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ROSAT


General characteristics


Field list

These are the fields for the ROSAT table.

Field descriptions

image

instrument

origin

rosat

exposure

ra

dec

l

b

target

Notes

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).

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 first volume of CDROMs of ROSAT X-ray images. Volume 1 contains all pointed PSPC and HRI images which were released to the public data archives between 01 Jan 1993 and 30 Jun 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.

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.

The Detectors

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 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.

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.

Detector Artifacts

In each image various detector/telescope effects are apparent. Three of the most noticeable are:
  1. a) 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. b) 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. c) 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.

Further Information

All the FITS images and exposure maps on these CDROMs, as well as many other files related to ROSAT and other High-Energy missions, are available electronically by using the `anonymous ftp' file transfer program on the internet. The PSPC broad band images and exposure maps (along with PSPC soft and hard band images) are available from legacy.gsfc.nasa.gov (128.183.8.233) in the directory rosat/data/pspc/images/fits and the HRI images can be found in the directory rosat/data/hri/images/fits . Note that all these files use the longer file naming convention, as listed in the crossref file in the docs directory on the CDROMs. All these ROSAT images are also available in the form of photon event lists, which give the arrival time and energy of each individual photon. A guide to all the available ROSAT data products can be found in the file rosat/doc/archive/archive_intro.txt also available from legacy.gsfc.nasa.gov .

Further information concerning the ROSAT mission can be found in the ROSAT Mission Description and 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.

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.
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