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 | Center of Ra Axis | |
| center_position | s_dec | Center of Dec Axis | |
| field_of_view | s_fov | 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 | 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 | 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 | Maximum of Spectral Axis | |
| em_res_power | Spectral CRDELn Keyword | ||
| em_xel | 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 | More 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 | 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 | |||
| rms_noise | |||
| 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 of interest upon that axis. For instance, the center of an axis longer than 1 is given by:
CRVALn + CRDELn*(NAXISn/2 - CRPIXn)
There 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 | Comment |
|---|---|---|---|
| image_set_id | obs_id | automatically generated | Unique Identifier for the Image Set |
| project_code | Required to facilitate Ingestion | ||
| configuration | The weblog contains min/max baseline information | This will need to hold the entire list used for the imaging. | |
| collection_name | obs_collection | ||
| calib_level |
VO ObsCore Remaining Fields:
| VO Requirement | Value | Source |
|---|---|---|
access_url | ||
| access_estsize | files.filesize | |
| dataproduct_type | 'image' | default |
| access_format | 'fits' | default |
obs_publisher_did | default | |
| facilty_name | 'NRAO' | default |