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 spatial 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 spatial region information
- Determination and acquisition of time-domain information.
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Images
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Images Database Table:
| Column Name | Units | VO Equivalent | Source | Comments |
|---|---|---|---|---|
| sourcename | target_name | OBJECT Keyword | ||
| ra |
| deg | s_ra | Center of Ra Axis |
| dec | deg | s_dec | Center of Dec Axis | |
| image_field_of_view | deg | s_fov |
| Quadratic Mean of the ra & dec extents (NAXISn*CDELTn) | ||||
| spatial_region | s_region | Derived from spatial Data | STC-S String Defined in the TAP (Dowler, et al 2010) | |
| ra_element_count | s_xel1 |
| Relevant NAXISn Keyword | ||||
| ra_pixel_size | abs(Relevant CDELTn) | |||
| dec_element_count | s_xel2 |
| Relevant NAXISn Keyword | ||||
| dec_pixel_size | abs(Relevant CDELTn) | |||
| spatial_resolution | arcsec | s_resolution |
| 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 | em_min | Minimum of Spectral Axis |
The spectral information is recoverable from the FITS header, and will identify the spectral coverage, and total bandwidth processed for the image.
| max_frequency |
| Hz | em_max | Maximum of Spectral Axis |
| rest_frequency | Hz | RESTFRQ Keyword | Used for transformations | |
| band_code | weblog | Requires outside information source for accuracy. | ||
polarization_id | pol_states |
Currently, there is insufficient information provided about the polarization information of the image in the quicklook FITS headers. There is a STOKES axis, but no further information is provided.
Since Single Epoch images will be created in 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. | ||||
| telescope | instrument_name | TELESCOP Keyword | 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 | o_ucd | BTYPE & BUNIT Keywords | Description of the physical quantity measured in the image | |
| max_intensity | image_units | image table values | ||
| min_intensity |
| image_units | image table values | |||
| rms_noise | 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:
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Maximum: CRVALn + CDELTn*(NAXISn - CRPIXn)
There are similar formulae for the minimum and maximum values of an axis.
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:
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| 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 | VlassMgr | This will need to hold the entire list used for the imaging. | |
| collection_name | obs_collection | 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 |
|---|---|---|
access_url | ||
| access_estsize | files.filesize, or combined value for an image set | |
| dataproduct_type | 'image' | default |
| access_format | 'fits' | default |
obs_publisher_did | Obtained upon registering with the Virtual Observatory | |
| faciltyfacility_name | 'NRAO' | default |
| t_resolution | images.image_integrationexposure_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