This page documents the columns of the exposure-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
Exposure Identity
Name | Unit/Type | Description |
---|---|---|
local_id | nonnegative integer | A unique number identifying this exposure within this daschlab session |
series | short string | The lowercase series name of the plate |
platenum | positive integer | The number of the plate within its series |
mosnum | nonnegative integer | The ID of the FITS mosaic image associated with this exposure's plate |
solnum | nonnegative integer | The ID of the WCS solution number associated with this exposure |
expnum | nonnegative integer | The exposure sequence number from contemporaneous logbooks |
Identifying exposures is somewhat tricky.
Every photographic plate worth scanning was exposed at least once, but some were exposed multiple times. This means that on a given plate image, the same pixel may correspond to multiple different RA/Dec values, and a single RA/Dec may occur at several different pixel locations, if the multiple exposures overlapped each other.
For each plate that was successfully scanned, there is at least one “mosaic”, which is the DASCH terminology for a single large FITS image made from a plate scan. There can in principle be multiple mosaics per plate. Each mosaic is analyzed to search for as many WCS solutions as can be found.
Ideally, each WCS solution corresponds to an exposure of the plate. However, we also have external information about what plates have exposures, based on historical observing logbooks. WCS solutions may or may not be identifiable with known exposures. Sometimes there are fewer WCS solutions than logged exposures; sometimes there are more. Furthermore, the logbooks may provide information about exposures on plates that haven't been scanned at all.
There are therefore essentially two "kinds" of exposures. There are exposures
associated with a particular WCS solution tied to a particular mosaic image
derived from a scan of a particular plate. These exposures are said to "have
imaging". They are uniquely identified by the combination of series
,
platenum
, mosnum
, and solnum
. For these exposures, the expnum
field
might be defined, if the exposure can be matched to an historical logbook
record. The field might be masked if it cannot.
Exposures without associated imaging data correspond to historical logbook
records, and they are uniquely identified by the combination of series
,
platenum
, and expnum
. These plates will have masked mosnum
and solnum
values. For these exposures, the plate in question might or might not have been
scanned: there might be a mosaic available, but no WCS solution that can be
matched to this exposure’s information.
Plate Attributes
Name | Unit/Type | Description |
---|---|---|
class | short string | A classification of the associated 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.
Name | Unit/Type | Description |
---|---|---|
exptime | minutes | The duration of the exposure |
obs_date | Astropy Time | The date of the exposure |
pos | Astropy SkyCoord | The center position of the exposure; can be undefined |
wcssource | short string | Description of the source of this exposure’s positional information |
When an exposure list is exported, the pos
column may need to be broken into
individual columns. In this case you will instead see:
Name | Unit/Type | Description |
---|---|---|
ra_deg | deg | The RA encoded in pos |
dec_deg | deg | The 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
Name | Unit/Type | Description |
---|---|---|
binflags | bit flags | Which rebinnings of the FITS mosaic have been created |
scannum | nonnegative integer | The ID of the scan associated with the digitized form of the plate |
scan_date | Astropy Time | The date-time at which the scan was made |
mos_date | Astropy Time | The date-time at which the mosaic was made |
rotation_deg | small integer | The rotation applied to the mosaic in order to make North be up |
This fields are only defined if the exposure has associated imaging.
Data Quality
Name | Unit/Type | Description |
---|---|---|
reject | bit flags | Flags that you set to mark points as rejected |
Search Information
Name | Unit/Type | Description |
---|---|---|
center_distance | cm | The physical distance between the search position and the plate center |
edge_distance | cm | The 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 exposures without imaging: 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 exposures without imaging: yes (but accuracy may be poor)
If you generate an exposure 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 exposure does not have associated imaging, this field will still be available. The logged exposure 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 exposures without imaging: 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 exposures without imaging: yes (but accuracy may be poor)
If you generate an exposure 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 exposure does not have associated imaging, this field will still be available. The logged exposure center will be used for this computation rather than a precise computation. The resulting number may have poor accuracy.
expnum
- Synopsis: The plate-relative ID of the exposure
- Units/type: Nonnegative integer
- Available for exposures without imaging: yes
Each plate in the DASCH database is associated with one or more exposures. Different exposures of the same plate are numbered in the database sequentially starting at 0. This field gives this number.
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 exposure
- Units/type: Minutes
- Available for exposures without imaging: yes
This field gives the duration of the exposure.
Exposure time 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 exposure within this daschlab session
- Units/type: Nonnegative integer
- Available for exposures without imaging: yes
Every exposure in a daschlab exposure list is assigned a unique "local id", which is its index within the session’s top-level exposure 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 exposures without imaging: 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 this exposure
- Units/type: Nonnegative integer
- Available for exposures without imaging: 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 exposure
- Units/type: Astropy Time
- Available for exposures without imaging: yes
This field gives the date of the exposure that caused this plate to match a
spatial query (see expnum
), as a geocentric JD of the exposure
midpoint.
Observation dates are not always known with precision. Due to technical
limitations, exposure-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 exposures without imaging: yes
The number of a plate within its series
. These two items uniquely
identify each physical plate.
pos
- Synopsis: The center position of the exposure
- Units/type: Astropy SkyCoord
- Available for exposures without imaging: yes (but accuracy may be poor)
This field gives the center position of the exposure, 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 observing logs. The resulting number may have poor accuracy.
On the other hand, this column will occasionally be undefined, even if a plate has been successfully scanned. In this case, the RA and Dec values will be NaN. This situation seems to arise when a multiple-exposure plate is only partially WCS-solved.
When an exposure 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 exposures without imaging: 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 exposure-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 exposures without imaging: 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 exposures without imaging: 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 exposures without imaging: 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 exposures without imaging: 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 associated with this exposure
- Units/type: Nonnegative integer
- Available for exposures without imaging: 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 associated with this exposure.
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 exposures without imaging: 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 exposure-list table is defined to contain only the following columns:
series
platenum
class
expnum
solnum
obs_date
exptime
pos
(orra_deg
anddec_deg
)wcssource
mosnum
rotation_deg
center_distance
edge_distance