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 images table was written specifically with the idea of spacial images in mind. Other data products (spectra, time series, etc) would have different amounts of granularity in the header information available.
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.
Images Table:
| Column Name | VO Equivalent | Source | Comments |
|---|
| target_name | target_name | OBJECT Keyword |
|
| center_position | s_ra |
Calculated from DataCalculated from DataCalculated from Spacial Axis Data| Average of NAXISn*CRDELn of the Spacial Axes |
|
| s_region | Derived from Spacial Data | STC-S String Defined in the TAP (Dowler, et al 2010) |
| s_xel1 | Relevent NAXISn Keyword |
|
| s_xel2 | Relevent NAXISn Keyword |
|
| spacial_resolution | s_resolution |
Calculated from Spacial Axis Data| Average of CRDELn of the Spacial Axes |
|
| t_max |
| Temporal domain information for an image is dependent upon the observation(s) used in its generation. Some controlling process (Like Vlass Manager, whatever imaging pipeline we create, and any image manipulation software we use) will need to supply it. Since we are intending to store cubes as a collection of images, t_xel will can be removed and automatically returned as 1 for any individual image.
|
| t_min |
|
| t_exptime |
|
| t_xel |
|
| observing_band | em_min |
Calculated from | Minimum of Spectral Axis |
The spectral information is recoverable from the FITS header, and will identify the observing receiver(s), and total bandwidth processed for the image. |
| observing_band | em_max |
Calculated from | Maximum of Spectral Axis |
| em_res_power |
Relevant | Spectral CRDELn Keyword |
| em_xel |
Relevent | Spectral 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 StokesMore clarity is required for polarization information, as the exemplar files (Vlass quicklook images) have limited information available. Some method of conveying information about images (in CTYPE/CUNIT keywords, or via outside information) is required to get anything other than an overly simplistic assumption (NAXISn = 1 means Stokes I and NAXISn = 4 means Full Stokes Parameters). |
| pol_xel | Relevent NAXISn Keyword |
| telescope | instrument_name | TELESCOP Keyword | The observing instrument(s) |
| image_id | obs_id | automatically generated | Unique Identifier for the Image |
| file_id |
| automatically generated | link to information about the physical file |
| max_intensity |
Calculated from FITS Data Unit
|
|
These values are can be determined from the data portion of the image file in conjunction with the BSCALE & BZERO keywords. The library which is used for FITS interaction for ingestion can perform these calculations, but no information about the speed of results is yet available.
|
| min_intensity |
Calculated from FITS Data UnitCalculated from FITS Data Unit |
|
|
| thumbnail |
| weblog, a PNG with identical name |
|
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 longer than 1 is given by:
CRVALn + CRDELn*(NAXISn/2 - CRPIXn)
This will be used in determining the the central position of the imageThere are similar formulae for the minimum and maximum values of an axis.
Image Sets 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)( and/or VLASS Manager)| contains min/max baseline information | This will need to hold the entire list used for the imaging. |
| collection_name | obs_collection |
(VLASS Manager initially) |
VO ObsCore Remaining Fields:
| VO Requirement | Value | Source |
|---|
| dataproduct_type | 'image' | default |
access_url |
|
|
| access_estsize |
| files.filesize |
| dataproduct_type | 'image' | default |
| access_format | 'fits' | default |
obs_publisher_did |
| default |
| facilty_name | 'NRAO' | default |
