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.
Below is the proposed new table structure for Images and Image Products in the metadata database. There are 4 classes of columns below: initial columns, new columns, columns adapted from (previous_column), and columns renamed to match their equivalents in the observation tables. Depending on the design decisions and the behavior of CASA, some of these fields could be removed (t_xel, em_xel, pol_xel) if they they will be invariant.
Images Database Table:
| Column Name | Units | VO Equivalent | Source | Comments | |||
|---|---|---|---|---|---|---|---|
| sourcename | target_name | OBJECT Keyword | |||||
| ra (center_position) | deg | s_ra | Center of Ra Axis | ||||
| dec (center_position)dec | deg | s_dec | Center of Dec Axis | ||||
| image_field_of_view | deg | s_fov | Average of NAXISn*CRDELn of the spatial AxesQuadratic 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 | Relevent Relevant NAXISn Keyword | |||||
| ra_pixel_size | abs(Relevant CDELTn) | ||||||
| dec_element_count | s_xel2 | Relevent 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.Average of CDELTn of the spatial Axes, converted from deg | |||||
| starttime | MJD | t_maxmin | These values should be obtained by some larger-scale process. VlassMgr, in the case of quicklook imagesTemporal 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. | ||||
| endtime | MJD | t_minmax | |||||
| exposure_time | s | t_exptime | |||||
| min_frequency observing_band) | Hz | em_min | Minimum of Spectral Axis | ||||
| max_frequency (observing_band) | Hz | em_max | Maximum of Spectral Axis | spectral_resolution | unitless | em_res_power | |
| rest_frequency | Hz | RESTFRQ Keyword | Used for transformations | ||||
| band_code | weblog | Requires outside information source for accuracy. | Center Frequency/CDELTn|||||
polarization_id (reverting to initial name) | pol_states | Polarization CRVALn value | CASA uses the CRVALn value to convey polarization information, with [1,2,3,4] mapped to [I,Q,U,V]. When we begin dealing with single-polarization images, this will need to be expanded. 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 | 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.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, a PNG with identical namefind 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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| 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 | |
| facility_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 |
Thumbnail
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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
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- of the sha1 sum, $2 the second two
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- , and $3 the third two.
- in the metadata database we will store the $1/$2/$3/$FILENAME path
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