FITS header format

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Introduction

This document [1] proposes FITS header keywords, value types and overall layout for storing metadata of digitized astronomical photographic plates. The header format is intended to suit various cases: direct images with single exposures, multiple exposures of a single object/field, exposures of different objects/fields, objective-prism spectra, etc.

A FITS header format for astronomical photographic plates has been proposed by Kirov et al. (2012, Serdica J. Computing, Vol. 6, 67–76) [2], hereafter Paper I. The introduced FITS keywords have been implemented in the header2011 software that creates headers for inclusion in the FITS files of scanned plates. The header2011 software is tightly related with the Wide-Field Plate Database (WFPDB) [3], as the software uses the WFPDB files [4] as a source of plate metadata.

In this document, we propose a refinement to the header format of Paper I, closely following the FITS Standard [5] (version 3.0). The modifications are described in Section 2, a complete sample header is shown in Section 3, and a sample header created with the header2011 software is given in Section 4.

Proposed FITS header format

For better readability of headers, we propose organizing keyword records into groups of related keywords and separating the groups with keywords that have a blank name:

KEYWORD1= 'value   '           / sample keyword
KEYWORD2= 'value   '           / sample keyword
        --------------------------------------- Original data of the observation
KEYWORD3= 'value   '           / sample keyword
KEYWORD4= 'value   '           / sample keyword
        ----------------------------------------------------- Photographic plate
KEYWORD5= 'value   '           / sample keyword
KEYWORD6= 'value   '           / sample keyword
        --------------------------------------- Computed data of the observation
KEYWORD7= 'value   '           / sample keyword
KEYWORD8= 'value   '           / sample keyword

In the next subsections we denote the omitted and missing keywords with a hyphen (-).

Lowercase n and i in keyword names denote numbering: for example, RAn becomes RA1, RA2, RA3, etc. Numbers in keywords are not padded with zeros.

Group 1 – mandatory and array-description keywords


Keyword Paper I Type Comment

SIMPLE SIMPLE logical (FITS Standard) File conforms to FITS Standard.
BITPIX BITPIX integer (FITS Standard) Number of bits per data pixel.
NAXIS NAXIS integer (FITS Standard) Number of data axes.
NAXIS1 NAXIS1 integer (FITS Standard) Length of data axis 1 (number of pixels in a row).
NAXIS2 NAXIS2 integer (FITS Standard) Length of data axis 2 (number of rows).
BSCALE BSCALE float (FITS Standard) For unsigned 16-bit integer data, the value should be 1.0.
BZERO BZERO integer (FITS Standard) For unsigned 16-bit integer data, the value should be 32768.
- EXTEND logical Not required by FITS Standard, even if the FITS file does contain extensions.
END END no value (FITS Standard) Marks the end of the header.

Main differences with Paper I:

  • EXTEND → omit
  • BZERO = 65536 → 32768

Example:

SIMPLE  =                    T / file conforms to FITS standard
BITPIX  =                   16 / number of bits per data pixel
NAXIS   =                    2 / number of data axes
NAXIS1  =                18904 / length of data axis 1
NAXIS2  =                18904 / length of data axis 2
BSCALE  =                  1.0 / physical_value = BZERO + BSCALE * array_value
BZERO   =                32768 / physical_value = BZERO + BSCALE * array_value
END

Group 2 – original data of the observation

This group of keyword records provides the original information about the observation, as described in the observation logbook and other sources.


Keyword Paper I Type Comment

DATEORIG - string Original recorded date of the observation (evening date)
TMS-ORIG ST string Original recorded time of the start of the observation (format "TZ hh:mm:ss", where TZ is time zone). Time zone can be 'ST' (sidereal time), 'UT' (universal time), or any time zone. Multiple time notations are separated with commas

(e.g. 'UT 18:13, ST 02:44').

TME-ORIG - string Original recorded time of the end of the observation. See TMS-ORIG for details.
TIMEFLAG - string Quality flag of the recorded observation time: 'error', 'missing', 'uncertain'.
RA-ORIG RAEPOBS string Original recorded right ascension of the telescope pointing (plate center)
DEC-ORIG DECEPOBS string Original recorded declination of the telescope pointing (plate center)
COORFLAG - string Quality flag of the recorded coordinates (right ascension and declination): 'error', 'missing', 'uncertain'.
OBJECT OBJECT

FIELD

string (FITS Standard) Name of the observed object or field. If there are more than one field observed, then the value shall be 'multiple' and individual names shall be given with the OBJECTn keywords.
OBJTYPE - string Object type (literal text), as listed in the WFPDB
EXPTIME EXPTIME float Exposure time of the first exposure, expressed in seconds
NUMEXP MULTIEXP integer Number of exposures
DATEORn - string Original recorded date of the n-th exposure (n = 1…99), if exposures were made on multiple nights. Not used, when all exposures are from one night, given by DATEORIG.
TMS-ORn - string Original recorded time of the start of the n-th exposure (n = 1…99). See TMS-ORIG for details.
TME-ORn - string Original recorded time of the end of the n-th exposure (n = 1…99). See TMS-ORIG for details.
RA-ORn - string Original right ascension of the telescope pointing during the n-th exposure (n = 1…99). Not used, if only one pointing was used.
DEC-ORn - string Original declination of the telescope pointing during the n-th exposure (n = 1…99). Not used, if only one pointing was used.
OBJECTn - string Object (field) name on the n-th exposure (n = 1…99). Not used, if only one object (field) was observed.
OBJTYPn - string Object type that corresponds to OBJECTn (n = 1…99)
EXPTIMn - float Exposure time of the n-th exposure (n = 1…99)
OBSERVAT OBSERVAT string Observatory name
SITENAME - string Observatory site name. Useful if the observatory has more than one observing site.
SITELONG SITELONG float East longitude of the observing site, in decimal degrees
SITELAT SITELAT float Latitude of the observing site, in decimal degrees
SITEELEV SITEALTI float Elevation of the observatory site [m]. Keyword SITEELEV is more widely used than SITEALTI.
TELESCOP TELESCOP string (FITS Standard) Telescope name
TELAPER TELAPER float Clear aperture of the telescope [m]
TELFOC TELFOC float Focal length of the telescope [m]
TELSCALE TELSCALE float Plate scale of the telescope [arcsec/mm]
INSTRUME INSTRUME string (FITS Standard) Instrument name
DETNAM DETNAM string Detector name: 'photographic plate'
METHOD - string Observation method (literal text). A list of possible values is given in the WFPDB.
FILTER FILTER string Filter type
PRISM - string Information about the objective prism used
PRISMANG PRIZMANG string Angle of the objective prism (format "deg:min")
DISPERS DISPERS float Dispersion [Angstrom/mm]
GRATING - string Information about the grating used
FOCUS - float Focus value (from logbook). Used when a single value is given in the logs.
FOCUSn - float Focus value of the n-th exposure (n = 1…99)
TEMPERAT - float Air temperature (from logbook).
CALMNESS - string Calmness (seeing conditions), scale 1–5 (German: Ruhe)
SHARPNES - string Sharpness, scale 1–5 (German: Schärfe)
TRANSPAR - string Transparency, scale 1–5 (German: Durchsicht, Klarheit)
SKYCOND - string Notes on sky conditions (from logbook)
OBSERVER OBSERVER string (FITS Standard) Observer name
OBSNOTES - string Observer notes (from logbook)
NOTES - string Miscellaneous notes

We propose expressing exposure times in seconds. This is different from Paper I and the WFPDB that specify exposure time in decimal minutes, rounded to the first decimal place.

In case of multiple exposures (NUMEXP is greater than 1), exposure times of all sub-exposures can be given with the EXPTIMn keywords, where n is the exposure number in the range 1…99. The EXPTIME and EXPTIM1 keywords have the same value.

Example:

EXPTIME =                600.0 / [s] exposure time (of exposure 1)
NUMEXP  =                    3 / number of exposures of the plate
EXPTIM1 =                600.0 / [s] exposure time of exposure 1
EXPTIM2 =                 60.0 / [s] exposure time of exposure 2
EXPTIM3 =                  2.0 / [s] exposure time of exposure 3

In case of a single exposure, EXPTIM1 keyword is omitted:

EXPTIME =               1800.0 / [s] exposure time (of exposure 1)
NUMEXP  =                    1 / number of exposures of the plate

We propose expressing SITELONG and SITELAT in decimal degrees, instead of a character string in hexagesimal format.

Main differences with Paper I:

  • new keywords DATEORIG, TMS-ORIG, TME-ORIG, TIMEFLAG, and COORFLAG
  • ST → TMS-ORIG (provided that the original time is given as sidereal time)
  • RAEPOBS, DECEPOBS → RA-ORIG, DEC-ORIG
  • FIELD → OBJECT
  • EXPTIME expressed in minutes → seconds
  • MULTIEXP → NUMEXP
  • new keywords DATEORn, TMS-ORn, TME-ORn, RA-ORn, DEC-ORn, OBJECTn, OBJTYPn, EXPTIMn, where n is the exposure number in the range of 1…99
  • new keywords OBJTYPE and METHOD, based on the WFPDB
  • new keyword SITENAME
  • SITELONG, SITELAT expressed in sexagesimal format → decimal degrees
  • SITEALTI → SITEELEV
  • PRIZMANG → PRISMANG
  • new keyword GRATING
  • new keywords TEMPERAT, SEEING, SHARPNES, TRANSPAR, SKYCOND, OBSNOTES, and NOTES

Example:

        --------------------------------------- Original data of the observation
DATEORIG= '1910-08-02'         / recorded date of the observation
TMS-ORIG= 'ST 18:11:16'        / recorded time of the start of the observation
TME-ORIG= '        '           / recorded time of the end of the observation
TIMEFLAG= 'uncertain'          / quality of the recorded time
RA-ORIG = '19:11:42'           / recorded right ascension of telescope pointing
DEC-ORIG= '15:04:00'           / recorded declination of telescope pointing
COORFLAG= 'uncertain'          / quality of the recorded coordinates
OBJECT  = 'SA 87   '           / name of the observed object or field
OBJTYPE = 'field   '           / object type
EXPTIME =               1800.0 / [s] exposure time (of exposure 1)
NUMEXP  =                    1 / number of exposures of the plate
OBSERVAT= 'Astrophysikalische Observatorium Potsdam' / observatory name
SITENAME= 'Potsdam-Telegrafenberg' / observatory site name
SITELONG=            13.064167 / [deg] East longitude of the observatory
SITELAT =            52.380556 / [deg] latitude of the observatory
SITEELEV=                  107 / [m] elevation of the observatory
TELESCOP= 'Zeiss Triplet 15 cm' / telescope name
TELAPER =                 0.15 / [m] clear aperture of the telescope
TELFOC  =                  1.5 / [m] focal length of the telescope
TELSCALE=               137.68 / [arcsec/mm] plate scale of the telescope
INSTRUME= '        '           / instrument
DETNAM  = 'photographic plate' / detector
METHOD  = 'direct photograph'  / method of observation
FILTER  = 'none    '           / filter type
PRISM   = '        '           / objective prism
PRISMANG= '        '           / prism angle “deg:min”
DISPERS =                      / [Angstrom/mm] dispersion
GRATING = '        '           / grating
FOCUS   =                 32.2 / focus value
TEMPERAT=                 21.8 / [deg C] air temperature (degrees Celsius)
CALMNESS= '2-3     '           / sky calmness (scale 1-5)
SHARPNES= '2       '           / sky sharpness (scale 1-5)
TRANSPAR= '1-2     '           / sky transparency (scale 1-5)
SKYCOND = 'moonlight'          / sky conditions
OBSERVER= 'W. Muench'          / observer name
OBSNOTES= 'bad guiding'        / observer notes
NOTES   = 'SA 87 = Kapteyn Selected Area 87' / miscellaneous notes

Example 2 (multiple time notations):

        --------------------------------------- Original data of the observation
DATEORIG= '1964-01-02'         / recorded date of the observation
TMS-ORIG= 'UT 18:13, ST 02:44' / recorded time of the start of exposure 1
TME-ORIG= 'UT 19:13, ST 03:44' / recorded time of the end of exposure 1
TIMEFLAG= '        '           / quality of the recorded time
EXPTIME =               3600.0 / [s] exposure time (of exposure 1)
NUMEXP  =                    1 / number of exposures of the plate

Example 3 (multiple exposures):

        --------------------------------------- Original data of the observation
DATEORIG= '1934-04-01'         / recorded date of the observation
TMS-OR1 = 'ST 10:52'           / recorded time of the start of exposure 1
TMS-OR2 = 'ST 10:54'           / recorded time of the start of exposure 2
TMS-OR3 = 'ST 10:57'           / recorded time of the start of exposure 3
TME-OR1 = 'ST 10:53'           / recorded time of the end of exposure 1
TME-OR2 = 'ST 10:56'           / recorded time of the end of exposure 2
TME-OR3 = 'ST 11:01'           / recorded time of the end of exposure 3
TIMEFLAG= '        '           / quality of the recorded time
RA-ORIG = '        '           / recorded right ascension of telescope pointing
DEC-ORIG= '        '           / recorded declination of telescope pointing
COORFLAG= 'missing '           / quality of the recorded coordinates
OBJECT  = 'RY UMa  '           / name of the observed object or field
OBJTYPE = 'variable star'      / object type
EXPTIME =                 60.0 / [s] exposure time (of exposure 1)
NUMEXP  =                    3 / number of exposures of the plate
EXPTIM1 =                 60.0 / [s] exposure time of exposure 1
EXPTIM2 =                120.0 / [s] exposure time of exposure 2
EXPTIM3 =                240.0 / [s] exposure time of exposure 3
OBSERVAT= 'Astrophysikalische Observatorium Potsdam' / observatory name
SITENAME= 'Potsdam-Telegrafenberg' / observatory site name
SITELONG=            13.064167 / [deg] East longitude of the observatory
SITELAT =            52.380556 / [deg] latitude of the observatory
SITEELEV=                  107 / [m] elevation of the observatory
TELESCOP= 'Zeiss Triplet 15 cm' / telescope name
TELAPER =                 0.15 / [m] clear aperture of the telescope
TELFOC  =                  1.5 / [m] focal length of the telescope
TELSCALE=               137.68 / [arcsec/mm] plate scale of the telescope
INSTRUME= '        '           / instrument
DETNAM  = 'photographic plate' / detector
METHOD  = 'direct photograph, multi-exposure' / method of observation
FILTER  = 'none    '           / filter type
PRISM   = '        '           / objective prism
PRISMANG= '        '           / prism angle “deg:min”
DISPERS =                      / [Angstrom/mm] dispersion
GRATING = '        '           / grating
FOCUS   =                 34.4 / focus value
TEMPERAT=                    8 / [deg C] air temperature (degrees Celsius)
CALMNESS= '        '           / sky calmness (scale 1-5)
SHARPNES= '        '           / sky sharpness (scale 1-5)
TRANSPAR= '        '           / sky transparency (scale 1-5)
SKYCOND = 'clouds  '           / sky conditions
OBSERVER= 'W. Muench'          / observer name
OBSNOTES= 'poor transparency'  / observer notes
NOTES   = '        '           / miscellaneous notes

Group 3 – information about the photographic plate


Keyword Paper I Type Comment

PLATENUM PLATENUM string Plate number in original observation catalogue
WFPDB-ID PLATE-ID string Plate identification in the WFPDB
SERIES - string Series or survey in which the plate belongs, e.g. Carte du Ciel, Kapteyn Selected Areas, etc.
PLATEFMT - string Plate format (e.g. '9x12', '20x20')
PLATESZ1 PLATESZ float Plate size along axis 1
PLATESZ2 PLATESZ float Plate size along axis 2
FOV1 CUNIT1 float Field of view along axis 1
FOV2 CUNIT2 float Field of view along axis 2
EMULSION EMULSION string Type of the photographic emulsion
- COLOR The use of this keyword is not explained in Paper I.
DEVELOP - string Plate development information (developer, time)
PQUALITY PQUALITY string Quality of the plate
PLATNOTE - string Notes about the plate (e.g. contact copy of the original plate)

We propose giving the plate size separately for both axes, thus replacing the PLATESZ keyword (character string) with PLATESZ1 and PLATESZ2 (floating-point numbers).

We specify the field of view along both axes with the FOV1 and FOV2 keywords, replacing CUNIT1 and CUNIT2 that are reserved for the WCS.

Main differences with Paper I:

  • PLATE-ID → WFPDB-ID
  • new keyword SERIES
  • PLATESZ → PLATESZ1, PLATESZ2
  • CUNIT1, CUNIT2 → FOV1, FOV2
  • COLOR → omit
  • new keyword PLATNOTE

Example:

        ----------------------------------------------------- Photographic plate
PLATENUM= '317     '           / plate number in original observation catalogue
WFPDB-ID= 'POT015_000317'      / plate identification in the WFPDB
SERIES  = 'Kapteyn Selected Areas' / plate series
PLATEFMT= '20x20   '           / plate format in cm
PLATESZ1=                 20.0 / [cm] plate size along axis 1
PLATESZ2=                 20.0 / [cm] plate size along axis 2
FOV1    =                 7.65 / [deg] field of view along axis 1
FOV2    =                 7.65 / [deg] field of view along axis 2
EMULSION= 'Schleussner'        / photographic emulsion type
DEVELOP = '        '           / plate development information
PQUALITY= 'broken  '           / quality of plate
PLATNOTE= 'contact copy of original plate that is not available' / plate notes

Group 4 – computed data of the observation

In this group of keyword records, we provide data of the observation that are computed on the basis of the original data.


Keyword Paper I Type Comment

DATE-OBS DATE-OBS

TIME-OBS

string (FITS Standard) UT date and time of the start of the observation (format "YYYY-MM-DDThh:mm:ss", or "YYYY-MM-DD" if time is not specified). The date may differ from DATEORIG, because the original date usually refers to the evening of the observing night.
DT-OBSn - string UT date and time of the start of the n-th exposure

(n = 1…99)

DATE-AVG UT string (FITS Standard) UT date and time of the mid-point of the first exposure (format "YYYY-MM-DDThh:mm:ss")
DT-AVGn - string UT date and time of the mid-point of the n-th exposure

(n = 1…99)

DATE-END DATE-OBS

TIME-END

string UT date and time of the end of the first exposure (format "YYYY-MM-DDThh:mm:ss")
DT-ENDn - string UT date and time of the end of the n-th exposure (n = 1…99)
YEAR - float Decimal year of the start of the first exposure
YEARn - float Decimal year of the start of the n-th exposure (n = 1…99)
YEAR-AVG EPOCH float Decimal year of the mid-point of the first exposure
YR-AVGn - float Decimal year of the mid-point of the n-th exposure

(n = 1…99)

JD - float Julian date at the start of exposure 1
JDn - float Julian date at the start of the n-th exposure (n = 1…99)
JD-AVG JD float Julian date at the mid-point of the first exposure
JD-AVGn - float Julian date at the mid-point of the n-th exposure (n = 1…99)
HJD-AVG - float Heliocentric Julian date at the mid-point of the first exposure
HJD-AVn - float Heliocentric Julian date at the mid-point of the n-th exposure (n = 1…99)
RA RA string Right ascension of the telescope pointing (equinox J2000, sexagesimal format "h:m:s")
DEC DEC string Declination of the telescope pointing (equinox J2000, sexagesimal format "d:m:s")
RAn - string Right ascension of the telescope pointing, n-th exposure

(n = 1…99). Used only when different fields were exposed on the same plate.

DECn - string Declination of the telescope pointing, n-th exposure

(n = 1…99). Used only when different fields were exposed on the same plate.

RA_DEG - float Right ascension of the telescope pointing in decimal degrees (equinox J2000)
DEC_DEG - float Declination of the telescope pointing in decimal degrees (equinox J2000)
RA_DEGn - float Right ascension of the telescope pointing in decimal degrees, n-th exposure (n = 1…99). Used only when different fields were exposed on the same plate.
DEC_DEn - float Declination of the telescope pointing in decimal degrees,

n-th exposure (n = 1…99). Used only when different fields were exposed on the same plate.


We replace the EPOCH keyword (Paper I) with YEAR-AVG. The EPOCH keyword is deprecated in the FITS Standard. It was previously used to give the equinox in years for the celestial coordinate system in which positions were expressed. We reserve the EQUINOX keyword for the World Coordinate System, as required by the FITS Standard.

The RA, DEC, RA_DEG, and DEC_DEG keywords provide the precessed coordinates of the original recorded coordinates to the equinox J2000.

Main differences with Paper I:

  • DATE-OBS, TIME-OBS → DATE-OBS
  • TIME-END → DATE-END
  • UT → DATE-AVG
  • date format "YYYY-MM-DD hh:mm:ss" → "YYYY-MM-DDThh:mm:ss"
  • EPOCH → YEAR-AVG
  • JD → JD-AVG
  • new keywords YEAR, HJD-AVG
  • new keywords RA_DEG, DEC_DEG
  • numbered keywords for multiple exposures
  • EQUINOX → if necessary, specify with the WCS keywords

Example:

        --------------------------------------- Computed data of the observation
DATE-OBS= '1910-08-02T22:21:01' / UT date of the start of the observation
DATE-AVG= '1910-08-02T22:36:01' / UT date of the mid-point of exposure 1
DATE-END= '1910-08-02T22:51:01' / UT date of the end of exposure 1
YEAR    =       1910.583561644 / decimal year of the start of exposure 1
YEAR-AVG=       1910.583561644 / decimal year of the mid-point of exposure 1
JD      =       2418886.441678 / Julian date at the start of exposure 1
JD-AVG  =       2418886.441678 / Julian date at the mid-point of exposure 1
HJD-AVG =       2418886.441678 / heliocentric JD at the mid-point of exposure 1
RA      = '19:15:48'           / right ascension of pointing (J2000) "h:m:s"
DEC     = '+15:13:20'          / declination of pointing (J2000) "d:m:s"
RA_DEG  =           288.950000 / [deg] right ascension of pointing (J2000)
DEC_DEG =            15.222222 / [deg] declination of pointing (J2000)

Example 2:

        --------------------------------------- Computed data of the observation
DATE-OBS= '1934-01-25T20:36:56' / UT date of the start of exposure 1
DT-OBS1 = '1934-01-25T20:36:56' / UT date of the start of exposure 1
DT-OBS2 = '1934-01-25T20:45:55' / UT date of the start of exposure 2
DT-OBS3 = '1934-01-25T20:55:53' / UT date of the start of exposure 3
DT-OBS4 = '1934-01-25T20:57:53' / UT date of the start of exposure 4
DATE-AVG= '1934-01-25T20:40:56' / UT date of the mid-point of exposure 1
DT-AVG1 = '1934-01-25T20:40:56' / UT date of the mid-point of exposure 1
DT-AVG2 = '1934-01-25T20:48:25' / UT date of the mid-point of exposure 2
DT-AVG3 = '1934-01-25T20:56:23' / UT date of the mid-point of exposure 3
DT-AVG4 = '1934-01-25T20:58:53' / UT date of the mid-point of exposure 4
DATE-END= '1934-01-25T20:44:55' / UT date of the end of exposure 1
DT-END1 = '1934-01-25T20:44:55' / UT date of the end of exposure 1
DT-END2 = '1934-01-25T20:50:54' / UT date of the end of exposure 2
DT-END3 = '1934-01-25T20:56:53' / UT date of the end of exposure 3
DT-END4 = '1934-01-25T20:59:52' / UT date of the end of exposure 4
YEAR    =        1934.06806018 / decimal year of the start of exposure 1
YEAR1   =        1934.06806018 / decimal year of the start of exposure 1
YEAR2   =        1934.06807726 / decimal year of the start of exposure 2
YEAR3   =        1934.06809621 / decimal year of the start of exposure 3
YEAR4   =        1934.06810001 / decimal year of the start of exposure 4
YEAR-AVG=        1934.06806779 / decimal year of the mid-point of exposure 1
YR-AVG1 =        1934.06806779 / decimal year of the mid-point of exposure 1
YR-AVG2 =        1934.06808202 / decimal year of the mid-point of exposure 2
YR-AVG3 =        1934.06809716 / decimal year of the mid-point of exposure 3
YR-AVG4 =        1934.06810192 / decimal year of the mid-point of exposure 4
JD      =        2427463.35898 / Julian date at the start of exposure 1
JD1     =        2427463.35898 / Julian date at the start of exposure 1
JD2     =        2427463.36522 / Julian date at the start of exposure 2
JD3     =        2427463.37214 / Julian date at the start of exposure 3
JD4     =        2427463.37353 / Julian date at the start of exposure 4
JD-AVG  =        2427463.36176 / Julian date at the mid-point of exposure 1
JD-AVG1 =        2427463.36176 / Julian date at the mid-point of exposure 1
JD-AVG2 =        2427463.36696 / Julian date at the mid-point of exposure 2
JD-AVG3 =        2427463.37249 / Julian date at the mid-point of exposure 3
JD-AVG4 =        2427463.37422 / Julian date at the mid-point of exposure 4
HJD-AVG =                      / heliocentric JD at the mid-point of exposure 1
RA      = '        '           / right ascension of pointing (J2000) "h:m:s"
DEC     = '        '           / declination of pointing (J2000) "d:m:s"
RA_DEG  =                      / [deg] right ascension of pointing (J2000)
DEC_DEG =                      / [deg] declination of pointing (J2000)

Group 5 – scan details

This group contains information about scanner hardware and software settings, also the name of the scan author and the date of the scan.


Keyword Paper I Type Comment

SCANRES1 SCANRES integer Scan resolution along axis 1 [dpi]
SCANRES2 SCANRES integer Scan resolution along axis 2 [dpi]
PIXSIZE1 XPIXELSZ float Pixel size along axis 1 [μm]
PIXSIZE2 YPIXELSZ float Pixel size along axis 2 [μm]
SCANSOFT - string Name of the scanning software
- SCANHCUT Omit: scan high-cut value
- SCANLCUT Omit: scan low-cut value
SCANGAM SCANGAM float Scan gamma value
SCANFOC SCANFOC string Scan focus (e.g. 'glass')
WEDGE WEDGE string Type of photometric step-wedge
DATESCAN DATE-SCN string Scan date and time (UTC, format "YYYY-MM-DDThh:mm:ss")
SCANAUTH AUTHOR string Author of the scan
SCANNOTE - string Notes about the scan (e.g. scan orientation)
- REFERENC We propose omitting this keyword

We propose replacing the SCANRES keyword with the SCANRES1 and SCANRES2 keywords, indicating scan resolution along both image axes separately.

By FITS Standard, the AUTHOR and REFERENCE keywords are used when the data in the FITS file were compiled from a publication or multiple sources. For digitized photographic plates, these keywords are not appropriate for specifying the author of the scan nor acknowledging any funding sources. We propose replacing the AUTHOR keyword with SCANAUTH and providing acknowledgments with the COMMENT keyword (Group 8).

Main differences with Paper I:

  • SCANRES → SCANRES1, SCANRES2
  • XPIXELSZ → PIXSIZE1
  • YPIXELSZ → PIXSIZE2
  • PIXSIZE1, PIXSIZE2 unit: [microns] → [um]
  • new keywords SCANSOFT, SCANNOTE
  • SCANHCUT, SCANLCUT → omit
  • SCANFOC type: float → string
  • date format "YYYY-MM-DD hh:mm:ss" → "YYYY-MM-DDThh:mm:ss"
  • AUTHOR → SCANAUTH
  • REFERENC → COMMENT

Example:

        ------------------------------------------------------------------- Scan
SCANNER = 'Epson Expression 10000XL' / scanner name
SCANRES1=                 2400 / [dpi] scan resolution along axis 1
SCANRES2=                 2400 / [dpi] scan resolution along axis 2
PIXSIZE1=              10.5833 / [um] pixel size along axis 1
PIXSIZE2=              10.5833 / [um] pixel size along axis 2
SCANSOFT= 'VueScan '           / name of the scanning software
SCANGAM =                  1.0 / scan gamma value
SCANFOC = 'glass'              / scan focus
WEDGE   = 'Danes-Picta TG21S'  / type of photometric step-wedge
DATESCAN= '2011-05-17T10:33:26' / scan date and time
SCANAUTH= 'K. Tsvetkova'       / author of scan

Group 6 – data files


Keyword Paper I Type Comment

FILENAME FILENAME string Filename of the plate scan (this file)
FN-SCNi - string Filename of the i-th scan of the same plate (i = 1…99)
FN-WEDGE - string Filename of the wedge scan
FN-PRE - string Filename of the preview image (annotated plate)
FN-COVER - string Filename of the plate cover (envelope) image
FN-LOGB - string Filename of the logbook image
FN-NTBi - string Filename of the i-th notebook image (i = 1…99)
- URL We propose omitting this keyword.
ORIGIN ORIGIN string (FITS Standard) Institute responsible for creating the FITS file
DATE DATE string (FITS Standard) Date and time of the last change of the file

Main differences with Paper I:

  • new keywords FN-SCNi, FN-WEDGE, FN-PRE, FN-COVER, FN-LOGB, and FN-NTBi
  • URL → omit

Example:

        ------------------------------------------------------------- Data files
FILENAME= 'POT015_000317.fits' / filename of this file
FN-WEDGE= 'POT015_000317w.fits' / filename of the wedge scan
FN-PRE  = 'POT015_000317_pre.jpg' / filename of the preview image
FN-COVER= '        '           / filename of the plate cover image
FN-LOGB = 'POT015_000317-000334.jpg' / filename of logbook image
ORIGIN  = 'Leibniz-Institut fuer Astrophysik Potsdam (AIP)'
DATE    = '2013-04-09T12:00:00' / last change of this file

Example 2:

        ------------------------------------------------------------- Data files
FILENAME= 'LA00508x.fits'      / filename of this file
FN-SCN1 = 'LA00508x.fits'      / filename of scan 1
FN-SCN2 = 'LA00508y.fits'      / filename of scan 2
FN-WEDGE= '        '           / filename of the wedge scan
FN-PRE  = 'LA00508_pre.jpg'    / filename of the preview image
FN-COVER= 'LA00508_cover.jpg'  / filename of the plate cover image
FN-LOGB = 'LA-PV01-LA00501_00510.jpg' / filename of logbook image
FN-NTB1 = 'LA-LB04-1916-10-18a.jpg' / filename of notebook image 1
FN-NTB2 = 'LA-LB04-1916-10-18b.jpg' / filename of notebook image 2
FN-NTB3 = 'LA-LB04-1916-10-18c.jpg' / filename of notebook image 3
FN-NTB4 = 'LA-LB04-1916-10-18d.jpg' / filename of notebook image 4
FN-NTB5 = 'LA-LB04-1916-10-18e.jpg' / filename of notebook image 5
FN-NTB6 = 'LA-LB04-1916-10-18f.jpg' / filename of notebook image 6
ORIGIN  = 'Hamburger Sternwarte' /
DATE    = '2013-12-12T13:42:00' / last change of this file

Group 7 – World Coordinate System (WCS)

The astrometric information are given with the World Coordinate System (WCS) keywords, as described in the FITS Standard.

If the EQUINOX keyword is not given, then coordinates are assumed to refer to the International Celestial Reference System (ICRS).

When the WCS solution is not possible through matching stars in the scan, the WCS keywords can be used to provide the approximate coordinates, based on the plate scale and the precessed coordinates of the original telescope pointing.

Example:

        ------------------------------------------ World Coordinate System (WCS)
WCSAXES =                    2 / number of axes in the WCS description
RADESYS = 'FK5     '           / name of the reference frame
EQUINOX =               2000.0 / epoch of the mean equator and equinox in years
CTYPE1  = 'RA---TAN'           / TAN (gnomonic) projection
CTYPE2  = 'DEC--TAN'           / TAN (gnomonic) projection
CUNIT1  = 'deg     '           / physical units of CRVAL and CDELT for axis 1
CUNIT2  = 'deg     '           / physical units of CRVAL and CDELT for axis 2
CRPIX1  =               9452.5 / reference pixel for axis 1
CRPIX2  =               9452.5 / reference pixel for axis 2
CRVAL1  =               288.95 / right ascension at the reference point
CRVAL2  =            15.222222 / declination at the reference point
CD1_1   =        -0.0004047524 / transformation matrix
CD1_2   =                  0.0 / transformation matrix
CD2_1   =                  0.0 / transformation matrix
CD2_2   =         0.0004047524 / transformation matrix
LONPOLE =                  0.0 / native longitude of the celestial pole

Group 8 – modification history and acknowledgements

Modification history is given with the HISTORY keyword.

Comments and acknowledgements are given with the COMMENT keyword.

Example:

        --------------------------------------------------- Modification history
HISTORY Header written with PyPlates at 2013-12-18T12:00:00
HISTORY WCS modified by T. Tuvikene (AIP) at 2013-12-19T12:00:00
        ------------------------------------------------------- Acknowledgements
COMMENT The digitization of this plate was funded by the German Research
COMMENT Foundation (DFG) grant STE: 710/6-1,20.11.2009 and partially by the
COMMENT grants of the Bulgarian Ministry of Education and Science
COMMENT DO-02-273/275,18.12.2009. The 2011 May-June stay of K. Tsvetkova at AIP
COMMENT was funded by DO-02-275 MON.
COMMENT
COMMENT Publications based on this digitized photographic plate are requested to
COMMENT include the following acknowledgement.
COMMENT
COMMENT Based on photographic data of the Leibniz-Institut fuer Astrophysik
COMMENT Potsdam (AIP). The Kapteyn Selected Areas Survey was obtained with the
COMMENT 80-cm Great Refractor and the 15-cm Zeiss Triplet telescope at
COMMENT Potsdam-Telegrafenberg in 1910-1933. The project of plate digitization
COMMENT was funded by the grants of the German Research Foundation (DFG) and the
COMMENT Bulgarian Ministry of Education and Science.

Complete sample header

SIMPLE  =                    T / file conforms to FITS standard
BITPIX  =                   16 / number of bits per data pixel
NAXIS   =                    2 / number of data axes
NAXIS1  =                18904 / length of data axis 1
NAXIS2  =                18904 / length of data axis 2
BSCALE  =                  1.0 / physical_value = BZERO + BSCALE * array_value
BZERO   =                32768 / physical_value = BZERO + BSCALE * array_value
        --------------------------------------- Original data of the observation
DATEORIG= '1910-08-02'         / recorded date of the observation
TMS-ORIG= 'ST 18:11:16'        / recorded time of the start of the observation
TME-ORIG= '        '           / recorded time of the end of the observation
TIMEFLAG= 'uncertain'          / quality of the recorded time
RA-ORIG = '19:11:42'           / recorded right ascension of telescope pointing
DEC-ORIG= '15:04:00'           / recorded declination of telescope pointing
COORFLAG= 'uncertain'          / quality of the recorded coordinates
OBJECT  = 'SA 87   '           / name of the observed object or field
OBJTYPE = 'field   '           / object type
EXPTIME =               1800.0 / [s] exposure time (of exposure 1)
NUMEXP  =                    1 / number of exposures of the plate
OBSERVAT= 'Astrophysikalische Observatorium Potsdam' / observatory name
SITENAME= 'Potsdam-Telegrafenberg' / observatory site name
SITELONG=            13.064167 / [deg] East longitude of the observatory
SITELAT =            52.380556 / [deg] latitude of the observatory
SITEELEV=                  107 / [m] elevation of the observatory
TELESCOP= 'Zeiss Triplet 15 cm' / telescope name
TELAPER =                 0.15 / [m] clear aperture of the telescope
TELFOC  =                  1.5 / [m] focal length of the telescope
TELSCALE=               137.68 / [arcsec/mm] plate scale of the telescope
INSTRUME= '        '           / instrument
DETNAM  = 'photographic plate' / detector
METHOD  = 'direct photograph'  / method of observation
FILTER  = 'none    '           / filter type
PRISM   = '        '           / objective prism
PRISMANG= '        '           / prism angle “deg:min”
DISPERS =                      / [Angstrom/mm] dispersion
GRATING = '        '           / grating
FOCUS   =                 32.2 / focus value
TEMPERAT=                 21.8 / [deg C] air temperature (degrees Celsius)
CALMNESS= '2-3     '           / sky calmness (scale 1-5)
SHARPNES= '2       '           / sky sharpness (scale 1-5)
TRANSPAR= '1-2     '           / sky transparency (scale 1-5)
SKYCOND = 'moonlight'          / sky conditions
OBSERVER= 'W. Muench'          / observer name
OBSNOTES= 'bad guiding'        / observer notes
NOTES   = 'SA 87 = Kapteyn Selected Area 87' / miscellaneous notes
        ----------------------------------------------------- Photographic plate
PLATENUM= '317     '           / plate number in original observation catalogue
WFPDB-ID= 'POT015_000317'      / plate identification in the WFPDB
SERIES  = 'Kapteyn Selected Areas' / plate series
PLATEFMT= '20x20   '           / plate format in cm
PLATESZ1=                 20.0 / [cm] plate size along axis 1
PLATESZ2=                 20.0 / [cm] plate size along axis 2
FOV1    =                 7.65 / [deg] field of view along axis 1
FOV2    =                 7.65 / [deg] field of view along axis 2
EMULSION= 'Schleussner'        / photographic emulsion type
DEVELOP = '        '           / plate development information
PQUALITY= 'broken  '           / quality of plate
PLATNOTE= 'contact copy of original plate that is not available' / plate notes
        --------------------------------------- Computed data of the observation
DATE-OBS= '1910-08-02T22:21:01' / UT date of the start of the observation
DATE-AVG= '1910-08-02T22:36:01' / UT date of the mid-point of exposure 1
DATE-END= '1910-08-02T22:51:01' / UT date of the end of exposure 1
YEAR    =       1910.583561644 / decimal year of the start of exposure 1
YEAR-AVG=       1910.583561644 / decimal year of the mid-point of exposure 1
JD-AVG  =       2418886.441678 / Julian date at the mid-point of exposure 1
HJD-AVG =       2418886.441678 / heliocentric JD at the mid-point of exposure 1 
RA      = '19:15:48'           / right ascension of pointing (J2000) "h:m:s"
DEC     = '+15:13:20'          / declination of pointing (J2000) "d:m:s"
RA_DEG  =           288.950000 / [deg] right ascension of pointing (J2000)
DEC_DEG =            15.222222 / [deg] declination of pointing (J2000)
        ------------------------------------------------------------------- Scan
SCANNER = 'Epson Expression 10000XL' / scanner name
SCANRES1=                 2400 / [dpi] scan resolution along axis 1
SCANRES2=                 2400 / [dpi] scan resolution along axis 2
PIXSIZE1=              10.5833 / [um] pixel size along axis 1
PIXSIZE2=              10.5833 / [um] pixel size along axis 2
SCANSOFT= 'VueScan '           / name of the scanning software
SCANGAM =                  1.0 / scan gamma value
SCANFOC = 'glass'              / scan focus
WEDGE   = 'Danes-Picta TG21S'  / type of photometric step-wedge
DATESCAN= '2011-05-17T10:33:26' / scan date and time
SCANAUTH= 'K. Tsvetkova'       / author of scan
        ------------------------------------------------------------- Data files
FILENAME= 'POT015_000317.fits' / filename of this file
FN-WEDGE= 'POT015_000317w.fits' / filename of the wedge scan
FN-PRE  = 'POT015_000317_pre.jpg' / filename of the preview image
FN-COVER= '        '           / filename of the plate cover image
FN-LOGB = 'POT015_000317-000334.jpg' / filename of logbook image
ORIGIN  = 'Leibniz-Institut fuer Astrophysik Potsdam (AIP)'
DATE    = '2013-04-09T12:00:00' / last change of this file
        ------------------------------------------ World Coordinate System (WCS)
WCSAXES =                    2 / number of axes in the WCS description
RADESYS = 'FK5     '           / name of the reference frame
EQUINOX =               2000.0 / epoch of the mean equator and equinox in years
CTYPE1  = 'RA---TAN'           / TAN (gnomonic) projection
CTYPE2  = 'DEC--TAN'           / TAN (gnomonic) projection
CUNIT1  = 'deg     '           / physical units of CRVAL and CDELT for axis 1
CUNIT2  = 'deg     '           / physical units of CRVAL and CDELT for axis 2
CRPIX1  =               9452.5 / reference pixel for axis 1
CRPIX2  =               9452.5 / reference pixel for axis 2
CRVAL1  =               288.95 / right ascension at the reference point
CRVAL2  =            15.222222 / declination at the reference point
CD1_1   =        -0.0004047524 / transformation matrix
CD1_2   =                  0.0 / transformation matrix
CD2_1   =                  0.0 / transformation matrix
CD2_2   =         0.0004047524 / transformation matrix
LONPOLE =                  0.0 / native longitude of the celestial pole
        --------------------------------------------------- Modification history
HISTORY Header written with PyPlates at 2013-12-18T12:00:00
HISTORY WCS modified by T. Tuvikene (AIP) at 2013-12-19T12:00:00
        ------------------------------------------------------- Acknowledgements
COMMENT The digitization of this plate was funded by the German Research
COMMENT Foundation (DFG) grant STE: 710/6-1,20.11.2009 and partially by the
COMMENT grants of the Bulgarian Ministry of Education and Science
COMMENT DO-02-273/275,18.12.2009. The 2011 May-June stay of K. Tsvetkova at AIP
COMMENT was funded by DO-02-275 MON.
COMMENT
COMMENT Publications based on this digitized photographic plate are requested to
COMMENT include the following acknowledgement.
COMMENT
COMMENT Based on photographic data of the Leibniz-Institut fuer Astrophysik
COMMENT Potsdam (AIP). The Kapteyn Selected Areas Survey was obtained with the
COMMENT 80-cm Great Refractor and the 15-cm Zeiss Triplet telescope at
COMMENT Potsdam-Telegrafenberg in 1910-1933. The project of plate digitization
COMMENT was funded by the grants of the German Research Foundation (DFG) and the
COMMENT Bulgarian Ministry of Education and Science.
END

Original sample header (created with the header2011 software)

SIMPLE  =                  T / file does conform to FITS standard
BITPIX  =                 16 / number of bits per data pixel
NAXIS   =                  2 / number of data axes
NAXIS1  =              18904 / length of data axis 1
NAXIS2  =              18904 / length of data axis 2
EXTEND  =                  T / FITS dataset may contain extensions
BZERO   =              65536 /
BSCALE  =                  1 /
INVERTED=                  T / T -  big-endian, F - little-endian
DATE    = '2011-06-30 10:53:40' / last change of file
FILENAME= 'POT015_000317.fits' / source file name
PLATENUM= '317             ' / in original observation catalogue
PLATE-ID= 'POT015_000317   ' / WFPDB ident. of plate
OBJECT  = 'SA 87           ' / field name and/or star name
EMULSION= '                ' / photoemulsion type
EXPTIME = 3.000000000000E+01 / exposure time [minutes]
DISPERS =                    / dispersion [A/mm]
MULTIEXP=                  1 / number of exposure of the plate
PQUALITY= '                ' / quality of plate
DATE-OBS= '1910-08-02      ' / date of observation
RA      = '19:15:48        ' / center of plate FK5
DEC     = '+15:13:20       ' / center of plate FK5
EQUINOX = 2.000000000000E+03 / equatorial coordinates definition
UT      = '1910-08-02 22:36:01' / date and UT at mean epoch
ST      = '18:11:16        ' / ST at start of the observation
JD      = 2.418886441678E+06 / JD at mean epoch
TIME-OBS= '22:21:01        ' / UT at start of observation
TIME-END= '22:51:01        ' / UT at end of observation
RAEPOBS = '                ' / center of plate at epoch of observation
DECEPOBS= '                ' / center of plate at epoch of observation
EPOCH   = 1.910583561644E+03 / epoch of plate
PLATESZ = '20x20           ' / plate size [cm]x[cm]
CUNIT1  = 7.650000000000E+00 / X field size [deg]
CUNIT2  = 7.650000000000E+00 / Y field size [deg]
DETNAM  = 'Photographic Plate' /
OBSERVER= 'W.Muench        ' / observer name
OBSERVAT= 'AO Potsdam      ' / observatory name
INSTRUME= '                ' /
TELESCOP= 'Zeiss Triplet 15 cm Potsdam-Telegrafenberg' / telescop name
SITELONG= '-13:03:51.0     ' / longitude of the obsrvatory
SITELAT = '+52:22:50.0     ' / latitude of the observatory
SITEALTI=                107 / altitude of the observatory
COLOR   = 'Pg              ' /
FILTER  = 'NO              ' / filter type
PRIZMANG= 'NO              ' / prism angle
TELAPER = 1.500000000000E-01 / clear aperture [m]
TELFOC  = 1.500000000000E+00 / focal length [m]
TELSCALE= 1.376800000000E+02 / telescope scale  [arcsec/mm]
SCANNER = 'EPSON EXPRESSION 10000XL' / scanner name
SCANRES =               2400 / scan resolution
XPIXELSZ= 1.058330000000E+01 / X pixel size [microns]
YPIXELSZ= 1.058330000000E+01 / Y pixel size [microns]
SCANHCUT=                255 / focal length [m]
SCANLCUT=                  0 / scan shadow value
SCANGAM = 1.000000000000E+00 / scan gamma value
SCANFOC = 0.000000000000E+00 / scan focus
DATE-SCN= '2011-05-17 10:33:26' / scan date and time
AUTHOR  = 'K. Tsvetkova    ' / author of scan
ORIGIN  = 'Leibniz IAP - WFPDB - Sofia' /
REFERENC= 'May-June DFG Stay of K.Tsvetkova  in AIP, DO-02-275 MON' / reference
URL     = 'vo.aip.de, www.wfpdb.org' / base URL of VO Service to retrieve data
COMMENT
       Based on photographic data of the Leibniz Astrophysical Observatory
       Potsdam - Kapteyn Selected Areas Survey obtained using the 80 cm Great
       Refractor and 15 cm Zeiss Triplet telescope at Telegrafenberg - Potsdam
       in the period 1910-1933. The plates were digitized using professional
       flatbed scanners EPSON 10000XL/V700 and processed in the present digital
       form. The project of Plate digitization was funded by a German DFG grant
       STE: 710/6-1,20.11.2009 and partially of the grants of Bulgarian
       Ministry of Education and Science DO-02-273/275,18.12.2009.

       Investigators using these scans are kindly requested to include the
       above acknowledgements in any publications.

       Copyright (c) Leibniz Astrophysical Institute Potsdam.
       All rights reserved.
END
  1. Online version of an unpublished paper written and edited by T.Tuvikene, S.Schmalz, H.Enke, A.Partl (AIP) in 2013–2014.
  2. Software Tools for Digitization of Astronomical Photographic Plates in: Serdica Journal of Computing, vol. 6, no. 1, pp. 67–76 (2012).
  3. WFPDB
  4. Wide-Field Plate Database (Tsvetkov+ 1997) on VizieR
  5. FITS Standard Document