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: http://www.adass.org/adass/proceedings/adass02/P1-3/
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The first task was to decide how to organize the observations into sky regions. Initially this task was done as part of the MAST Scrapbook project where ``representative'' observations were chosen in a set of well defined ``bands'' for a sky region for the WFPC2 instrument. Each sky region was called a ``pointing.'' A ``pointing'' is defined as an area of the sky falling within an instrument's field of view. The project realized that once we had divided the sky into pointings, it would be a simple matter to count the observations and filters falling within a pointing, thus enabling the questions our users had been asking to be answered.
The first step in creating the tables is to determine the sky regions that have been observed by a specific HST imaging instrument. The coordinates for all observations made with a specific instrument are selected and sorted. The first set of coordinates in the list are automatically defined as the coordinates for the first pointing.
All coordinates within the defined ``field of view'' for that instrument are removed from the list of potential pointings. The next unselected observation becomes the second pointing and the same procedure is followed until all observations are assigned as a member of a pointing. It should be noted that while the method for defining a pointing is defined, the individual pointings may change over time as new observations are added for active instruments. During the second step, individual observations are assigned to pointings. An observation may fall in more than one pointing and is then counted as a member of all appropriate pointings. The filter is used to determine which ``band'' the observation is assigned to. The values for the total exposure time for each band in the pointing, the first and last observation date/time, the total number of exposures and the total number of different bands found for that pointing are also calculated. A table containing the list of dataset names with the pointing and band assignments is also created. Currently, pointings tables are available for the WFPC2, STIS (images), and FOC instruments.
The interface for the pointings table is a WWW form which can be found at http://archive.stsci.edu/cgi-bin/point, together with the definition of the pointing radii and filter/band assignments. Users select the instrument pointings table they wish to search. On the subsequent form, users may select a specific target, coordinate ranges, or might decide to search above/below the Galactic or ecliptic plane.
They may choose to look for pointings with observations in more than one filter with or without exposure time constraints in each filter. Users may choose to do time-variability searches by specifying the number of days between the first and last observations in a pointing. See Figure 1 for an example of the form. The search results are displayed as two tables (see Figure 2). The top table is a summary of all pointings found by the search. The second table has a row for each pointing found fitting the search criteria. The number of observations within each band is also a link. When this link is executed, a list of the specific datasets for that pointing/band is displayed. Users may look at previews and also submit archive requests for those data.
We plan to add pointings tables for NICMOS and ACS instruments in the next six months.
In the future, users will be able to search for pointings of ``secondary'' instruments in the context of pointings for a ``primary'' instrument. For instance, perhaps a user is interested in WFPC2 observations, but would also like to know if there are any STIS image observations within the field of view of the pointing. A search will be made of the WFPC2 pointings table. Then a secondary search of the STIS pointings table will be made looking for STIS pointings within the WFPC2 field of view for each WFPC2 pointing. Users will be able to specify filters and exposure times for all secondary instruments in addition to those specified for the primary instrument.
