25.4 WFPC2 Paper Products

The current paper product package consists of two parts. The first is a visit-level overview of the datasets in the package, meant to be similar to the observing log in ground-based observations. The information is presented in three separate tables: Target List, Observation List, and Observation Statistics. The Target List gives the targets observed in this visit, with their positions. The Observation List includes essential information on each dataset, such as rootname, target name, operating mode, aperture, filter, exposure time, and a number of flags intended to alert the user to any procedural problem with the data. The flags are represented graphically by circles, open when the data are okay, filled if any (potential) problem is encountered, to be detailed in the second part. The Observation Statistics is a table with basic image statistics for each dataset, such as mean counts, background level, and a rough estimate of the limiting magnitude that can be reached with respect to the background level in the image. Note that this limiting magnitude is computed with a rather simple, all-purpose algorithm, and is meant only as an approximate indication; it should not be used for any detailed calculations. Image statistics are given separately for each CCD.

The second part of the package presents the data exposure by exposure. Usually there are three pages for each exposure: the first two pages are grayscale representations of the image (one containing all four chips together, the other featuring the PC only), and the third summarizes various exposure characteristics, flags and error conditions that might arise. The grayscale presentation on the first page of the four chips together is a crude mosaic; each chip is rotated and placed in the correct position with respect to the others, but the overlap and the small differential rotations between chips (see Chapter 24) are not corrected for. The next page presents the PC by itself, at a larger scale that brings out smaller details not evident in the mosaic. This third page, giving dataset information, is divided into several zones, each summarizing information on a specific topic: the HST Spacecraft Performance Summary, with information on pointing and jitter; the Pipeline Processing Summary, with information on whether the file was processed properly by the pipeline (see Chapter 26); the Calibration Data Quality Summary, where any inconsistencies in the choices of calibration files are highlighted; the Exposure Summary, which details when and how the exposure was taken, and the Calibration Status Summary, which reports all reference files used in the calibration and their pedigree, if available. Some basic consistency checks are carried out, and the user is alerted to any potential problems thus discovered.

A few notes are in order regarding the grayscale representation of the data. The images are presented as they appear after the standard pipeline processing (see Chapter 26). Bias, standard dark, and flatfield corrections have been applied, but hot pixels and cosmic rays are not removed; hence the mottled appearance of many images, especially those with exposure times longer than a few hundred seconds. In order to limit the size of the printouts, images are block-averaged (2 x 2) before printing, causing some details to be lost. In general, because of the limited resolution of even good black-and-white printers, finer image details are lost; the grayscale images cannot in any way replace even a quick look on the screen.

Paper products shipped before August 1997 differ in two main respects. First, they do not have the visit-level summary of observations; some of the information is available via the tape log, but without any data quality flags or image statistics. Second, the exposure-level information is presented in a different way: the dataset information page consists simply of a printout of the PDQ file (See "Data Files and Suffixes" on page 25-1.). The grayscale images are produced by a different task, which gives them a different appearance, but the overall information content in them is very similar. Sample paper products in the new format are shown in Figure 25.3. Users who have received paper products in the old format can regenerate them in the new format using pp_dads.

Figure 25.3: Paper Products: List of Targets and Observations