UseCase: SelectAstrometricStandards

PrimaryActor:

Astronomer: either researcher or data curator.

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EndResult:

List of objects from one selected catalogue with good astrometry.

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OtherActors:

None.

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PreConditions:

Astronomer has an image in a FITS file. The file has an approximate WCS worked out from the telescope telemetry (pointing, PA, etc) and Astronomer wants to compute an accurate WCS.

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FlowOfEvents:

1. Astronomer invokes the command on his/her UserAgent "Get astrometric standards for (image)" naming the image-file in question.
2. System looks at the WCS in the file and derives a search area that enclose the entire image.
3. System selects from the ResourceCatalogue archive tables holding objects with astrometry better than some standard cut-off point.
4. System queries the selected catalogues and receives a table of results for each.
5. System retrieves to Astronomer's local computer the table with the most stars in the search area, and stores the table in a local file.

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PostCondition:

Astronomer has a file of standards to be used as input to his/her local astronometry programme.

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BasicAssumptions:

1. Some standard metadata exist by which System can identify which catalogues are good for astrometry. These are recorded in the ResourceCatalogue.
2. The astrometric solution should be made with results from only one astrometric catalogue. Merging different catalogues introduces unacceptable systematic-errors.
3. Given a set of astrometric catalogues covering the field, the catalogue with the most stars in the field is always the one to use.
4. The UserAgent has controls for a number of standard operations of which this is one.

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AlternativeFlows:

After the main flow, above:

1. System takes a copy of the original file and stores it on the Grid.
2. System run the VO's standard astrometric software on the image copy and the chosen catalogue-extract deriving the WCS.
3. System write the WCS into the copy of the image-file.

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Discussion:

This use case was suggested by the reduction procedures for the INT WFS. In the current procedure, the catalogue is usaully the APM catalogue, and the HST GSC is used only when APM doesn't have enough stars in the field. Doing the solution is a solved problem for the INT WFS. Hence, the alternate flow isn't especially useful.

There is no requirement to obtain and use uncertainties in position for individual objects in the catalogue.

This use case overlaps with SetImageWCS, but the emphasis is different.

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Links to ScienceProblems:

type here

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KeyReferences:

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GoodStyle: Please add comments below. This area should be used for refinement of the above document. If you want to ask questions or start a dialogue with the author, please use (or create) a topic in the Use Cases Forum.
Author: Once the refinements here and comments in the forum die down, perhaps you could rewrite the problem, incorporating the comments and refinements.

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-- GuyRixon - 12 Feb 2002 (as told by JimLewis^?)