PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM DATA_SET_ID = "HST-S-WFPC2-3-RPX-V1.0" OBJECT = TEXT PUBLICATION_DATE = 1999-12-06 AUTHOR_FULL_NAME = "Mark R. Showalter" NOTE = "HST/WFPC2 Data Set Tutorial" END_OBJECT = TEXT END HST/WFPC2 Data Set Tutorial =========================== This document provides a tutorial on how to make optimal use of this data set. It is organized as follows: 1. Data Set Overview 2. File Names and Directory Structure 3. File Types 4. HST/WFPC2 Images and Filters 5. FITS Labels 6. PDS Labels 7. PDS Keywords 8. Image Geometry 9. Image Calibration 1. Data Set Overview -------------------- This data set contains images of the Saturn system taken by the Wide Field/Planetary Camera 2 (WFPC2) aboard the Hubble Space Telescope (HST) during the Saturn ring plane crossings of 1995. These data are supplemented by all images of Saturn and/or its rings acquired by the same instrument in 1994. In many cases, the supplementary images were not specifically intended for ring observations; however, all WFPC2 images in which Saturn or its rings appear through the end of 1995 are included. The HST observing programs whose results are included in this data set are as follows: Proposal: 5219 Title: Saturn's Aurora, Airglow, and Extended Atmosphere PI: John Trauger (Jet Propulsion Laboratory) Proposal: 5508 Title: Tracking Features in Titan's Lower Atmosphere and on its Surface PI: Peter Smith (Lunar and Planetary Laboratory) Proposal: 5776 Title: Observations of the 1994 Spot on Saturn PI: Reta Beebe (New Mexico State University) Proposal: 5782 Title: Saturn Ring-Plane Crossing, May 1995 PI: Amanda Bosh (Lowell Observatory) Proposal: 5824 Title: Occultation of GSC5249-01240 by Saturn: Atmospheric and Ring Studies PI: Amanda Bosh (Lowell Observatory) Proposal: 5836 Title: Saturn Ring Plane Crossing Observations in August and November 1995 PI: Philip Nicholson (Cornell University) Proposal: 6030 Title: Clouds and Aerosols on Saturn and Uranus PI: Martin Tomasko (Lunar and Planetary Lab) Proposal: 6215 Title: Saturn's Aurora and Airglow PI: John Trauger (Jet Propulsion Laboratory) Proposal: 6216 Title: Saturn's Ring Atmosphere during Ring Plane Crossing PI: John Trauger (Jet Propulsion Laboratory) Proposal: 6295 Title: Imaging the Atmosphere of Titan and its Surface PI: John Caldwell (York University) Proposal: 6328 Title: Imaging and Spatial Scans of Saturn Near Ring-Plane Crossing PI: John Caldwell (York University) Abstracts of all these proposals are found in DOCUMENT/PROPnnnn.TXT, where nnnn is replaced by the 4-digit proposal identifier. In the specific case of Titan proposals 5508 and 6295, some images show Titan but not Saturn or its rings. These particular images have not been archived in this data set; however, they are obtainable from the HST Data Archive (http://archive.stsci.edu/). Files are distributed across this set of five CDROMs roughly chronologically, as follows: Volume Directory Proposal Dates # Images RPX_0001 199410XX/U2IQXXXX 5219, Trauger 1994-10-09 7 199410XX/U2IZXXXX 5508, Smith 1994-10-05 6 1994-10-06 6 199412XX/U2KRXXXX 5776, Beebe 1994-12-01 9 199505XX/U2OOXXXX 6216, Trauger 1995-05-22 7 RPX_0002 199505XX/U2ONXXXX 5782, Bosh 1995-05-22 48 RPX_0003 199508XX/U2TFXXXX 5836, Nicholson 1995-08-09 8 1995-08-10 42 1995-08-11 4 RPX_0004 199508XX/U2OOXXXX 6216, Trauger 1995-08-10 4 199508XX/U2QEXXXX 6030, Tomasko 1995-08-06 8 199510XX/U2VIXXXX 6295, Caldwell 1995-10-01 8 1995-10-09 8 199510XX/U2WCXXXX 6215, Trauger 1995-10-15 4 1995-10-31 4 199511XX/U2QEXXXX 6030, Tomasko 1995-11-17 16 RPX_0005 199511XX/U2OOXXXX 6216, Trauger 1995-11-20 3 199511XX/U2TFXXXX 5836, Nicholson 1995-11-21 20 1995-11-27 8 199511XX/U2ZNXXXX 6328, Caldwell 1995-11-18 10 199511XX/U2ZYXXXX 5824, Bosh 1995-11-21 2 1995-11-22 4 Because timing was critical for many of the observations in this data set, different PIs occasionally needed to obtain data during the same HST orbit. Space Telescope Science Institute (STScI) is unable to allocate fractions of an HST orbit to different investigators, so on a few occasions investigators traded images among themselves as follows: Volume Directory Actual Proposal Date File RPX_0003 199508XX/U2TFXXXX 6216, Trauger 1995-08-10 020E*.* RPX_0003 199508XX/U2TFXXXX 6216, Trauger 1995-08-10 020F*.* RPX_0004 199508XX/U2OOXXXX 5836, Nicholson 1995-08-10 0301*.* RPX_0004 199508XX/U2OOXXXX 5836, Nicholson 1995-08-10 0302*.* RPX_0004 199508XX/U2OOXXXX 5836, Nicholson 1995-08-10 0303*.* RPX_0005 199511XX/U2ZYXXXX 5836, Nicholson 1995-11-21 A301*.* RPX_0005 199511XX/U2ZYXXXX 5836, Nicholson 1995-11-21 A302*.* Because this data set is organized around STScI's original file names, these images are found among images belonging to a different PI. However, the PDS labels indicate the correct PI. The file DOCUMENT/EZINDEX.TXT contains an easy-to-read table of the key parameters for every image archived in this data set. The INDEX directory contains more detailed indices which may, however, be more difficult to use. An on-line database and query system for this data set is currently in development at the PDS Rings Node. This database will enable users to select WFPC2 images based on a broad variety of constraints including time, filter, target, PI, instrument mode, etc. See the Rings Node home page (http://ringmaster.arc.nasa.gov/) for up-to-date information. 2. File Names and Directory Structures -------------------------------------- STScI has a standard method for naming data files such that every data file ever produced by HST has a unique name. Here is a typical name: U2ON0102T.C0F. (This particular name is from the Saturn observations by Amanda Bosh in May 1995). The name breaks down as follows: U Instrument ID; U for WFPC2. 2ON Program ID. Every STScI observing proposal gets a unique 3-character ID. 01 Observation set ID. For this particular data set, there are files with set IDs 01, 02, A1, A2 and B1. Sets are divided up by HST orbit, purpose, or other criteria. 02 Observation ID, which specifies the unique image. T Source of transmission; T for tape-recorded. .C0 C0 indicates that this is a calibrated image. Other codes are used for other file types: D0 for raw images, Q0 for raw data quality mask, etc. F F is for a FITS-format file consisting of a FITS header plus data. H is for header alone; D is for data alone. PDS and ISO-9660 Level 1 CD-ROM standards require that file names be a maximum of eight characters and extensions a maximum of 3 characters. Furthermore, PDS standards require that the last 3 characters describe the file format, e.g. ".IMG" for images; ".TAB" for ASCII tables, ".DAT" for other binary formats. The aforementioned file has been archived in this data set under the name "199505XX/U2ONXXXX/CALIMAGE/0102_C0F.IMG". This name breaks downs as follows: 199505XX/ Files are organized first by year and month; this directory contains images from May 1995. U2ONXXXX/ Files are then grouped by the program ID; this directory contains images acquired by PI Bosh. CALIMAGE/ Files are then grouped by type; this directory contains calibrated images. Other file types include RAWIMAGE for raw images, ENGDATA for engineering data, and CALMASK for image quality masks flagging bad pixels. 0102_C0F Finally, files are named by the observation set ID, observation ID and STScI file type. .IMG This is the default PDS file extension for images. 3. File Types ------------- As shown above, image files in this data set are grouped by month, investigation and then by type. Eight different file types are found in this data set, indicated by the name of the directory in which they reside: Subdirectory STScI file File type extension BROWSE ---- reduced resolution GIF files. CALIMAGE .c0f calibrated images. CALMASK .c1f quality masks for calibrated images. ENGDATA .x0f extracted engineering data. ENGMASK .q1f quality masks for engineering data. HEADER .shf standard header containing observation parameters. RAWIMAGE .d0f raw images. RAWMASK .q0f data quality masks for raw images. The vast majority of users will only need to look inside the CALIMAGE and BROWSE directories. Files in the additional directories are only for users who have reason to re-process and/or re-calibrate the images. Users who wish to do so should consult the HST Data Handbook (DOCUMENT/HSTHB.LBL and related files), the WFPC2 Instrument Handboook (DOCUMENT/WFPC2.LBL and related files), and the STScI web site (http://www.stsci.edu) for additional guidance. The CALIMAGE directory contains calibrated WFPC2 images, suitable for scientific processing and analysis. The files named "*.IMG" are image files in FITS format (see Section 4 below). For each image, an associated PDS label file named "*.LBL" contains a complete description of the file in PDS format, with a variety of useful additional parameters. The BROWSE directory contains reduced-resolution, specially enhanced GIF images providing the user with a quick view of what the image contains. Many widely-available applications (such as web browsers) are available for displaying GIF images. 4. HST/WFPC2 Images and Filters ------------------------------- Users experienced with astronomical image analysis should have little trouble working with calibrated WFPC2 images. The one key property of WFPC2 images that may be slightly unconventional is that a WFPC2 image file actually contains four separate 2-D images, each 800x800 pixels. First is the Planetary Camera (PC1), with an image scale of 0.046 arcseconds/pixel. The next three are the Wide Field Cameras (WF2, WF3, and WF4), with image scales of 0.1 arcseconds/pixel. In the browse images provided, these four images have been mosaicked together in a standard way: +-----------------------------+ | | | | | | | | | ^ | | | | | | | | WF2 | +--------------+ | | | | | S | ^ | | | | | | <--------L | | PC1 | | | L | | | S----> | | | | +-----------------------------+--------------+--------------+ | | | | | | | | | | <--------S | L--------> | | | | | L | S | | | | | | | WF3 | | | WF4 | | | | | | | | | | | | v | v | | | | | | | | | | +-----------------------------+-----------------------------+ The direction of the S and L axes is also shown; note the 90 degree rotation of each image relative to the previous one. Here S refers to the "sample" (i.e., most rapidly-changing) axis in the file, and L refers to the "line" (i.e., slowest-changing) axis. This means that that the first pixel of the image's second line falls immediately after the last sample of the first line in the image file. However, when a single image is displayed by itself, it is conventional to let the S-axis increase to the right and the L-axis to increase upward. All four cameras are shuttered simultaneously and are exposed through a common filter. WFPC2 has a set of 48 possible filters on two filter wheels. Most filter names are of the form Fnnnx, where nnn is the approximate center wavelength of the filter's bandpass in nanometers, and x is one of W=wide, M=medium, and N=narrow. For example, F469N is a narrow filter centered at 469.5 nm, and F450W is a wide B filter centered at 444.5 nm, similar B of the standard UVBRI filter set. A few additional filters are of particular interest to the users of this data set. Their names begin FQCH4 and they are centered near 893 nm, within a methane absorption band. Images of Saturn are especially dark relative to the rings and moons through this filter, making faint rings and small moons easier to detect. However, images through these filters can have substantial vignetting, such that often only one of the four individual images is usable. Consult the WFPC2 Instrument Handbook (DOCUMENT/WFPC2.LBL and related files) for a more thorough discussion of the WFPC2 imaging system and filters. 5. FITS Labels -------------- All data files on this disk (except GIF images) are in FITS format. Files in the FITS format are structured as 2880-byte records, an inheritance from the days when files were typically stored on tapes. The FITS file is organized as an alternating sequence of headers and data objects, each beginning on a 2880-byte record boundary. Headers are in turn organized in 80-byte lines, each of the form keyword=value / comment Here the keyword is always exactly 8 characters, padded with blanks if necessary. The end of the header is indicated by a line that contains, simply, "END" and the data object then begins on the next 2880-byte record boundary. Further details of the FITS standard are found in the document "Definition of the Flexible Image Transport System (FITS) Standard", archived on this volume; see DOCUMENT/FITSINFO.LBL and related files. The FITS standard puts very few requirements on the names of the keywords used and what they mean. However, STScI has developed a large set of standard keywords for HST data. Brief definitions of these keywords are found in the file DOCUMENT/FITSDEFS.TXT. Because some of them are found only in headers for specific data types, file DOCUMENT/FITSXREF.TXT tabulates which FITS keywords can be found in which headers. More information about STScI's definitions of the FITS keywords can be found in the HST Data Handbook (DOCUMENT/HSTHB.LBL and related files). The calibrated image files are structured as follows: header #1 four images header #2 extension table The four images (PC1, WF2, WF3, WF4) are run together without any gaps. The extension table contains a variety of additional parameters. Various software packages will read FITS files. Two of these are IRAF (see http://iraf.noao.edu/) and IDL (see http://www.rsinc.com/). 6. PDS Labels ------------- PDS has developed a more sophisticated and flexible set of labeling standards than FITS. These are fully documented on-line at the PDS Central Node web site, http://pds.jpl.nasa.gov/. In this data set, all PDS labels are in the form of detached labels. This means that a file xxxxxxxx.LBL will be present for every data file of the same name but a different 3-character extension. For example, a file 0101_C0F.LBL contains a full PDS-compliant description of the FITS data file 0101_C0F.IMG. By using these detached labels, all files are compatible with both FITS and PDS standards. The PDS label file consists of a sequence of expressions of the form "keyword = value". These keywords are sometimes nested inside data objects, indicated by "OBJECT = xxx" and terminated by "END_OBJECT = xxx". These data objects describe specific sections of the data files. For example, the calibrated images labels contain six data objects: FITS_HEADER PC1_IMAGE WF2_IMAGE WF3_IMAGE WF4_IMAGE FITS_EXTENSION_HEADER EXTENSION_TABLE The PDS keyword values indicate where to find each data object in the associated FITS file and how to interpret it. The PDS tool NASAView will display images in PDS format. It is available from the PDS Central Node (http://pds.jpl.nasa.gov/). However, at the time of this writing many of the files on this data set are incompatible with NASAView; fixes are in progress. In addition, the PDS has developed toolkits called the Label Library (L3) and the Object Access Library (OAL), which makes it easy to read, manipulate and write PDS labels and data files in PDS format, using programs written in C or FORTRAN. For less ambitious users, the label files on this data set have all been structured identically. For example, the label for every calibrated image file is exactly 211 records long, and a given keyword value always appears at the same location of the same record. Hence, a relatively simple program could be written to extract needed parameters from the label, although such a program might then fail for other PDS labels. 7. PDS Keywords --------------- The PDS labels contain a large amount of useful ancillary information about each image, including geometry and calibration information that is either not found in the FITS headers, or which can only be extracted from the headers with considerable effort. In some cases, the PDS labels correct incorrect information in the FITS headers. For this reason, it is strongly recommended that users consult the PDS labels rather than the FITS headers in this data set. All PDS keywords used have standardized definitions in the "PDS Data Dictionary" (PDSDD); the file DOCUMENT/PDSDD.TXT contains the official definition of every keyword appearing in a label in this data set. However, a few require elaboration in the context of this data set. SOURCE_FILE_NAME STScI's name for this data file. FILTER_NAME Because WFPC2 has two filter wheels, pairs of filters are occasionally superimposed. When this happens, the filter name is generated by appending the WFPC2 filter names with an underscore "_" in between. NOTE The note field contains a variety of useful WFPC2 parameters that have no direct equivalent in the PDSDD. They are: HST Proposal ID P.I. Last name HST Target name Instrument mode Aperture type Exposure type FGS lock type Data quality Comment (3 lines) WAVELENGTH The median wavelength in microns of the filter/instrument response, after convolution with the solar spectrum. MINIMUM_WAVELENGTH The wavelengths in microns bounding 90% of the MAXIMUM_WAVELENGTH filter's bandpass, after convolution with the solar spectrum. NAME Each of the four image objects (PC1, WF2, WF3, and WF4) is given a name in the label. That name is "TARGETED_IMAGE" for the image specifically targeted by the P.I., and "UNTARGETED_IMAGE" for the other three. Generally, the targeted image will be of greatest interest to the user, although untargeted images often contain interesting features. 8. Image Geometry ----------------- All geometric quantities appearing in the labels are in J2000 coordinates. In this coordinate frame, the z-axis points northward along the Earth's rotation axis and the x-axis points toward the First Point of Aries. Each image label contains several key vectors in this coordinate frame, needed if the user wishes to reconstruct the image viewing geometry. These vectors are the position and velocity of the Saturn center with respect to the observer (SC_TARGET_POSITION_VECTOR, SC_TARGET_VELOCITY_VECTOR) and of the Sun with respect to the observer (SC_SUN_POSITION_VECTOR, SC_SUN_VELOCITY_VECTOR). Positions are given in units of km; velocities are given in units of km/sec. This information is compiled for the mid-time of each exposure. However, note that these quantities apply to the center of the Earth; they have not been corrected for the position of HST relative to Earth's center. Pointing information for each image is provided in the form of three Euler angles called RIGHT_ASCENSION, DECLINATION, and TWIST_ANGLE, all in units of degrees. From these it is possible to construct the C matrix, which rotates vectors from J2000 coordinates to image plane coordinates. In other words, C X_j2000 = X_ipc, where C is the matrix, X_j2000 is a three-vector in J2000 coordinates, and X_ipc is the same vector in image plane coordinates. The RIGHT_ASCENSION and DECLINATION values are simply the direction in space corresponding to the central pixel of each image. The image plane coordinate system (IPC) is one in which the x-axis points toward the right in the image, the y-axis points downward (not upward!) in the image, and the z-axis points along the optic axis of the camera. This assumes that the image has been displayed in its conventional orientation, with samples increasing toward the right and lines increasing upward. The reason the IPC y-axis points downward rather than upward is to ensure that IPC is a right-handed coordinate system. The C matrix is defined as follows: |-sinR cosT - cosR sinD sinT cosR cosT - sinR sinD sinT cosD sinT| | sinR sinT - cosR sinD cosT -cosR sinT - sinR sinD cosT cosD cosT| | cosR cosD sinR cosD sinD | where R refers to right ascension, D to declination, and T to twist angle. For users' convenience, the image labels also contain two other useful angles, which can be derived from the three Euler angles above. These are CELESTIAL_NORTH_CLOCK_ANGLE and BODY_POLE_CLOCK_ANGLE, both in degrees. The first is the "clock angle" (measured clockwise from the upward direction in the conventionally oriented image) of the projected direction of celestial north in the image. The latter is the clock angle of the projected direction of Saturn's north pole in the image. The pole of Saturn is from French R. G., et al., "Geometry of the Saturn System from the 3 July 1989 occultation of 28 Sgr and Voyager observations", Icarus 103, 163-214, 1993. French et al.'s model for Saturn's pole precession is also included. Note that these clock angles are derived from the value of the FITS keyword PA_V3, found in the image labels. We have found the value of this particular keyword to be reliable in all of the images inspected. However, a related FITS keyword, ORIENTAT, is often erroneous and should not be used for image interpretation. Note also that HST's orientation is constrained by the requirement that its solar panels point toward the Sun. As a result, the orientation of Saturn within the WFPC2 field of view changes significantly between images and undergoes a 180 degree flip at oppositions (in September 1994 and September 1995). Be aware that the Euler angles provided have some uncertainty, which can amount to several pixels in the images. The pointing should always be corrected based on visible fiducial features in an image before precise positional measurements are obtained. The last piece of information one needs to construct the precise geometry of an image is the mapping from image plane coordinates to pixel coordinates. This involves the spatial size of each pixel, which is approximately 0.046 seconds of arc in PC1 and 0.1 in WF2, WF3, and WF4. However, a more precise mapping involves third-order polynomials, as discussed by: Holtzman, J., et al., "The performance and calibration of WFPC2 on the Hubble Space Telescope", Publ. Astron. Soc. Pacific 107, 156-178, 1995. 9. Image Calibration -------------------- Two keywords in the PDS label files give the approximate conversion factor between the pixel values in the image and physically meaningful units. They are: RADIANCE_SCALING_FACTOR REFLECTANCE_SCALING_FACTOR The former converts from pixel values to units of erg/s/cm^2/micron/ster. The second converts to values of I/F, where the target intensity I is expressed in relation to F, where pi*F is the incident solar flux density at Saturn. By this definition, I/F is unity for a perfectly diffusing "Lambert" surface with normal illumination. The value for the RADIANCE_SCALING_FACTOR is based on a calibration parameter called PHOTFLAM found in the FITS labels, via RADIANCE_SCALING_FACTOR = PHOTFLAM / (texp * fov) where texp is the exposure time in seconds, and fov is the field-of-view of an individual pixel in steradians. More information about PHOTFLAM and other calibration issues can be found in the WFPC2 Instrument Handbook (DOCUMENT/WFPC2.LBL and related files). However, it should be noted that values for PHOTFLAM found in the images through the FQCH4 filters are incorrect; the correct values are 1.918E-16 for FQCH4P15 and 1.953E-16 for FQCH4N (Patrick Seitzer, personal communication, 1996). The value for the REFLECTANCE_SCALING_FACTOR is found from REFLECTANCE_SCALING_FACTOR = RADIANCE_SCALING_FACTOR / F where pi*F is the incident solar flux density at Saturn, in units of erg/s/cm^2/micron. This value can be calculated from the solar spectrum (CALIB/SOLAR.TAB) convolved with the normalized filter bandpass. Bandpass profiles for most of the filters used in this data set can be found in CALIB/Fnnnx.TAB, where Fnnnx is replaced by the name of the WFPC2 filter.