Howto create FITS images/spectra to be VO-compatible

FITS format is a popular output used in astronomy for spectra and images. FITS recommendations when applied enable to make a clean mapping between FITS header and ObsCore DM.

Mapping FITS with ObsCore

Some Obscore items can be assigned by provider (i.e.: obsid, access_estimated_size, ..) and other can be detected in FITS using different keywords.
We can classify them into 3 categories (ordered by relevance):

  1. WCS keywords
    Available for different topics (position, electromagnetics band..). WCS is a serialisation based on FITS keywords that make a bijection between a simple set of values and the true values (see the official document: https://fits.gsfc.nasa.gov/fits_wcs.html )
    An automated workflow requires all WCS keywords :
    i.e.: CRVAL, CTYPE, CRPIX, CDELT ,CUNIT, [CDi_j]
  2. NASA standards and extended keywords list
    See https://fits.gsfc.nasa.gov/
  3. Other keywords in usage
    Other keywords used FITS, not described by NASA

Proposed keywords grouped by topics

Obscore Description Mandatory WCS possible Automated assignment possible NASA FITS standard keywords NASA FITS exended keywords Others keywords
dataproduct_type Data product (file content) primary type YES YES
dataproduct_subtype Data product specific type
calib_level Calibration level of the observation YES YES
target_name Object of interest YES YES OBJECT OBJNAME OBJECT, TARGET
target_class Class of the Target object as in SSA
obs_id Internal ID given by the ObsTAP service YES YES OBS-ID
obs_title Brief description of dataset in free format TITLE
obs_collection Name of the data collection YES YES
obs_creation_date Date when the dataset was created
obs_creator_name Name of the creator of the data
obs_creator_did IVOA dataset identifier given by the creator
obs_release_date Curation.releaseDateunitlessdateObservation release date (ISO 8601)
obs_publisher_did ID for the Dataset given by the publisher. YES YES
publisher_id IVOA-ID for the Publisher
bib_reference Service bibliographic reference
data_rights Public/Secure/Proprietary/
access_url URL used to access dataset YES YES
access_format Content format of the dataset YES YES
access_estsize Estimated size of dataset: in kilobytes YES YES
s_ra Right ascension YES YES RA, RA_NOM, RA_OBJ, RA_PNT, RA_SXS, RA_SCY,RA_SCZ RA, RA2000, RAOBJ, RA1950
s_dec Declination YES YES DEC, DEC_NOM, DEC_OBJ, DEC_PNT, DEC_SXS, DEC_SCY, DEC_SCZ DEC, DEC2000, DEC1950, DE2000, DE1950, DEOBJ
s_fov Estimated size of the covered region as the diameter of a containing circle YES YES FOV
s_region Sky region covered by the data product (expressed in ICRS frame) YES YES REGION
s_resolution Spatial resolution of data as FWHMof PSF YES YES
s_xel1 Number of elementsalong the first coordinate of the spatial axis YES YES (NAXIS)
s_xel2 Number of elementsalong the second coordinate of the spatial axis YES YES (NAXIS)
s_ucd UCD for the nature of the spatial axis
s_unit Unit used for spatial axis
s_resolution_min Resolution min value on spatial axis (FHWM of PSF)
s_resolution_max Resolution max value on spatial axis
s_calib_status Type of calibration along the spatial axis
s_stat_error Astrometric precision along the spatial axis
s_pixel_scale Sampling period in world coordinate units along the spatial axis
t_xel Number of elementsalong the time axis YES
t_refpos Time Axis Reference Position as defined in STC REC
t_min Start time inMJD YES DATE-OBS TIME-OBS TSTART, OBSSTART, OBS-DATE, DATE-OBS, DATEOBS, OBSDATE, MJDMJDOS, TIME
t_max Stop time in MJD YES TIME-END, DATE-END TSTOP, OBSEND, ENDOBS, ENDDATE, DATEEND
t_exptime Total exposure time YES EXPOSURE, EXPTIME, TELAPSE, ELAPTIME, ONTIME, LIVETIME EXPTIME, TEXP, OBSTIME, TIMEOBS
t_resolution Temporal resolution FWHM YES
t_calib_status Type of time coordinate calibration
t_stat_error Time coord statistical error
em_xel Number of elementsalong the spectral axis YES YES (NAXIS)
em_ucd Nature of the spectral axis
em_unit Units along the spectral axis
em_calib_status Type of spectral coord calibration
em_min start in spectral coordinates YES YES
em_max stop in spectral coordinates YES YES
em_res_power Value of the resolving power along the spectral axis. (R) YES YES
em_res_power_min Resolving power min value on spectral axis
em_res_power_max Resolving power max value on spectral axis
em_resolution Value of Resolution along the spectral axis
em_stat_error Spectral coord statistical error
o_ucd Nature of the observable axis YES
o_unit Units used for the observable values
o_calib_status Typeof calibration for the observable coordinate
o_stat_error Statistical error on the Observable axis
pol_xel Number of elements along the polarization axis YES
pol_states List of polarization states present in the data file YES POL, STOKES
instrument_name The name of the instrument used for the observation YES INSTRUME INSTRUME
facility_name Name of the facility used for this observation YES TELESCOP FACILITY, TELESCOP
dataproduct_subtype Data product specific type

Validator tools

Tools like the CDS fits validator (based on Saada) checks the VO-compatibility according to WCS and a keyword list based on the FITS extended keywords.

http://cdsarc.unistra.fr/vizier.submit/fitsvalidator.html

The service was built in the VizieR ingestion context. The background process is similar as the VizieR web service provided for authors.

FITS (Nasa) recomendations

Note: NASA FITS url: https://fits.gsfc.nasa.gov/fits_dictionary.html , HEASARC URL : https://heasarc.gsfc.nasa.gov/docs/fcg/ (idem than FITS)

Obscore Description Mandatory WCS possible Automated assignment possible NASA FITS standard keywords NASA FITS exended keywords Others keywords
dataproduct_type Data product (file content) primary type YES YES
dataproduct_subtype Data product specific type
calib_level Calibration level of the observation YES YES
target_name Object of interest YES YES OBJECT OBJNAME OBJECT, TARGET
target_class Class of the Target object as in SSA
obs_id Internal ID given by the ObsTAP service YES YES OBS-ID
obs_title Brief description of dataset in free format TITLE
obs_collection Name of the data collection YES YES
obs_creation_date Date when the dataset was created
obs_creator_name Name of the creator of the data
obs_creator_did IVOA dataset identifier given by the creator
obs_release_date Curation.releaseDateunitlessdateObservation release date (ISO 8601)
obs_publisher_did ID for the Dataset given by the publisher. YES YES
publisher_id IVOA-ID for the Publisher
bib_reference Service bibliographic reference
data_rights Public/Secure/Proprietary/
access_url URL used to access dataset YES YES
access_format Content format of the dataset YES YES
access_estsize Estimated size of dataset: in kilobytes YES YES
s_ra Right ascension YES YES RA, RA_NOM, RA_OBJ, RA_PNT, RA_SXS, RA_SCY,RA_SCZ RA, RA2000, RAOBJ, RA1950
s_dec Declination YES YES DEC, DEC_NOM, DEC_OBJ, DEC_PNT, DEC_SXS, DEC_SCY, DEC_SCZ DEC, DEC2000, DEC1950, DE2000, DE1950, DEOBJ
s_fov Estimated size of the covered region as the diameter of a containing circle YES YES FOV
s_region Sky region covered by the data product (expressed in ICRS frame) YES YES REGION
s_resolution Spatial resolution of data as FWHMof PSF YES YES
s_xel1 Number of elementsalong the first coordinate of the spatial axis YES YES (NAXIS)
s_xel2 Number of elementsalong the second coordinate of the spatial axis YES YES (NAXIS)
s_ucd UCD for the nature of the spatial axis
s_unit Unit used for spatial axis
s_resolution_min Resolution min value on spatial axis (FHWM of PSF)
s_resolution_max Resolution max value on spatial axis
s_calib_status Type of calibration along the spatial axis
s_stat_error Astrometric precision along the spatial axis
s_pixel_scale Sampling period in world coordinate units along the spatial axis
t_xel Number of elementsalong the time axis YES
t_refpos Time Axis Reference Position as defined in STC REC
t_min Start time inMJD YES DATE-OBS TIME-OBS TSTART, OBSSTART, OBS-DATE, DATE-OBS, DATEOBS, OBSDATE, MJDMJDOS, TIME
t_max Stop time in MJD YES TIME-END, DATE-END TSTOP, OBSEND, ENDOBS, ENDDATE, DATEEND
t_exptime Total exposure time YES EXPOSURE, EXPTIME, TELAPSE, ELAPTIME, ONTIME, LIVETIME EXPTIME, TEXP, OBSTIME, TIMEOBS
t_resolution Temporal resolution FWHM YES
t_calib_status Type of time coordinate calibration
t_stat_error Time coord statistical error
em_xel Number of elementsalong the spectral axis YES YES (NAXIS)
em_ucd Nature of the spectral axis
em_unit Units along the spectral axis
em_calib_status Type of spectral coord calibration
em_min start in spectral coordinates YES YES
em_max stop in spectral coordinates YES YES
em_res_power Value of the resolving power along the spectral axis. (R) YES YES
em_res_power_min Resolving power min value on spectral axis
em_res_power_max Resolving power max value on spectral axis
em_resolution Value of Resolution along the spectral axis
em_stat_error Spectral coord statistical error
o_ucd Nature of the observable axis YES
o_unit Units used for the observable values
o_calib_status Typeof calibration for the observable coordinate
o_stat_error Statistical error on the Observable axis
pol_xel Number of elements along the polarization axis YES
pol_states List of polarization states present in the data file YES POL, STOKES
instrument_name The name of the instrument used for the observation YES INSTRUME INSTRUME
facility_name Name of the facility used for this observation YES TELESCOP FACILITY, TELESCOP
dataproduct_subtype Data product specific type
Topic revision: r2 - 2019-11-21 - GillesLandais
 
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