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Astronomical Data Analysis Software and Systems XII ASP Conference Series, Vol. 295, 2003 H. E. Payne, R. I. Jedrzejewski, and R. N. Hook, eds.

The MAST Pointings Tables Pro ject
Karen Levay, Paolo Padovani1 , Randall Thompson, Megan Donahue, Michael Corbin Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 Abstract. We have undertaken a pro ject to create a database of all HST imaging observations organized by position on the sky. A World Wide Web (WWW)-based interface to this database has also been created, in supplement to the existing interface to the HST archive. These "pointings tables" enable quick identification of overlapping fields that can be used for multi-wavelength studies of ob jects and for variability studies by comparing images at given positions over different epochs. They will also allow for "mini-surveys" by providing lists of images over a specified coordinate range, such as above and below the Galactic plane. We plan to develop pointings tables for other MAST missions besides HST and expect these tables to form an important part of the upcoming virtual observatories.

1.

Introduction

A question often asked of the HST archive has been: "Is there an easy way to find out how many times a region of the sky has been observed with more than one filter?" Now that question and others may be easily answered using the HST "pointings" tables and interface. The first task was to decide how to organize the observations into sky regions. Initially this task was done as part ofthe MAST Scrapbook pro ject 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 pro ject 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. 2. Creating the Tables

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
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ESA Space Telescope Division

77 c Copyright 2003 Astronomical Society of the Pacific. All rights reserved.


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Levay, Padovani, Thompson, Donahue & Corbin

Figure 1.

Pointing search specification page.

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 MAST Pointings Tables Pro ject

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Figure 2. 3.

Pointings search output.

The Interface

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.


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Levay, Padovani, Thompson, Donahue & Corbin Future Plans

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 ofpointings 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.