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HEASARC ROSAT CD Volume 2, 1 May 1994

Prepared by: M. Corcoran, B. O'Neel, B. Perry, K. Smale, W. Pence, N. White, and R. Petre 



	Laboratory for High Energy Astrophysics and
	High Energy Astrophysics Science Archive Research Center
	NASA Goddard Space Flight Center
	Greenbelt, Maryland 20771

  • Front Cover: A PSPC image of the galaxy NGC 253
  • Back Cover: An HRI image of the galaxy NGC 253
  • Disc 1: An HRI image of the supernova remnant Cas A
  • Disc 2: An HRI image of the Tycho supernova remnant
  1. Introduction

    The images on these disks were obtained by the X-ray detectors on board the ROSAT X-ray satellite observatory. ROSAT, an abbreviation of Rontgensatellit, consists of an X-ray Telescope (XRT) and two X-ray detectors, the Position Sensitive Proportional Counter (PSPC) and the High Resolution Imager (HRI). Both detectors are sensitive to X-rays in the energy range 0.1 to 2.4 keV (corresponding to a wavelength range of 124.0 to 5.1 Angstrom). A third instrument, the Wide Field Camera (WFC), is co-aligned with the XRT and extends wavelength coverage to the far UV.

    This is the second volume of CDROMs of ROSAT X-ray images. Volume 2 contains all pointed PSPC and HRI images which were released to the public data archives between 01 Jul 1993 and 31 Dec 1993 inclusive. Future volumes containing additional public images will be issued; it is the present plan to eventually have all public ROSAT X-ray images available on CD.

    The images are stored on these CDROMs in the standard FITS (Flexible Image Transport System) image format and also as GIF (Graphic Interchange Format) files. Exposure maps for each FITS image are also included. The exposure maps were compressed from the original FITS data using the Unix compress utility.

  2. Mission Overview

    ROSAT is a joint collaboration between the Max Planck Institut fur Extraterrestrische Physik (MPE) in Germany, the Goddard Space Flight Center (GSFC) and the Center for Astrophysics (CfA) in the US, and Leicester University in the UK. MPE provided the spacecraft and the PSPC, CfA provided the HRI, and Leicester provided the WFC. Data obtained by ROSAT are processed at MPE and GSFC for distribution to principal investigators and the data archives which reside at MPE, GSFC, and Leicester.

    ROSAT was launched on 01 Jun 1990 into a 90 minute ecliptic-polar orbit. After an initial verification and calibration phase, ROSAT obtained the first all-sky survey in X-rays. After the conclusion of the 6-month all-sky survey phase, ROSAT began a phase of pointed observations. Pointed observations with the PSPC will end in early 1994, when the proportional counter gas supply is exhausted. It is expected that pointed observations with the HRI (and WFC) will continue for some time after 1994.

  3. The Detectors

    1. The PSPC

      The Positional Sensitive Proportional Counter, or PSPC, is a circular detector of 8 cm diameter (which subtends 2 degrees at the focus of the XRT). X-rays enter the front side of the PSPC and photoionize the gas which fills the body of the detector. The photoelectrons drift toward a grid of anode wires, where they produce a charge avalanche and induce a charge in the read-out cathodes. The energy of the incident X-ray is proportional to the size of the electron pulse produced, and the 2-dimensional location of the X-ray is determined from the position of the pulses on the read-out cathodes. Thus position on the detector, pulse amplitude (or energy) and time of arrival is recorded for each photon detected by the PSPC. The spatial resolution of the PSPC is about 1 arcminute.

      The PSPC window is held against vacuum by 3 separate support structures. Main support is provided by the PSPC ribs, which consist of an inner ring of diameter approximately 1 degree, an outer ring of diameter approximately 2 degrees and 8 equally-spaced ribs extending radially between the inner and outer rings. Below the ribs are a coarse wire mesh and a fine wire mesh. At the center of the field of view the focus of the X-ray beam from a point source is so sharp that significant shadowing by the coarse wire mesh can occur. In order to reduce the spatial variation of shadowing by the coarse mesh the spacecraft is ``wobbled'', that is, the spacecraft performs a small-amplitude oscillation about the pointing direction. For the PSPC, the amplitude of the wobble is about +/- 3 arcminutes and the wobble period is about 400 seconds.

      A boron filter may be placed in front of the PSPC. The boron filter is used to provide better energy resolution near 0.3 keV. The boron filter is a circular disk of about 1 degree in diameter and thus only covers the inner 1 degree of the PSPC field of view.

    2. The HRI

      The High Resolution Imager, or HRI, is a square microchannel plate detector subtending about 38 arcminutes at the focus of the XRT. X-rays impinging on one of the channels produce electrons which are accelerated down the channel and then detected by the read-out system. The HRI records the 2-dimensional position of the detected X-ray and its time of arrival but does not provide any real energy resolution. The spatial resolution of the HRI is about 1 arcsec. Because the HRI is a solid-state detector, it does not require any support against vacuum, unlike the PSPC. However, there are quantum efficiency variations (at about the 10\% level) over the detector which can cause spatial variations in the effective exposure. During observations with the HRI, the spacecraft is``wobbled'', that is, the spacecraft performs a small-amplitude oscillation about the pointing direction. For the HRI, the amplitude of the wobble is about +/- 1.5 arcminutes and the wobble period is about 400 seconds.

    3. Detector Artifacts

      In each image various detector/telescope effects are apparent. Three of the most noticeable are:

      1. Vignetting. The ROSAT mirror assembly has more effective area on-axis than off-axis, i.e. the mirrors are more efficient at collecting photons from on-axis sources than from off-axis sources. This effect is known as vignetting. Vignetting causes the effective exposure time to decrease with increasing off-axis angle.

      2. Point-spread function variations. The ROSAT point spread function is also dependent on off-axis position. The width of the point spread function is smallest for sources observed on-axis and increases rapidly with off-axis angle. Thus point sources observed at large off-axis angles look much larger than they would if observed on-axis.

      3. The``wobble''. The direction of the spacecraft wobble is usually taken parallel to one of the PSPC support ribs. Any X-ray source under this rib will be occulted by the rib for the entire observation. Thus the area of the sky along this rib receives almost no exposure and appears dark in PSPC images. Other sources in the field not under this rib will in general be occulted by some part of the PSPC window support structure for only part of the observation. The wobble is much less noticeable in HRI images because the HRI does not have any support structure.

  4. The CDROM files
    1. The Directory Structure

      Each CDROM is organized into the following 2 directories:

      • docs - contains documentation about the files on this CDROM

      • images - subdirectories under this directory contain the actual images. Images are stored based on the RA of the center of the field of view. Directly below the images directory are subdirectories of right ascension in hours. For example, the directory 01h contains all images pointed in the interval 01h <= RA < 02h.

    2. The Documentation files The docs directory contains 3 text files describing the files contained on this CDROM set:
      • readme - this file which you are currently reading

      • headers - sample FITS headers for the PSPC and HRI data processed at MPE and at GSFC

      • crossref - a cross-reference table giving the names of the images on the CDROM and the names of the archived data sets from which the images were obtained

      The original ROSAT image file names had to be shortened to 8 characters for this CDROM version. The cross reference table may be used to determine the original name of each CDROM image file and the sequence number of the processed data set.

      There are 3 versions of each of these text files which only differ in the type of special characters used to indicate the end of each line of text. The *.txt files use a LF as the end of line character and are suitable for reading on most Unix and VMS machines. The *.dos files are suitable for MS-DOS machines and use a LF,CR as end of line characters. Finally, the *.mac files are for display on Macintosh machines and use CR as the end of line character.

      VAX/VMS users should note that it may be necessary to preprocess these text files with the CONVERT utility after they are copied from the CDROM before they may be displayed or printed properly. To use CONVERT, type: 

      CONVERT/FDL=FILE.FDL
      where FILE.FDL is a file containing the following two lines: 
      record
format stream_lf
    3. The Images

      All the image files have names of the form IRRRSDDX.EXT where 

             I      instrument: p = PSPC, f = filtered PSPC, h = HRI
       RRR    degrees of RA of pointing direction (000 -- 359)
       S      sign of declination, s for south, n for north
       DD     degrees of declination of pointing direction
       X      a letter (starting with "a") to uniquely
              identify pointings with the same RA and Dec
       EXT    extension, either `fit' for FITS image files, or
              `z' for the compressed FITS exposure maps, or
              `gif' for GIF-format image files
      For example, the first PSPC image of EF Eri with RA(2000) = 3h 14m 12s and Dec(2000) = -23d 15m 36s in FITS format would be named: p048s23a.fit, its compressed exposure map would be p048s23a.z and the GIF formatted image would be p048s23a.gif. Note that file names had to be shortened to 8 characters on the CDROM, so the crossref file in the docs directory may be used to determine the original sequence identifier of the processed data set from which these files were derived.

      All images use a sky coordinate system in which North is to the top and East to the left. Plate scales are 15 arcsec per pixel for the PSPC and 8 arcsec per pixel for the HRI. The units in the images are counts per detector pixel. The units in the uncompressed exposure maps are seconds of time. Each image displays photons from the broad ROSAT energy range (about 0.1 to 2.4 keV).

    4. The FITS Images The FITS (Flexible Image Transport System) format is described in Astronomy and Astrophysics Supplement, 44, 363. Each FITS image on these CDROMs consists of a header followed by the 512 x 512 pixel image data. The pointing information and plate scale are given in the headers where the CRVAL1 and CRVAL2 keywords give the right ascension and declination values (in degrees) of the reference pixel defined by CRPIX1 and CRPIX2 keywords. The CDELT1 and CDELT2 keywords provide the plate scales in the 2 directions.

      The FITS files may be displayed using any data display package which accepts FITS data. Some useful and popular packages for different operating systems are SAOIMAGE and XIMAGE under UNIX and VMS, NIHImage for MacOS, and IMDISP under DOS. Interactive Data Language (IDL) is also useful for the display and analysis of FITS data and runs under all these operating systems.

      Additional information about the FITS format in general may be obtained from: 

       NASA/OSSA Office of Standards and Technology
 Code 633.2
 NASA Goddard Space Flight Center
 Greenbelt MD 20771
 Internet: nost@nssdca.gsfc.nasa.gov
 DECNET: NCF::NOST
 Phone: 301-286-3575
    5. The Compressed PSPC Exposure Maps

      Each PSPC image on this disk has associated with it an exposure map file which gives the variation of the exposure time over the field of view of the PSPC. To save space, these maps have been compressed using the Unix compress facility. To uncompress them, either use the Unix "zcat" command, or copy the compressed file to magnetic disk, renaming it so that it end in an upper case .Z, and then use the Unix "uncompress" command.

    6. The GIF Images

      All the images on these CDROMs are also provided as GIF (Graphic Interchange Format) images. The GIF images have been created from a display of the FITS data using the XIMAGE display package. Each GIF image shows the ROSAT field of view with an appropriate color lookup table and scaling. Right ascension and declination coordinate grids (in J2000 coordinates) have been drawn on each image. These GIF images are provided to allow a quick look at the data. In order to determine observed counts, analysis of the FITS data is necessary.

      The GIF files can be displayed with the xv package under UNIX and VMS, ColorView under DOS, and Giffer under MacOS. For more information on displaying GIF files, a frequently asked questions(FAQ) list (in 3 parts) about GIF and other popular image formats is available via anonymous FTP from rtfm.mit.edu (18.70.0.209) in the directory 

           /pub/usenet/news.answers/pictures-faq
      in the files ``part1'',``part2'', and ``part3''.

  5. For Further Information All X-ray data obtained by ROSAT (including all the images on these CDROMs) are available electronically from

    heasarc.gsfc.nasa.gov (128.183.8.233)

    by using the `anonymous ftp' file transfer program on the internet. The data can be found under the directory

    rosat/data/I/processed_data/XXXXXX/YYYYYYYY

    where 

     I   	  = either hri or pspc
 XXXXX 	  = sequence id group (from 100000 to 900000)
 YYYYYYYY = sequence id
    for example, data associated with the observation having sequence id wp200197 (a PSPC observation of 15 Mon, see image p100n09a on Volume 1) are located in the directory

    rosat/data/pspc/processed_data/200000/wp200197

    To minimize storage space, the data are stored on the jukebox in UNIX compressed format. Users can uncompress the data automatically when ftp'ing. To do this, simply "get" the file by specifying the filename WITHOUT the .Z extension.

    A guide to all the available ROSAT data products can be found in the file rosat/doc/archive/archive_intro.txt also available from

    heasarc.gsfc.nasa.gov

    Further information concerning the ROSAT mission can be found in the ROSAT Mission Description. Further information concerning the description and use of ROSAT data products can be found in the ROSAT Data Products Guide. Both these documents are available from the ROSAT Science Data Centers. In the US, contact the ROSAT Guest Observer Facility, Code 668, NASA/GSFC, Greenbelt, MD 20771 (rosat@heasrc.gsfc.nasa.gov) for further information.

  6. Acknowledgements

    We wish to thank the the scientists and staff at the ROSAT Data Centers at MPE, CfA and GSFC for producing the FITS images presented here and for their continuing efforts to ensure timely access to processed ROSAT data. Production of these CDROMs would not have been possible without the able assistance of the National Space Science Data Center (NSSDC) Data Archive and Distribution Service (NDADS) at GSFC. In particular we thank J. Behnke, C. Cheung and G. Bimson for their unflagging support. Software to update the headers of the images processed at MPE with the AIPS-style WCS keywords was provided by P. Tyler. This help was most appreciated.