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.
Images Table:
| Column Name | VO Equivalent | Source | Comments |
|---|---|---|---|
| target_name | target_name | OBJECT Keyword | |
| center_position | s_ra | Calculated from Axis Data | |
| center_position | s_dec | Calculated from Axis Data | |
| field_of_view | s_fov | Calculated from Spacial Axis Data | |
| 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 | |
| t_max | |||
| t_min | |||
| t_exptime | |||
| t_xel | |||
| observing_band | em_min | Calculated from Spectral Axis | |
| observing_band | em_max | Calculated from Spectral Axis | |
| em_res_power | Relevant CRDELn Keyword | ||
| 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 |
| 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 | ||
| min_intensity | Calculated from FITS Data Unit | ||
| rms_noise | Calculated from FITS Data Unit | ||
| thumbnail | weblog, a PNG with identical name |
FITS Data Description Keywords:
For the purpose of generality, FITS provides a detail-independent method of data access. It's easier to think of the data axis descriptors in groupings by their axis number. The NAXIS keyword provides the total number of dimensions within the data. For the nth dimension of the data, we have a set of descriptor keywords which should be considered and used together:
- NAXISn - Total data size along this axis
- CRPIXn - Our reference location
- CRVALn - The physical value at our reference location
- CDELTn - The increment along the axis
- CTYPEn - The axis label
- CUNITn - The axis units
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 upon that axis. For instance, the center of an axis is given by:
CRVALn + CRDELn*(NAXISn/2 - CRPIXn)
This will be used in determining the the central position of the image.
Image Sets Table:
| Column Name | VO Equivalent | Source | Description |
|---|---|---|---|
| image_set_id | obs_id | automatically generated | Unique Identifier for the Image Set |
| project_code | Provided to Ingestion (VLASS Manager to start) | ||
| configuration | (weblog and/or VLASS Manager) | ||
| collection_name | obs_collection | (VLASS Manager initially) | |
| calib_level |
VO ObsCore Remaining Fields:
| VO Requirement | Value | Source |
|---|---|---|
| dataproduct_type | 'image' | default |
access_url | ||
| access_estsize | files.filesize | |
| access_format | 'fits' | default |
obs_publisher_did | default | |
| facilty_name | 'NRAO' | default |