The Wide-Field Plate Database (WFPDB) 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: maindata, notes, quality and observer. (The other file archives mentioned there is strictly speaking intended for the Catalogue of WFPA.) 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.
- 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