RDF is Coming! ARE YOU READY?

--M. F. Corcoran, Ed Seufert

Introduction

Starting in the spring of 1994, the FITS data files distributed on tape by the US Roast Data Processing Center (USRSDPC) and by the Max Planck Institut fur Extraterrische Physik (MPE) will conform to the Rationalized Data File (RDF) format as specified by the High Energy Astrophysics Science Archive Research Center (HEASARC), the Center for Astrophysics (CfA) and MPE. This represents a major change in the organization of the FITS files as compared to the previous (REV0) FITS formats distributed prior to spring 1994. In addition to distributing all data obtained after spring 1994 in RDF format, all ROSAT PSPC and HRI data obtained prior to this time will be reprocessed into the RDF format. Eventually, all data obtained by ROSAT will have been distributed and archived in RDF format, and the RDF versions of the data products will constitute the official ROSAT data archive.

Advantages of RDF:

The change to the RDF format was made in order to present ROSAT data in a more user-friendly, multi-mission context. The RDF format offers the following advantages:

Important tabulated quantities are more easily identified by the user through use of more descriptive column names.
Primary headers and extension headers contain more useful information.
COMMENT and HISTORY cards are extensively used to provide the user with detailed information regarding the information contained in the files.

In addition to the advantages list above, the format of the ROSAT data products will be independent of where the data was processed because both MPE and the USRSDPC have adopted the RDF format. Therefore, users of ROSAT data processed at MPE will be able to interpret and use ROSAT datasets processed in the US easily, and vice-versa. This independence is a great improvement over the current situation, in which similar quantities have different names/locations depending upon which site processed the data.

Because the RDF formats are designed to be flexible, they can accommodate data from other high-energy astronomy observatories. Currently, data from EINSTEIN, ASCA, CGRO and XTE are being or will be archived in the RDF format. This archiving strategy means that analysis software designed to work with the ROSAT RDF data also will be able to handle data from these other satellites in a straightforward way. Users of these data will see common information presented in a common way, thus greatly facilitating the comparison of data from different high-energy observatories.

Impact on the user

Changes in file content and format mean that previous versions of data analysis software in general will need to be modified in order to work with the RDF formatted data. However, starting with version 2.3, PROS will allow analysis of the RDF files. For more information, see the PROS article elsewhere in this issue. EXSAS users also will have the capability to analyze RDF-formatted data.

RDF Datasets for PSPC and HRI Observations

The PSPC fileset

This section gives more detailed information concerning the structure of the data files for PSPC data distributed under the RDF format.

The BASIC file

Each data set contains a BASIC file, which contains the bare-bones, calibrated science data. This is the essential information that anyone needs to know to perform standard scientific analyses. On the data tape, the BASIC file can be identified by its official name, which ends in _bas.fits. Basic data generally consist of times that define the length of the observation, and the photon event list that gives (for the PSPC) time of arrival, sky position, detector position, and raw and corrected amplitudes for each photon. Status histories that can be used to filter the data also are given in the BASIC file.

For PSPC data, the BASIC file consists of a NULL primary array and the following six binary table (XTENSION = BINTABLE) extensions:

The standard good-time intervals extension EXTNAME = STDGTI This extension gives the time intervals that the standard analysis software system (SASS) has deemed to contain good science time as defined by the standard screening criteria (see below), and the corresponding observation interval (OBI) number.
The STANDARD events list extension EXTNAME = STDEVT This extension contains the list of photons that have passed the standard screening. For each detected photon, the list contains the (x, y) position, the arrival time, the pulse height, the gain-corrected pulse height, and the linearized detector coordinates. For each photon, a status is given designating whether that photon passed or failed the standard screening. By definition, all photons in the standard events list passed standard screening.
The REJECTED events list extension EXTNAME = REJEVT This extension contains the list of photons that were rejected by the standard screening. For each detected photon, the list contains the (x, y) position, the arrival time, the pulse height, the gain-corrected pulse height, and the linearized detector coordinates. For each photon, a status is given designating whether that photon passed or failed the standard screening. By definition, all photons in the standard events list failed standard screening.
Temporal Status History extension EXTNAME = TSI This table lists the quality flags as a function of time. The quality flags consist of PASSFAILS and LOGICALS. PASSFAILS indicate why data were rejected at a given time. LOGICALS indicate the status of various spacecraft subsystems. PASSFAILS and LOGICALS are bit-encoded. The description of the coding used to define both PASSFAILS and LOGICALS is described in the extension header.
Quality Limits extension EXTNAME = QLIM This table gives the allowable values of the quality flags.
All Quality limits extension EXTNAME = ALLQLIM This table gives the quality limits for accepting the data as part of the observation, regardless of whether the data is scientifically useful. These limits are used in determining the entire set of photons (accepted + rejected events) detected during the observation.

The ANCILLARY file

Each PSPC data set distributed using the RDF format contains a file of ANCILLARY data. The ANCILLARY file can be recognized by its official name, which ends in _anc.fits. The data provided in the ANCILLARY file, while not essential for scientific analysis of the BASIC data, often are very useful if more detailed analyses are required. Ancillary data include orbit and spacecraft pointing information, and other engineering or housekeeping information.

For PSPC data, the ANCILLARY file consists of a NULL primary array and the following five binary table (XTENSION = BINTABLE) extensions:

The spacecraft ephemeris extension EXTNAME = EPHEM This extension gives information about the orbital ephemeris of the spacecraft as a function of time. It includes the satellite position vector, satellite longitude, latitude and altitude.
The spacecraft pointing extension EXTNAME = ASPECT This extension gives information about the pointing of the spacecraft as a function of time. It includes spacecraft right ascension, declination and roll.
The Housekeeping extension EXTNAME = HKP This extension lists values of engineering and other housekeeping data as a function of time. For the PSPC, the instrument-in-use flag, the detector temperature, pressure, and high voltage, the missing housekeeping flag, and the filter type are given.
The Event Rate extension EXTNAME = EVRATE This table lists rates of detected events as a function of time. Accepted or rejected event rates, SAA rates, and other quantities are given.
The housekeeping status extension EXTNAME = HKSTA This table records the values of the status of the housekeeping parameters, such as the high voltage status, the carrousel status, the detector status, etc. These values are also available in the temporal status history (TSI) extension. They are given here for compatibility with the EXSAS software package.

Images

Images are stored as 512 x 512 primary arrays in separate FITS files. All images use the convention in which (1,1) is the CENTER of the pixel in the LOWER left corner. Derived images include both total (source+background) and background images.

There are eight PSPC image files included as part of the standard processing -- source + background images in three energy bands (total, hard, and soft), three background images in three bands, the broad-band exposure map, and the energy-encoded color map. The images are a compression of the focal plane scale by a factor of 30 (i.e. each pixel has a dimension of 15 x 15 arc seconds, rather than the 0.5 x 0.5 arc second pixels used in the STDEVT and REJEVT tables). The header keywords PIMIN, PIMAX give the PI values used to construct the images.

The BROAD-BAND image file FILENAME: r*_im1.fits This file contains as the 512 x 512 primary array the X-ray image derived from all accepted photons in the range 0.07 - 2.4 keV. The file also contains two binary table extensions, the first of which gives the source detections from the local detect algorithm in the broad band, and the second of which gives the broad-band source detections from the map detect algorithm.
The BROAD-BAND background file FILENAME: r*_bk1.fits This file contains the broad-band background map as a 512 x 512 primary array. This background image was constructed from the BROAD-BAND image file after removal of detected X-ray sources.
The BROAD-BAND exposure map file FILENAME: r*_mex.fits This file contains the exposure map as a 512 x 512 primary array. This map was created by casting the BROAD-BAND instrument map in sky coordinates using the information in the aspect histogram and represents the amount of exposure each area of the field of view received taking into account vignetting and the spacecraft wobble.
The HARD-BAND image file FILENAME:,b> r*_im2.fits This file contains as the 512 x 512 primary array the X-ray image derived from all accepted photons in the range 0.4 - 2.4 keV. The file also contains two binary table extensions, the first of which gives the source detections from the local detect algorithm in the hard band, and the second of which gives the hard-band source detections from the map detect algorithm.
The HARD-BAND background file FILENAME: r*_bk2.fits This file contains the hard-band background map as a 512 x 512 primary array. This background file was constructed from the HARD-BAND image file after removal of detected X-ray sources.
The SOFT-BAND image file FILENAME: r*_im3.fits This file contains as the 512 x 512 primary array the X-ray image derived from all accepted photons in the range 0.07 - 0.4 keV. The file also contains two binary table extensions, the first of which gives the source detections from the local detect algorithm in the soft band, and the second of which gives the soft-band source detections from the map detect algorithm.
The SOFT-BAND background file FILENAME: r*_bk3.fits This file contains the soft-band background map as a 512 x 512 primary array. This background file was constructed from the SOFT-BAND image file after removal of detected X-ray sources.
The ENERGY-CODED COLOR map FILENAME: r*_ime.fits This file contains the energy-coded image as a 512 x 512 array. Each pixel in the image represents the mean photon energy detected (and not intensity as in the other images).

The SOURCE file

The SOURCE file contains information about sources in the field of view of the detector. The source file can be recognized by its official name, which ends in _src.fits. This file contains the following binary table (BINTABLE) extensions:

The DETECTED X-RAY source extension EXTNAME = SRCTBL This table contains information about sources in the field of view as detected by the standard processing system. Positions, counting rates, variability flags, hardness ratios, etc. are included.
The SIMBAD source list EXTNAME = SKYTBL This table contains information derived from the SIMBAD (Set of Identifications, Measurements, and Bibliographies for Astronomical Data) database about all catalogued sources in the field of view of the detector. Positions, names, spectral type or morphology, V and B magnitudes are among the information provided.
The SPECTRAL FIT results table EXTNAME = FITSS This table contains the results of the fitting of bright sources with standard spectral models (absorbed power laws or thermal models). Source and background position, brightness, and the spectral fit values are provided. Note that this table will not be provided if there are no sources bright enough for the spectral fitting package (at present, spectral fitting is only done for sources with greater than 200 net counts and a likelihood greater than 50).
The source VARIABILITY table EXTNAME = VARY This table contains the results of the standard source variability analysis. Variability tests include chi-square, KS and FFT�s. These tests are performed on all sources with a likelihood greater than 50; if no such sources appear in the field of view the variability table will not be provided.
The BACKGROUND SPECTRUM table EXTNAME = BKGSPC This table contains the background spectrum (counts per channel) over the whole field of view as defined by SASS after removal of detected sources.
The BACKGROUND LIGHTCURVE table EXTNAME = BKGRTE This table contains the background lightcurve (rate plus error vs. time) over the whole field of view as defined by SASS after removal of detected sources.
SOURCE SUMMARY table EXTNAME = SUMMARY This extension contains a 132-character per row ASCII copy of the MASTER SOURCE LIST (MASOL), which also is provided as hardcopy.
MAXIMUM LIKELIHOOD table EXTNAME = ML_SOURCES This extension contains a 132-character per row ASCII copy of the Maximum Likelihood List (MLLST) which contains the results of the maximum likelihood analysis used by SASS to determine positions and source rates. Note that the MLLST is also provided as hardcopy.
MERGED SOURCE table EXTNAME = MERGED_SOURCES This extension contains a 132-character per row ASCII copy of the Merged Source List (MPLST), which contains the results of the merger of the sources detected by the local detect (LDETECT) and map detect (MDETECT) algorithms used by SASS. Note that the MPLST is also provided as hardcopy.
Extracted source spectra/Off-axis histograms Spectra (rates versus channel plus other information) extracted by SASS for each source with a likelihood greater than 50 and more than 200 net counts are provided in individual binary table extensions at the end of the SOURCE file (one extension per source). Each spectrum extension is designated by an EXTNAME of the form EXTNAME = SPnnn where nnn is a three-digit number corresponding to the source ID number given in the MPLSX source lists and recorded in the SRCTBL extension.
Each extracted source spectrum has associated with it an extension which gives the off-axis histogram (fraction of time versus off-axis angle). Each off-axis histogram can be identified by the EXTNAME of the form EXTNAME = OAHnnn where nnn is the three-digit number corresponding to the source ID number given in the MPLSX source lists and recorded in the SRCTBL extension.

The HISTORY file FILENAME: r*_his.fits

This file contains the processing log and parameter files generated/used by SASS during sequence (SEQ) and OBI processing as 80-ASCII-characters-per-row binary table extensions. These extensions provide some documentation of the SASS processing setup. However, the information contained in these extensions is fairly cryptic; interpretation of this information generally requires at least some familiarity with the SASS processing system.

the PRINT file FILENAME: r*_prt.fits

This file contains ASCII text and POSTSCRIPT results produced by the standard processing (source lists, analysis results, images, etc.) These files appear in the PRINT file in individual FITS BINTABLE extensions. In order to view the contents of any extension, the user should read the binary data table using any standard FITS BINTABLE reader, convert the binary data to ASCII, and write the data to a file. The text contents then can be viewed using standard text editors (for the simple ASCII data) or standard viewers for the POSTSCRIPT files (such as ghostscript or pageview), or printed by the user. A list of EXTNAMEs for the PRINT file for the data set rp110590 is shown below:

EXTNAME = SUMPG.trl -- Summary Pages (PostScript)
EXTNAME = NIM1.trl -- Normalized Image1 Plot (PostScript)
EXTNAME = NIM2.trl -- Normalized Image2 Plot (PostScript)
EXTNAME = NIM3.trl -- Normalized Image3 Plot (PostScript)
EXTNAME = ZNIM2.trl -- Zoomed Normalized Image2 Plot (PostScript)
EXTNAME = MEXMP.trl -- Exposure Map Plot (PostScript)
EXTNAME = NBKMP1.trl -- Normalized Background Map Plot (PostScript)
EXTNAME = MXPLT.trl -- Sources Merged after Extn. Analysis Plot (PS)
EXTNAME = MXLST.trl -- Sources Merged after Extn. Analysis List (Text)
EXTNAME = MPLST.trl -- Sources Merged after Map and Local Detect List
EXTNAME = MAXPLT.trl -- Max. Likelihood Sources Plot (PostScript)
EXTNAME = MAXLST.trl -- Max. Likelihood Sources List (Text)
EXTNAME = SOPLT.trl -- Accepted Sources Plot (PostScript)
EXTNAME = ZSOPLT.trl -- Zoomed Accepted Sources Plot (PostScript)
EXTNAME = SMBPLT.trl -- SIMBAD Sources in Field of View Plot (PS)
EXTNAME = SMBID.trl -- SIMBAD Sources Identification Plot (PostScript)
EXTNAME = SMBLST.trl -- SIMBAD Sources in Field of View List (Text)
EXTNAME = SMBPS.trl -- SIMBAD Sources in Field of View List (PS)
EXTNAME = CORLST.trl -- ROSAT/SIMBAD Source Correlation List (Text)
EXTNAME = CORPS.trl -- ROSAT/SIMBAD Source Correlation List (PS)
EXTNAME = MEXBM.trl -- Blinded Map Plot (PostScript)
EXTNAME = SOLST.trl -- Final Source List (Text)
EXTNAME = MASOL.trl -- Master Source List (Text)
EXTNAME = DETSA.trl -- Detailed Source Analysis List (Text)
EXTNAME = EXTEN.trl -- Extended Source Analysis List (Text)
EXTNAME = SBPLT.trl -- Background Spectrum Plot (PostScript)
EXTNAME = HRDRT.trl -- Hardness Ratio List (Text)
EXTNAME = SSLST.trl -- Source Spectral Fitting List (Text)
EXTNAME = SSPLT.trl -- Source Spectral Fitting Plot (PostScript)
EXTNAME = SVLST.trl -- Source Variability Tests List (Text)
EXTNAME = SVPLT.trl -- Source Variability Tests Plot (PostScript)
EXTNAME = SAAQ.trl -- Aspect, Aspect Quality, Accepted Times Plot(PS)
EXTNAME = EVRH.trl -- Event Rates - Housekeeping Information Plot(PS)

The HRI fileset

This section gives more detailed information concerning the structure of the data files for HRI data distributed under the RDF format.

The BASIC file

Each data set contains a BASIC file, which contains the "bare-bones," calibrated science data. This file has the essential information that anyone needs to know to perform standard scientific analyses. On the data tape, the BASIC file can be identified by its official name, which ends in _bas.fits. Basic data generally consist of times that define the length of the observation, and the photon event list that gives (for the HRI) the time of arrival, the sky position, the raw (uncorrected) detector position, and the raw amplitudes for each photon. Status histories that can be used to filter the data also are given.

For HRI data, the BASIC file consists of a NULL primary array and the following five binary table (XTENSION = BINTABLE) extensions:

The standard good time intervals extension EXTNAME = STDGTI This extension gives the time intervals that the standard analysis software system (SASS) has deemed to contain good science time as defined by the standard screening criteria (see below), and the corresponding observation interval (OBI) number.
The STANDARD events list extension EXTNAME = STDEVT This extension contains the list of photons that have passed the standard screening. For each detected photon, the list contains the (x, y) position, the arrival time, the pulse height, and the raw (uncorrected) detector coordinates. For each photon, a status is given designating whether that photon passed or failed the standard screening. By definition, all photons in the standard events list passed standard screening.
The REJECTED events list extension EXTNAME = REJEVT This extension contains the list of photons that were rejected by the standard screening. For each detected photon, the list contains the (x, y) position, the arrival time, the pulse height, and the raw (uncorrected) detector coordinates. For each photon, a status is given designating whether that photon passed or failed the standard screening. By definition, all photons in the standard events list passed standard screening.
Temporal Status History extension EXTNAME = TSI This table lists the quality flags as a function of time. The quality flags consist of PASSFAILS and LOGICALS. PASSFAILS indicate why data were rejected at a given time. LOGICALS indicate the status of various spacecraft subsystems. PASSFAILS and LOGICALS are bit-encoded. The description of the coding used to define both PASSFAILS and LOGICALS is described in the extension header.
Quality Limits extension EXTNAME = QLIM This table gives the allowable values of the quality flags. For the HRI these values are actual minimum and maximum values within which the quality flags must fall.
All Quality limits extension EXTNAME = ALLQLIM This table gives the quality limits for accepting the data as part of the observation, regardless of whether the data is scientifically useful. These limits are used in determining the entire set of photons (accepted + rejected events) detected during the observation.

The ANCILLARY file

Each HRI data set distributed using the RDF format contains a file of ANCILLARY data. The ANCILLARY file can be recognized by its official name, which ends in _anc.fits. The data provided in the ANCILLARY file, while not essential for scientific analysis of the BASIC data, is often very useful if more detailed analyses are required. Ancillary data include orbit and spacecraft pointing information, and other engineering or housekeeping information.

For HRI data, the ANCILLARY file consists of a NULL primary array and the following four binary table (XTENSION = BINTABLE) extensions:

The spacecraft ephemeris extension EXTNAME = EPHEM This extension gives information about the orbital ephemeris of the spacecraft as a function of time. It includes the satellite position vector, and the satellite longitude, latitude and altitude.
The spacecraft pointing extension EXTNAME = ASPECT This extension gives information about the pointing of the spacecraft as a function of time. It includes the spacecraft right ascension, declination and roll.
The housekeeping extension EXTNAME = HKP This extension lists values of engineering and other housekeeping data as a function of time. For the HRI, three detector temperatures are given.
The event rate extension EXTNAME = EVRATE This table lists rates of detected events as a function of time. Primary and secondary science rates, background levels, and SAA rates are among the tabulated quantities.

Images

There are two HRI image files included as part of the standard processing - one total image and one derived background image. The images use a compression of the scale by a factor of 16 (i.e. each pixel has a dimension of 8 x 8 arc seconds, rather than the 0.5 x 0.5 arc second pixels used in the STDEVT and REJEVT tables). Compression of the focal plane scale saves an enormous amount of space for an image containing the entire HRI field of view. However, given the intrinsic spatial resolution of the HRI (which is a few arc-seconds) compression by a factor of 16 is somewhat unsatisfactory and may result in the loss of information. Detailed analysis of the HRI usually requires the user to create an image of a sub-region of the HRI field of view at a smaller compression factor.

The TOTAL image file FILENAME: rh*_im1.fits This file contains the total image constructed from all photons that passed the standard screening as a 512 x 512 primary array.
The TOTAL background image file FILENAME: rh*_bkg.fits This file contains the total background image constructed from the TOTAL image after removal of detected X-ray sources as a 512 x 512 primary array.

The SOURCE file

The detected X-RAY source extension EXTNAME = SRCTBL This table contains information about sources in the field of view as detected by the standard processing system. Positions, counting rates, variability flags, etc. are included.
The SIMBAD source list EXTNAME = SKYTBL This table contains information derived from the SIMBAD (Set of Identifications, Measurements, and Bibliographies for Astronomical Data) database about all catalogued sources in the field of view of the detector. Positions, names, spectral type or morphology, V and B magnitudes are among the information provided.
Source summary table EXTNAME = HOUT This extension contains a 132-character-per-row ASCII copy of the HRI output file (HOUT.SEQ), which also is provided as hardcopy.

The HISTORY file

The HISTORY file contains the processing log and parameter files generated/used by SASS during sequence (SEQ) and OBI processing as 80-ASCII-characters-per-row binary table extensions. These extensions provide some documentation of the SASS processing setup. However, the information contained in these extensions is fairly cryptic; interpretation of this information generally requires at least some familiarity with the SASS processing system.

The PRINT file FILENAME: r*_prt.fits

EXTNAME = SUMPG.trl -- Summary Page (PostScript)
EXTNAME = OUTPUT.trl -- Source, Background, OBI, Time history (Text)
EXTNAME = SKYLST.trl -- Field Sky Catalog Summary List (Text)
EXTNAME = FLDPLT.trl -- Field Plot (PostScript)
EXTNAME = SRCPLT.trl -- Source Plot (PostScript)
EXTNAME = SKYMAP.trl -- Sky Map (PostScript)
EXTNAME = SMBPLT.trl -- Plot of SIMBAD Sources in Field of View (PS)
EXTNAME = SMBLST.trl -- List of SIMBAD Sources in Field of View (Text)
EXTNAME = CORLST.trl -- ROSAT/SIMBAD Source Correlation List (Text)
EXTNAME = CORPS.trl -- ROSAT/SIMBAD Source Correlation List (PS)
EXTNAME = STRPLT.trl -- Strip plots (PostScript)
EXTNAME = HIMAGE.trl -- Gray Scale Image (PostScript)

RDF File naming conventions

Starting in March 1994, revision 2 processing of ROSAT data will begin. All previously processed data - revision 0 (for AO1 and AO2 data observed from Feb. 1991 through Jun. 1992), revision 1 (for AO3 and AO4 data observed from Jun. 1992 through Dec. 1993), and the PCV (post-launch calibration and verification) data will be reprocessed using the revision 2 software. The revision 2 reprocessing of Guest Observer data will proceed in the order of observation and will be distributed to the US Guest Observers. The revision 2 reprocessing is expected to take approximately 75 weeks to finish (assuming no major problems are encountered; such as, a bug in the software that requires some of the data be redone). All new observations (those observed after Dec. 21, 1993) will be processed using the revision 2 software as the data is received from Germany.

The biggest difference between revision 2 processing and older data will be the use of the Rationalized Data Format FITS (RDF) for the data products distributed to users and the public archive. RDF will be used by both the German and US data centers and will be the final format for all ROSAT data in the German, US, and UK public archives.

Below is a brief description of the naming convention that will be used for the RDF data products produced by the revision 2 processing:

        r_.fits

                r               - For ROSAT.

                  - Code for detector/filter:
                                        h - HRI
                                        p - PSPC
                                        f - Filtered PSPC.

                           - 6 digit ROR number.


		

			n00  - as planned, first segment (n = Normal)

			a01,a02, - as first, second, and subsequent
                                       segments added to the "normal"
                                       segment (a = Add-on)

			a00,a01,�  - indicate 1st, 2nd and subsequent
                                       segments for an observation that was first processed as a single
              	               observation in Revision 0 processing.

			m01,m02,�  - mispointed segment, numbering begins
                                       at 1 (m = Mispointed)

                 - 3 letter abbreviation for the RDF file type:

                                  HRI
                                  --ANC - ancillary
                                  BAS - basic
                                  BK1 - background
                                  HIS - history
                                  IM1 - total image
                                  PRT - print files - PostScript and text
                                  SRC - derived source information

                                  PSPC
                                  ---ANC - ancillary
                                  BAS - basic
                                  BK1 - hard background
                                  BK2 - soft background
                                  BK3 - total background
                                  MEX - exposure map
                                  HIS - history
                                  IM1 - total image
                                  IM2 - hard image
                                  IM3 - soft image
                                  IME - energy encoded image
                                  PRT - print files - PostScript and text
                                  SRC - derived source information

A detailed description of the content of each of the files can be found in the RDF Data Products Guide, which will be available March 94.

Examples:

rp123456n00_anc.fits - PSPC Ancillary file for a �normal� segment of ROR 123456

rp123456a01_anc.fits - PSPC Ancillary file for �first� add-on segment of ROR 123456

rp654321a00_anc.fits - PSPC Ancillary file for �first� segment resulting from the splitting of ROR 654321 that was originally processed under Revision 0 as one chuck.

rp654321a01_anc.fits - PSPC Ancillary file for �second� segment resulting from the splitting of ROR 654321 that was originally processed under Revision 0 as one chuck.

rp123456m01_anc.fits - PSPC Ancillary file for �mispointed� segment of ROR 123456