Table of Contents

The header has an unlimited number of parameters in the form
*key=value / comment* that may be associated with and describe the data.
The header consists of ASCII records of 80 characters (with optional line
separators) each; both header and data have a blocksize of a 2880 bytes
(or a small integer (1-10) factor thereof - the blocking factor). All major
astronomical data reduction packages (AIPS, GIPSY, MIRIAD, IRAF, MIDAS
etc.). can import/export their data from/to FITS format in some extent. However,
there are some known FITS ‘‘dialects’’.

The FITS format was originally described
in a classic paper by D.H. Wells, E.W. Greisen and R.H. Harten in *Astronomy
and Astrophysics Supplement Series, Vol. 44 (1981), pp. 363-370*. A number of
addenda have been published since then, which describe various extensions
to the format. There is often information in the manuals of packages like
AIPS and IRAF. An updated version is published by R. Hanish *et al.* in *Astronomy
and Astrophysics, Vol. 376 (2001), pp. 359*. (bibcode: **2001A&A...376..359H**)

NEMO
has two library routines available to process *fits* files: *fits(3NEMO)*
and
*fitsio(3NEMO)*
, the latter more specialized to read and write images and
derived (and compatible with) from the MIRIAD fitsio interface.

SIMPLEFor SIMPLE=T fits file the size of the data section (in bytes) can always be computed from the following formula:T (standard FITS file) BITPIXNumber of bits per datum [choice of 8, 16, 32, -32, -64, (64 experimental)(-32,-64 IEEE floating point, 8 unsigned, 16,32 twos-complement ints) NAXIStotal number of axes (0, 1, 2, ... 999) in cube NAXISnsize of a particular axis (n=1 through NAXIS) in cube PCOUNTnumber of extra parameters associated with cube [0 if not present] GCOUNTnumber of repetitions of the cube+parameter [1 if not present] CRVALnvalue of the coordinate at the reference pixel CRPIXnthe reference pixel (often in range 1..NAXISn) CDELTnincrement per pixel CTYPEncoordinate type (an ascii string) DATE-OBSObserving date: new Y2K format "yyyy-mm-ddThh:mm:ss" TIME-OBSObserving time, "hh:mm:ss", deprecated after new Y2K HISTORYhistory records (no ’=’ needed) COMMENTcomment cards (no ’=’ needed) ENDlast keyword in header

|BITPIX/8| * GCOUNT * (PCOUNT + NAXIS1*NAXIS2* ... *NAXISm)where

date +%Y-%m-%dT%H:%M:%S.%N 2020-12-18T13:15:49.648961250and this should be the new format (with as many sub-second digits as are needed).

Some common coordinate systems:

LINEAR Simple linear coordinate system RA, DEC Equatorial (followed by a projection type) ELON, ELAT Ecliptic (followed by a projection type) GLON, GLAT Galactic (followed by a projection type)

Some common coordinate system projection types:

RP Name ___ __ ___________________________________ AZP 90 Zenithal perspective TAN 90 Gnomic (AZP w/ mu = 0) SIN 90 Orthographic (AZP w/ mu = infty) NCP 90 North celestial pole (special case of SIN) STG 90 Stereographic (AZP w/ mu = 1) ARC 90 Zenithal equidistant ZPN 90 Zenithal polynomial ZEA 90 Zenithal equal-area AIR 90 Airy CYP 0 Cylindrical perspective CAR 0 Cartesian MER 0 Mercator CEA 0 Cylindrical equal area COP 90 Conical perspective COD 90 Conical equidistant COE 90 Conical equal-area COO 90 Conical orthomorphic BON 90 Bonne’s equal area PCO 0 Polyconic SFL 0 Sanson-Flamsteed GLS 0 GLobal Sinusoidal (Similar to SFL) PAR 0 Parabolic AIT 0 Hammer-Aitoff equal area all-sky MOL 0 Molweide CSC 0 Cobe Quadrilateralized Spherical Cube QSC 0 Quadrilateralized Spherical Cube TSC 0 Tangential Spherical Cube

30-may-88writtenPJT 20-mar-90Nemo’s FITS I/O library is being testedPJT 3-jul-94doc updated with PCOUNT/GCOUNTPJT 28-sep-01experimental bitpix 64 addedPJT 18-dec-20y2k date-obs reminderPJT