WFPDB metadata

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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.

Maindata

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:

SIMBAD: star                            WFPDB: S1
SIMBAD: nova                            WFPDB: S1
SIMBAD: Be star                         WFPDB: S1
SIMBAD: emission-line star              WFPDB: S1
SIMBAD: S star                          WFPDB: S1
SIMBAD: high proper-motion star         WFPDB: S1
SIMBAD: star in double system           WFPDB: S2
SIMBAD: double or multiple star         WFPDB: S2
SIMBAD: spectroscopic binary            WFPDB: S2
SIMBAD: classical cepheid               WFPDB: S3
SIMBAD: eclipsing binary                WFPDB: S3
SIMBAD: variable star                   WFPDB: S3
SIMBAD: semi-regular pulsating star     WFPDB: S3
SIMBAD: pulsating variable star         WFPDB: S3

WFPDB's codes for observation method are:

 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)

Example:

POT017 004781              M193403052016   Y Dra               S30101  60.0Superpan   G        091211100

This metadata set describes a 9×12 cm plate with Superpan emulsion obtained with a 17-cm telescope at the Potsdam observatory (assigned a database plate number 'POT017 004781') on the 5th of March 1934 at 20:16 UT with Y Dra as observation target, which is a variable star (code S3). The observation is a single 60 minute exposure direct image (code 01) with a G filter. Files quality, notes and observer are available for this plate, too. Coordinates are missing (code M).

Notes

The file notes contains metadata for one plate per line. Each line is 80 characters long. It will include any kind of important metadata which doesn't fit into the content framework of other three files, but primarily 1) exposure times in case of multi-exposure plates, 2) objective prism details, 3) observer's notes from observational books.

 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  A1       ---      CONT     Continuation sign (1, 2, ...) or blank
 16-80  A65      ---      NTEXT    Text of note

Example:

POT017 004822  focus: 34.4; observer note: X UMa not found;

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

Quality

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.

 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  A1       ---      CONT     Continuation sign (1, 2, ...) or blank
 16-80  A65      ---      QTEXT    Text of quality information

Example:

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

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

Observer

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

 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  A1       ---      CONT     Continuation sign (1, 2, ...) or blank
 16-57  A42      ---      OBSNAM   Observer's name(s)

Example:

POT017 004781  R.Mueller

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

Discussion

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

Notes

  1. WFPDB
  2. Wide-Field Plate Database (Tsvetkov+ 1997) on VizieR
  3. Catalogue of WFPA