Specification for the Siril HEALpixel Catalog Format

This document specifies the structure of the Siril Catalog Format, designed to support efficient storage and querying of nested HEALpixel based astronomical catalogs such as Gaia DR3. The format includes a fixed length header, an index for rapid look-up, and structured data records optimized for space and speed. The document also outlines recommended search strategies for utilizing the format effectively.