure review.