Archives of Photographic PLates for Astronomical USE

WFPDB metadata

The Wide-Field Plate Database (WFPDB)[1] contains descriptive information for astronomical wide-field (>1°) photographic observations stored in numerous archives all over the world.

The WFPDB provides for each observation information for the corresponding archive, the parameters of the observational instrument, the observation parameters (position on sky, observation time, object name, method, exposure time, emulsion type, filter type, spectral band, plate size), as well as data on the plate quality, comments, and observers.

The entire observational information is stored in four metadata files as published on VizieR[2]: maindata, notes, quality and observer. (The other file archives mentioned there is strictly speaking intended for the Catalogue of WFPA[3].) These files are written in plain ASCII text und use fixed formats. Below is a description for each file structure with examples.


The file maindata contains metadata for one plate per line. Each line is 104 characters long.

General notes:

  • the column Bytes shows the character positions assigned to a metadata unit in the line
  • the column Format shows the type and character length of a metadata unit
  • the column Units shows the measurement units used for a metadata unit
  • the column Label gives an arbitrary label of a metadata unit
  • the column Explanation gives a short description of a metadata unit
  • WFPDB observatory identifier is a three-letter obervatory code as given in the Catalogue of WFPA
  • WFPDB plate number is not necessarily the same with the exact original plate number, sometimes differences in suffix letters do arise
  Bytes Format    Units     Label      Explanation
  1-  3  A3        ---      IDobs     WFPDB observatory identifier
  4-  6  I3        cm       IDins     Instrument aperture
      7  A1       ---       IDsuf1    [A-Z] Suffix to the instrument identifier
  8- 13  I6       ---       IDno      WFPDB plate number
     14  A1       ---       IDsuf2    [A-Z] Suffix to the WFPDB plate number
 15- 16  I2       h         RAh       Right ascension (hours) (J2000.0)
 17- 18  I2       min       RAm       Right ascension (minutes)
 19- 20  I2       s         RAs       Right ascension (seconds)
     21  A1       ---       DE-       Declination sign (J2000.0)
 22- 23  I2       deg       DEd       Declination (degrees)
 24- 25  I2       arcmin    DEm       Declination (arcminutes)
 26- 27  I2       arcsec    DEs       Declination (arcseconds)
     28  A1       ---       CCOD      [EMU] Code for Error, Missing data, or Uncertainty of coordinates
 29- 32  I4       yr        DATEy     Date of observation, year (UT)
 33- 34  I2       month     DATEm     Date of observation, month
 35- 36  I2       d         DATEd     Date of observation, day
 37- 38  I2       h         UTh       Observation time (hour) (UT)
 39- 40  I2       min       UTm       Observation time (min)
 41- 42  I2       s         UTs       Observation time (sec)
     43  A1       ---       TCOD      [EMU] Code for Error, Missing data, or Uncertainty of observation time
 44- 63  A20      ---       OBJNAM    Object or field designation
 64- 65  A2       ---       OBJTYP    Object type code
 66- 67  I2       ---       METHOD    Method of observation code
 68- 69  I2       ---       MULTEX    Multiplicity of exposure
 70- 75  F6.1     min       EXP       Exposure time
 76- 86  A11      ---       EMULS     Emulsion type
 87- 93  A7       ---       FILT      Filter type
 94- 95  A2       ---       SPEC      Spectral band
 96- 97  I2       cm        DIMx      X dimension of plate
 98- 99  I2       cm        DIMy      Y dimension of plate
    100  I1       ---       PQUAL     [0,1] Pointer to file 'quality'
    101  I1       ---       PNOT      [0,1] Pointer to file 'notes'
    102  I1       ---       POBS      [0,1] Pointer to file 'observer'
    103  I1       ---       PAVA      [0,1] Pointer to file 'availability'
    104  I1       ---       PDIG      [0,1] Pointer to file 'digitization'

WFPDB's codes for object type are:

A1 - planet
A2 - moon
A3 - sun
A4 - asteroid
A5 - comet
A6 - meteor
A7 - artificial satellite
S1 - star
S2 - double or multiple star
S3 - variable star
S4 - stellar cluster
S5 - HII region
S6 - nebula
S7 - planetary nebula
S8 - supernova or SN remnants
S9 - fundamental star
SA - stellar association
SD - dark nebula
SH - Herbig-Haro object
SM - molecular cloud
SP - pulsar
SR - reference star around a radio source
G1 - galaxy
G2 - QSO
G3 - group of galaxies
G4 - cluster of galaxies
G5 - supercluster
G6 - void
G7 - radio galaxy
GR - gamma-ray source
F  - field
XR - x-ray source
RS - radio source
IR - infrared source
U  - object of unknown nature

For stars (codes S1-S3), it is advisable to follow SIMBAD's object type of each particular star and assign WFPDB's object type codes as follows:

 1 - direct photograph
 2 - direct photograph (multi-exposure)
 3 - stellar tracks
 4 - objective prism
 5 - objective prism (multi-exposure)
 6 - Metcalf's method
 7 - proper motions
 8 - no guiding
 9 - out of focus
10 - test plate
11 - Hartmann test
12 - with mask
13 - focusing
14 - sub-beam (Pickering) prism
15 - raster scan/trail
24 - objective grating
25 - objective grating (multi-exposure)


POT017 004781              M193403052016   Y Dra               S30101  60.0Superpan   G        091211100

This metadata set describes the focus value during the observation and an observer note.


The file quality contains metadata for one plate per line. Each line is 80 characters long. The idea behind this file is to provide information about the quality of the plate itself, observing conditions and any other aspects which might have affected the observation.


POT017 004798  temperature: +6; cirri during the last 10 minutes

This metadata set provides air temperature and sky conditions during the observation.


The file observer contains metadata for one plate per line. Each line is 57 characters long. Its only purpose is to mention observers' names.


POT017 004781  R.Mueller

This metadata set just tells the name of the person, who performed the observation of the given plate.


The WFPDB approach to handle metadata certainly has many advantages, the most important of which is the easier possibility to parse metadata by software. Nevertheless, there are still a lot of unclear, problematic or redundant points therein. Some of them are:

  • the allocated length of text in each line in the files notes and quality is restricted to only 65 characters, which is often not enough; a similar shortage of space is present for the object and emulsion names in the file maindata
  • there is no ready solution within the WFPDB metadata structure to handle plates with two different overexposed fields (e.g., the field of the observation target and a comparison field)
  • if multiple close objects are simultaneously the observation target, then it's impossible to assign a single object type code, when these objects are of different types
  • the exposure time in WFPDB has to be given in minutes with one decimal, which is not a standard way in astronomy (seconds)
  • the files notes, quality and observer only have a continuation sign at the byte position 15, it is not included in the file maindata, which is a rather confusing fact
  • the code M for missing data (coordinates and/or observation time) is strictly speaking not necessary, because the absence of data itself is already self-descriptive
  • the explanations published on VizieR include original plate number, which is just incorrect, because it has to be a WFPDB plate number, which can be different from the original plate number
  • the future status and structure of the files availability and digitization is still unclear

See also

Extended WFPDB metadata structure



Wide-Field Plate Database (Tsvetkov+ 1997) on VizieR

Catalogue of WFPA