ING Banner
Home > Astronomy > William Herschel Telescope > ING FITS Headers

ING FITS Headers

Data obtained with ING common-user instruments are accompanied by FITS headers(refs 1,2,5) summarising the state of the telescope, instrument and detector, e.g.:

    AZSTART =           298.644448 / Azimuth at start of observation (degrees)
    CAT-DEC = '+39:55:13.00'       / Target Declination
    CAT-EPOC=              2000.00 / Target epoch of proper motions
    CAT-EQUI= 'J2000.00'           / Equinox of target coordinates
    CAT-NAME= 'HDS930            ' / Target input-catalogue name

The pages linked below show an example FITS header from an ACAM arc exposure taken 19 December 2010, with the entries sorted in different ways:

The rather unhelpful order of the headers results from them being written out in a series of pre-defined packets (called OBSVATON, TELESCOP, AUTOGUID etc.). The format of each header follows the usual FITS standards(ref 3).

The names of headers relating to a given instrument usually begin with the name of the instrument (or an abbreviation) e.g. ACAM or ISI (for ISIS). Headers whose names begin 'CAGA' refer to components within the WHT Cassegrain acquisition and guiding box. The names of most other headers are standard FITS keywords ('DATE-OBS') or are based on those in use at AAO (e.g. 'RUN') and UKIRT, and on the names expected by iraf data-reduction programs (e.g. 'AIRMASS', 'DARKTIME', 'DISPAXIS', 'EPOCH', 'EXPTIME', 'IMAGETYP', 'IRAFNAME', 'OBSERVAT').

To search for a header containing a given string, grep the output from imhead. E.g. to search for headers recording the CCD window in use when obtaining image, issue the following command at the iraf prompt:

ecl> imhead r1234567[1] l+ | grep WIN

Below are listed current WHT headers which are common to data from most instruments (the INT ones are similar).

Telescope headers
CAT-RA, CAT-DEC, CAT-EQUI (= EQUINOX), CAT-EPOC, PM-RA, PM-DEC, RATRACK, DECTRACK are the 'catalogue' RA, Dec, equinox, epoch, proper motion and differential tracking used to point the telescope to target CAT-NAME.

RA and DEC record the position of the telescope after any 'offset' command. A change in position due to a 'tweak' command is not reflected in the headers. The applied offsets are recorded by RAOFF, DECOFF (offsets in tangent plane), XAPNOM and YAPNOM (nominal aperture offsets), and XAPOFF, YAPOFF (total aperture offsets).

ROTSKYPA is the sky position angle, ROTTRACK records whether the rotator is tracking sky.

The angular positions of the telescope, dome and rotator relative to the ground, at the start and end of the exposure, are recorded by: AZSTART, AZEND, ZDSTART, ZDEND (telescope); DAZSTART, DAZEND (dome); AMSTART, AMEND (and AIRMASS = effective mean airmass); and MNTPASTA, MNTPAEND (rotator mount PA). PARANSTA, PARANEND record the parallactic angle. VIGNETTE records whether the telescope is vignetted by the dome. TELSTAT records the telescope status, e.g. 'MOVING' at the end of the observation. The values of most of the other telescope headers mentioned here are recorded at the start of the exposure.

The name and geographical coordinates of the telescope are recorded in TELESCOP (and SLATEL, the name by which it is known to the SLALIB package), LATITUDE, LONGITUD and HEIGHT.

TELFOCUS is the nominal position of the WHT focus drive, prior to applying corrections FOCUSTMP, FOCUSALT and FOCUSTMP for temperature, elevation and filter respectively. The actual focus position (sum of the above 4 numbers) is given by ACTELFOC.

FSTATION records the focal station. The allowed values for the WHT are 'Cassegrain', 'NasmythD' (for Nasmyth drive side), 'NasmythCW' (for Nasmyth cable-wrap side) or 'Prime'. PLATESCA is the plate scale at this focal station.

AUTOMODE, AUTOX and AUTOY record the guide loop status at the start of the observation, and the x,y reference position on the autoguider. At WHT Cassegrain, CAGARADI, CAGATHET, CAGAFOCU and CAGAFILT record the autoguider r and theta coordinates, focus and filter.

Other telescope headers: PARALLAX, RADECSYS and RADVEL.

Time-related headers
DATE-OBS, JD and MJD-OBS are the calendar date, Julian date and modified Julian date at the start of observation.

UTOBS, UTSTART and ST (same as STSTART) record the time at the start of the observation. See the 'UT in the FITS headers' page for a discussion of the differences between UTOBS and UTSTART.

CCD and data-acquisition headers
The main CCD readout parameters are recorded by CCDSPEED, CCDXBIN, CCDYBIN, CCDSUM (iraf-style record of the binning), NWINDOWS (number of readout windows defined), WINSEC1, WINSEC2, WINSEC3 and WINSEC4 (coordinates, in unbinned pixels, of each defined window) and WINNO (readout window number). For some instruments, COAVERAG, NUMREADS, READMODE and STORMODE will be recorded.

The exposure time is recorded in EXPOSED = EXPTIME, ELAPSED, REQTIME and DARKTIME.



The run number is recorded by RUN, IRAFNAME (same as RUN, but with preceding 'r') and RUNSET.

The value of OBSTYPE depends on whether the commands run, arc, flat, sky or dome were used at the telescope to obtain the data. The information is duplicated in IMAGETYP (required by iraf). The command used to obtain the exposure does not affect the values of any other FITS headers.

World-coordinates headers
The world coordinates(ref 4) are recorded by the CD*, CRP*, CRV*, CUNIT*, PROJ* and PV* headers.

General headers
OBSERVAT, PROPOSAL, OBSERVER, INSTRUM and SYSVER record the observatory name ('LAPALMA'), proposal number, observer surname, instrument used to obtain the observation, and version number of the observing system.

Observing logs
A subset of the FITS headers for a given observation is reproduced in the corresponding line in the WHT or INT automatic observing log. The log also records meteorological data. For a description of the format, see the 'Log description' page.


  1. FITS headers for WHT FITS tapes. Unger S.W., Rixon G.T., Gribbin F., 1996.
  2. FITS headers for INT observations. Rixon G.T., Irwin M., Bunclark P., Lewis J., 1997.
  3. NASA/OSSA Office of Standards and Technology, Implementation of the Flexible Image Transport System (FITS) (draft standard) NASA document NOST 100-0.3b, 6th November 1991. [reference taken from an ancient ING document]
  4. Representation of world coordinates in FITS. Greisen E.W., Calabretta M.R., 2002, A&A, 395, 1061.
  5. Definition of the Flexible Image Transport System (FITS). Pence W.D. et al, 2010, A&A, 524, A42

Last updated: 25 January 2013

Click here to return to WHT home page

Top | Back

Contact:  (WHT Manager)
Last modified: 17 September 2015