Data Products

The UVOT can produce seven types of data products: four science products and three engineering products. A description of the science produces is given below.

Event List.

Each photon event is reported by its position on the detector and the associated CCD frame time-stamp. The accuracy of photon arrival time is dominated by the resolution of the CCD frame integration period ($\approx 11$ ms).

Image.

Image data consist of a 2-D histogram (image) of the event stream taken over the specified integration period (generally 10-1000 s). Image pixels correspond to individual detector sub-pixels ($1 \times 1$ binning) or binned detector sub-pixels (e.g., $2 \times 2$ or $4 \times 4$ binning). For large numbers of events, image data require much less telemetry than event data, at the expense of greatly reduced time resolution.

Finding Chart.

Finding Chart data consist of a small subset of the pixels in an image, carefully chosen to lie under and near the brightest point sources. The number of pixels transmitted per source depends on the total number of sources above a threshold brightness. Pixels that are not in an N $\times$ N grid centered on the brightest pixel in a source are not transmitted to the ground. N is 1, 3, or 5 depending on the number of bright sources in the field. A ``sparse'' version of the full image can be constructed on the ground, providing a coarse representation of the detected point sources while consuming modest telemetry volume. Finding chart data are sent immediately via TDRSS to the GCN, whereas most other science data are stored for later downlinks via a ground station.

GRB Neighborhood Image (GeNIe).

All of the data in the $324 \times 324$ Finding Chart pixels around the XRT position are sent to the ground. The purpose of the GeNIe, also called the dark burst image, is to transmit quickly to the ground more definitive information about the sources close to the XRT or BAT GRB position. Instead of transmitting a small number of pixels for each source, the GeNIe is a full image, with each pixel populated with the actual counts received by the detector. The GeNIe is sent immediately after the Finding Chart via TDRSS to the GCN.