Archives of Photographic PLates for Astronomical USE

FITS Header for Photoplates

1 Introduction
2 Proposed FITS header format
2.1 Group 1 – mandatory and array-description keywords
2.2 Group 2 – original data of the observation
2.3 Group 3 – information about the photographic plate
2.4 Group 4 – computed data of the observation
2.5 Group 5 – scan details
2.6 Group 6 – data files
2.7 Group 7 – World Coordinate System (WCS)
2.8 Group 8 – modification history and acknowledgements
3. Sample Header
3.1 Complete sample header (new)
3.2 Complete sample header (2011)

1. 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, sample headers are shown in Section 3, comparing the suggested new one (3.1) and a sample header created with the header2011 software (3.2).

2. 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 (8 space characters):

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.

2.1 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
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

2.2 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             string  (FITS Standard) Name of the observed object or field. 
             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

2.3 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         

2.4 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  string	 (FITS Standard) UT date and time 
TIME-OBS                         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  string    UT date and time of the end       
TIME-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 "h: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 
                                 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
"YYYY-MM-DD hh:mm:ss"   →      "YYYY-MM-DDThh:mm:ss" (date format)
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

PLATENUM     = '317 '          / plate number in original observation catalogue
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)

2.5 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
"YYYY-MM-DD hh:mm:ss"    → "YYYY-MM-DDThh:mm:ss" (date format)
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

2.6 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

==================

2.7 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

2.8 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.