DRnext: Plate List Data Table Columns

This page documents the columns of the plate-list data tables exposed by the daschlab analysis toolkit. Different data sources (e.g. the “Cannon” data access portal) may use different names or expose somewhat different sets of data.

Columns by Category

Plate Identity

NameUnit/TypeDescription
platenumpositive integerThe number of the plate within its series
local_idnonnegative integerA unique number identifying this plate within this daschlab session
seriesshort stringThe lowercase series name of the plate

Plate Attributes

NameUnit/TypeDescription
classshort stringA classification of the plate’s type, assigned historically

Exposure-Specific Attributes

Some plates were exposed multiple times. Each exposure has its own position on the sky, exposure time, etc.

NameUnit/TypeDescription
expnumnonnegative integerThe ID of the exposure that caused this plate to match the query
exptimeminutesThe duration of the matching exposure
obs_dateAstropy TimeThe date of the matching exposure
posAstropy SkyCoordThe center position of the matching exposure
solnumnonnegative integerThe ID of the WCS solution number associated with this exposure
wcssourceshort stringDescription of the source of the WCS information

When a plate list is exported, the pos column may need to be broken into individual columns. In this case you will instead see:

NameUnit/TypeDescription
ra_degdegThe RA encoded in pos
dec_degdegThe declination encoded in pos

If obs_date cannot be represented as an Astropy Time object, it will be expressed as a geocentric Julian Date.

Digitization

NameUnit/TypeDescription
binflagsbit flagsWhich rebinnings of the FITS mosaic have been created
scannumnonnegative integerThe ID of the scan associated with the digitized form of this plate
scan_dateAstropy TimeThe date-time at which the scan was made
mosnumnonnegative integerThe ID of the FITS mosaic associated with the digitized form of this plate
mos_dateAstropy TimeThe date-time at which the mosaic was made
rotation_degsmall integerThe rotation applied to the mosaic in order to make North be up

Data Quality

NameUnit/TypeDescription
rejectbit flagsFlags that you set to mark points as rejected

Search Information

NameUnit/TypeDescription
center_distancecmThe physical distance between the search position and the plate center
edge_distancecmThe physical distance between the search position and the nearest plate edge

Detailed Column Information (Alphabetical)

binflags

  • Synopsis: Internal field; which rebinnings of the FITS mosaic have been created
  • Units/type: Bit flags
  • Available for unscanned plates: no

This is an internal field used to track which rebinnings of the FITS plate mosaic have been created. In the modern DASCH pipeline, the only binnings used are 1×1 (full resolution) and 16×16, and this field should always be 9 for plates that have been scanned. This column is provided for completeness but is not expected to be useful for analysis.

center_distance

  • Synopsis: The physical distance between the search position and the plate center
  • Units/type: cm
  • Available for unscanned plates: yes (but accuracy may be poor)

If you generate a plate list based on a spatial query (which is the typical case), this column will give the physical distance between the query center and the center of the plate. This quantity is particular to the matching exposure. If the plate contains multiple exposures, it is possible that your query center might appear on the plate in two different physical locations!

You can use this information to identify plates where a target position is too close to the plate edge, where the data are likely to be of poor quality.

If the plate has not been scanned and WCS-solved, this field will still be available. The logged plate center will be used for this computation rather than a precise computation. The resulting number may have poor accuracy.

class

  • Synopsis: A classification of the plate’s type, assigned historically
  • Units/type: Short string
  • Available for unscanned plates: yes

See the Plate Stacks summary of plate classes for information about the historical classification scheme. Plate classifications conveyed a grab-bag of information about sky position, imaging type (direct, spectra, etc.), emulsion, exposure count, and so on.

Plate class information is often missing from the DASCH databases, and when present it may not be of high accuracy. If you are doing anything that relies on the classes, you should examine photos of the plate jackets to confirm that your data are reliable.

Most plate class information is not very useful in the modern context, but it can provide information about which photographic emulsion was used, which determines the color response of each plate. At the moment, checking the plate class is one of the ways to address the Unspecified Emulsions known issue.

edge_distance

  • Synopsis: The physical distance between the search position and the nearest plate edge
  • Units/type: cm
  • Available for unscanned plates: yes (but accuracy may be poor)

If you generate a plate list based on a spatial query (which is the typical case), this column will give the physical distance between the query center and the nearest edge of the plate. This quantity is particular to the matching exposure. If the plate contains multiple exposures, it is possible that your query center might appear on the plate in two different physical locations!

You can use this information to identify plates where a target position is too close to the plate edge, where the data are likely to be of poor quality.

If the plate has not been scanned and WCS-solved, this field will still be available. The logged plate center will be used for this computation rather than a precise computation. The resulting number may have poor accuracy.

expnum

  • Synopsis: The ID of the exposure that caused this plate to match the query
  • Units/type: Nonnegative integer
  • Available for unscanned plates: yes

Each plate in the DASCH database is associated with one or more exposures. They are numbered in the database sequentially starting at 0. This field indicates which exposure caused the plate to match a query.

Exposure information is obtained from historical logbooks, and so is available even for plates that haven't been scanned.

For plates that have been scanned, the pipeline attempts to match every exposure to a WCS solution obtained from analysis of its image. The ordering of the WCS solutions (see solnum) may not match the ordering of the exposures. Sometimes, no empirical WCS solution is obtained for a known exposure. Other times, a WCS solution is obtained that does not correspond to any known exposure. In the latter case, this field will be undefined.

exptime

  • Synopsis: The duration of the matching exposure of this plate
  • Units/type: Minutes
  • Available for unscanned plates: yes

This field gives the duration of the exposure that caused this plate to match a spatial query (see expnum).

Exposure information is obtained from historical logbooks, and so is available even for plates that haven't been scanned.

local_id

  • Synopsis: A unique number identifying this plate within this daschlab session
  • Units/type: Nonnegative integer
  • Available for unscanned plates: yes

Every plate in a daschlab plate list is assigned a unique "local id", which is its index within the session’s top-level plate list. Local IDs should only be used internally in an analysis; they are not meaningful except with reference to the particular session in which they were generated.

mos_date

  • Synopsis: The date-time at which the FITS mosaic of the plate’s scan was made
  • Units/type: Astropy Time
  • Available for unscanned plates: no

This is the time when the FITS mosaic of this plate was created. It is included for completeness but is not expected to be relevant to data processing.

mosnum

  • Synopsis: The ID of the FITS mosaic associated with the digitized form of this plate
  • Units/type: Nonnegative integer
  • Available for unscanned plates: no

After DASCH scans a plate, the raw scan data are “mosaicked” to create a large FITS image of the plate. Sometimes, multiple mosaics of the same plate need to be created. These mosaics are distinguished by a small increasing integer, the mosaic number. This column gives the relevant mosaic number for this plate.

This field is used to identify and obtain the FITS mosaic files but otherwise shouldn’t be relevant to your analysis.

obs_date

  • Synopsis: The date of the matching exposure of this plate
  • Units/type: Astropy Time
  • Available for unscanned plates: yes

This field gives the date of the exposure that caused this plate to match a spatial query (see expnum), as a geocentric JD.

Observation dates are not always known with precision. Due to technical limitations, plate-list tables do not currently provide information about the accuracy of this field. Lightcurve tables provide a time_accuracy column that quantifies the date accuracy.

For precise timing of astronomical events, observation times need to be converted into a barycentric or heliocentric frame. Note that many plates are sufficiently large on the sky that this conversion will give appreciably different results for sources at different plate locations.

Exposure information is obtained from historical logbooks, and so is available even for plates that haven't been scanned.

platenum

  • Synopsis: The number of the plate within its series
  • Units/type: Positive integer
  • Available for unscanned plates: yes

The number of a plate within its series. These two items uniquely identify each physical plate.

pos

  • Synopsis: The center position of the matching exposure on this plate
  • Units/type: Astropy SkyCoord
  • Available for unscanned plates: yes (but accuracy may be poor)

This field gives the center position of the exposure that caused this plate to match a spatial query (see expnum), in the ICRS J2000 frame.

If the plate has not been scanned and WCS-solved, this field will still be available. It will provide the plate center as recorded in the hand-written osberving logs. The resulting number may have poor accuracy.

When a plate list is exported, the pos column may need to be broken into individual columns. In this case you will instead see ra_deg and dec_deg.

reject

  • Synopsis: Flags that you set to mark points as rejected
  • Units/type: Bit flags
  • Available for unscanned plates: yes

This column stores user-defined rejection flags as explained in the Lightcurve Reduction Guide. While that documentation refers to lightcurve data tables, the same philosophy and machinery applies to plate-list tables. If you do not perform any rejections, this column will be filled with zeroes.

rotation_deg

  • Synopsis: The rotation applied to the mosaic in order to make North be up
  • Units/type: small integer
  • Available for unscanned plates: no

During the DASCH scanning and astrometric-solving process, the FITS mosaic of each plate is rotated to ensure that North is up in the final image. This rotation is done coarsely, in increments of 90 degrees. This field gives that level of rotation. It will be equal to one of 0, 90, 180, or 270.

The amount of rotation needed depends on factors like the orientation of the plate in the scanner, and so will not generally be of interest for scientific analysis. However, for some legacy purposes, the rotation value is needed to look up FITS mosaic files.

scan_date

  • Synopsis: The date-time at which the scan was made
  • Units/type: Astropy Time
  • Available for unscanned plates: no

This is the time when the plate was scanned. It is included for completeness but is not expected to be relevant to data processing. Note that plates were sometimes scanned more than once (see scannum).

scannum

  • Synopsis: The ID of the scan associated with the digitized form of this plate
  • Units/type: Nonnegative integer
  • Available for unscanned plates: no

DASCH scanned some plates more than once. Different scans of the same plate are distinguished by a small increasing integer, the scan number. This column gives the relevant scan number associated with the digitized version of this plate.

There is generally a 1:1 correspondence between scan numbers of mosaic numbers, but some scans did not have any mosaics made of them, and some scans had multiple mosaics made. Because the mosnum is what identifies the specific FITS file associated with a plate, it is generally the more important quantity, although neither field should matter for scientific analysis.

series

  • Synopsis: The lowercase series name of the plate
  • Units/type: Short string
  • Available for unscanned plates: yes

In conjunction with the plate number, the series uniquely identifies a physical plate.

See the Plate Stacks summary of plate series for some summary information about the different series.

solnum

  • Synopsis: The ID of the WCS solution that caused this plate to match the query
  • Units/type: Nonnegative integer
  • Available for unscanned plates: no

Each plate in the DASCH database is associated with one or more exposures. For plates that have been scanned, the pipeline attempts obtain detailed WCS solutions for each exposure. Successful solutions are numbered in the database sequentially starting at 0. This column gives the sequence number of the solution that caused this plate to match the query.

The ordering of the WCS solutions may not match the ordering of the exposures (see expnum). Sometimes, no empirical WCS solution is obtained for a known exposure. In this case, position-based matching can still occur using the logged exposure location. Such matches will appear as rows in the plate list where expnum is defined but this field is not.

Other times, a WCS solution is obtained that does not correspond to any known exposure. In those cases, this field will be defined but expnum will be blank.

wcssource

  • Synopsis: Description of the source of the WCS information
  • Units/type: Short string
  • Available for unscanned plates: yes

This is a string describing the origin of the positional information for this match. Allowed values are "none", "logbook", or "imwcs".

The value "logbook" indicates that the positional information comes from a transcribed observing log. Due to rounding and transcription errors, this information may not be very accurate.

The value "none" does not imply that there is no positional information for the plate. Instead, it is equivalent to "logbook", but for plates that haven't yet been scanned.

The value "imwcs" indicates that the positional information comes from an astrometric fit to the stars in a scanned plate image.

Different exposures on the same plate may have different WCS source values, depending on which ones were able to be solved and which ones weren't.

Column Subsets

“Medium” Columns

A “medium”-width plate-list table is defined to contain only the following columns: