Below are two sets of metadata which will be stored in the new archive in regard to Images and the Image Sets to which they belong. Included where relevant are the equivalent fields in the Virtual Observatory ObsCore Data Model, a potential source from which to obtain the data, and a comment. The existing images table covers much of the desired data. The modifications we require are:
- Split center_position and observing_band fields
- Retaining image size data (the VO _xel fields)
- Add fields for spectral resolution, physical data description, and spacial region information
- Determination and acquisition of time-domain information.
was written specifically with the idea of spatial images in mind. Other data products (spectra, time series, etc) would have different amounts of granularity in the header information available.
Images Database
...
Table:
| Column Name | Units | VO Equivalent | Source | Comments |
|---|
target_name| sourcename |
| target_name | OBJECT Keyword |
|
| ra |
center_positionCalculated from Axis Data | center_positionCalculated from Data |
|
| image_field_of_view | deg | s_fov |
Calculated from Spacial Axis Data| Quadratic Mean of the ra & dec extents (NAXISn*CDELTn) |
|
| spatial_region |
| s_region | Derived from |
Spacial | spatial Data | STC-S String Defined in the TAP (Dowler, et al 2010) |
| ra_element_count |
| s_xel1 |
Relevent | Relevant NAXISn Keyword |
|
| ra_pixel_size |
|
| abs(Relevant CDELTn) |
|
| dec_element_count |
| s_xel2 |
Relevent spacial |
|
| dec_pixel_size |
|
| abs(Relevant CDELTn) |
|
| spatial_resolution | arcsec | s_resolution |
Calculated from Spacial Axis Data | t_max | t_min | t_exptime | t_xel | | sqrt(BMAJ*BMIN) | Geometric mean of the synthesized beam axes. |
| beam_axis_ratio |
|
| BMAJ/BMIN | Ratio of the synthesized beam axes. |
| starttime | MJD | t_min |
| These values should be obtained by some larger-scale process. VlassMgr, in the case of quicklook images. |
| endtime | MJD | t_max |
|
| exposure_time | s | t_exptime |
|
| min_frequency | Hz |
observing_bandCalculated from observingbandCalculated from em_res_power | Relevant CRDELn Keyword |
|
| rest_frequency | Hz |
| RESTFRQ Keyword | Used for transformations |
| band_code |
|
| weblog | Requires outside information source for accuracy. |
em_xel | Relevent NAXISn Keyword | o_ucd | BTYPE & BUNIT Keywords | Description of the physical quantity measured in the image |
polarization_id |
| pol_states |
Deduced from NAXISn Keyword | NAXISn = 1 → I NAXISn = 4 → Full Stokes | | Polarization CRVALn value | CASA uses the CRVALn value to convey polarization information, with [1,2,3,4] mapped to [I,Q,U,V]. Default to 'None' in case of other values. |
pol_xel | Relevent NAXISn Keyword |
| telescope |
| instrument_name | TELESCOP Keyword |
The observing instrument(s) | | This must accommodate images for multiple instruments (i.e. VLA + Single Dish) |
| file_id |
|
| automatically generated | link to information about the physical file |
| image_id |
| obs_id | automatically generated | Unique Identifier for the Image |
| image_units |
fileiducd | BTYPE & BUNIT Keywords | Description of the physical quantity measured in the image |
automatically generated | link to information about the physical fileCalculated from FITS Data Unit| image_units |
| image table values |
|
| min_intensity |
| image_units |
| image table values |
Calculated from FITS Data UnitCalculated from FITS Data Unit | thumbnail | weblog, a PNG with identical name | image_units |
| weblog | Both of these will need to come from the last stage of the imaging process, to maintain accuracy. For quicklook images, that is stage7. That may change (and will likely be different for the Single Epoch products). |
| thumbnail |
|
| weblog, find the matching thumbnail image generated. |
| tags |
|
|
| Internal tagging system to facilitate searches. |
| ra_pixel_size |
|
| Appropriate CDELTn |
|
| dec_pixel_size |
|
| Appropriate CDELTn |
|
FITS Data Description Keywords:
...
The CTYPEn and CUNITn values provide information about the axis to which this group of values applies. The rest of the keywords can then be used to calculate points of interest upon that axis. For instance, the center of an axis is given by:, in axes longer than 1, we have:
Minimum: CRVALn + CDELTn*(1-CRPIXn)
Center: CRVALn + CDELTnCRVALn + CRDELn*(NAXISn/2 - CRPIXn)
This will be used in determining the the central position of the image.
Maximum: CRVALn + CDELTn*(NAXISn - CRPIXn)
For the Frequency axis, which only has a single point (NAXISn=1), our calculations are simpler:
Minimum: CRVALn - CDELTn/2
Center : CRVALn
Maximum: CRVALn +CDELTn/2
Image Sets Database Table:
The Image Set information will need to come from outside sources, as most of the information is not guaranteed to be in the FITS files themselves. Vlass Manager holds all the needed information for their images, but future development will need to provide the relevant metadata as image sources broaden beyond VLASS.
...
| Column Name | VO Equivalent | Source |
|---|
Description| Comment |
|---|
| image_set_id | obs_id | automatically generated | Unique Identifier for the Image Set |
| project_code |
Provided
| Required to facilitate Ingestion |
(VLASS Manager to start)(weblog and/or VLASS Manager)
| VlassMgr | This will need to hold the entire list used for the imaging. |
| collection_name | obs_collection |
(VLASS Manager initially) | VlassMgr |
|
| calibration_level | calib_level | VlassMgr | As defined by the VO in their 0-4 system |
| product_file_id |
| automatically generated | Link to the imaging products tar file |
| tags |
|
| internal tags which apply to all images of the set |
VO ObsCore Remaining Fields:
| VO Requirement | Value | Source |
|---|
dataproduct_type | 'image' | default |
|---|
access_url |
|
|
| access_estsize |
| files.filesize, or combined value for an image set |
| dataproduct_type | 'image' | default |
| access_format | 'fits' | default |
obs_publisher_did |
default | facilty
| Obtained upon registering with the Virtual Observatory |
| facility_name | 'NRAO' | default |
| t_resolution |
| images.exposure_time |
| t_xel | 1 | default |
| em_res_power | null | default |
| em_xel | 1 | default |
| pol_xel | 1 | default |
Thumbnails
We won't store thumbnails in NGAS as such, for each we will:
- compute the sha1 hash of the file
- store the file in a filesystem at $ROOT/$1/$2/$3/$FILENAME, where $ROOT is a CAPO property that maps to the root of the filesystem, $1 is the first two characters of the sha1 sum, $2 the second two, and $3 the third two.
- in the metadata database we will store the $1/$2/$3/$FILENAME path
