In-Orbit Vignetting Calibrations of XMM-Newton Telescopes

We describe measurements of the mirror vignetting in the XMM-Newton Observatory made in-orbit, using observations of SNR G21.5-09 and SNR 3C58 with the EPIC imaging cameras. The instrument features that complicate these measurements are briefly described. We show the spatial and energy dependences of measured vignetting, outlining assumptions made in deriving the eventual agreement between simulation and measurement. Alternate methods to confirm these are described, including an assessment of source elongation with off-axis angle, the surface brightness distribution of the diffuse X-ray background, and the consistency of Coma cluster emission at different position angles. A synthesis of these measurements leads to a change in the XMM calibration data base, for the optical axis of two of the three telescopes, by in excess of 1 arcmin. This has a small but measureable effect on the assumed spectral responses of the cameras for on-axis targets. This revised version was published online in July 2006 with corrections to the Cover Date.     

A realistic model for point-sources imaged on array detectors: The model and initial results

We have constructed a computer model for simulation of point-sources imaged on two-dimensional detectors. An attempt has been made to ensure that the model produces data that mimic real data taken with 2-D detectors. To be realistic, such simulations must include randomly generated noise of the appropriate type from all sources (e.g. source, background, and detector). The model is generic and accepts input values for parameters such as pixel size, read noise, source magnitude, and sky brightness. Point-source profiles are then generated with noise and detector characteristics added via our model. The synthetic data are output as simple integrations (onedimensional), as radial slices (two-dimensional), and as intensity-contour plots (three-dimensional). Each noise source can be turned on or off so that they can be studied separately as well as in combination to yield a realistic view of an image. This paper presents the basic properties of the model and some examples of how it can be used to simulate the effects of changing image position, image scale, signal strength, noise characteristics, and data reduction procedures.Use of the model has allowed us to confirm and quantify three points: 1) The use of traditionalsize apertures for photometry of faint point-sources adds substantial noise to the measurement which can significantly degrade the quality of the observation; 2) The number of pixels used to estimate the background is important and must be considered when estimating errors; and 3) The CCD equation normally used by the astronomical community consistently overestimates the signal-to-noise obtainable by a measurement while a revised equation, discussed here, provides a better estimator.     

Optimizing CCDs for spectrographs

For a spectrograph giving a fixed format spectrum,the quantum efficiency (QE) can be optimized for the different wavelengthsacross the CCD. It is shown that a slight modification of the conventionalsingle layer anti-reflection coating can give major improvements in QE forsuch instruments, while at the same time minimizing problems with fringingand stray light from the CCD.     

A Golden Era for Astronomy: The Advent of CCDs and Infrared Arrays

The advent of solid-state imaging devices transformed astronomy. Beginning with the introduction into astronomy of charge-coupled devices in 1976, followed a decade later by infrared arrays, astronomers gained access to near-perfect imaging devices. The consequences have been nothing short of revolutionary, perhaps especially so in the infrared. Witness, for example, the spectacular pictures from the Hubble Space Telescope cameras, or the impressive infrared imagery from the 2MASS project. Within the last decade CCD formats deployed or planned for use in ground-based cameras have become huge. Infrared mosaics, stimulated by the Next Generation Space Telescope, are coming soon. In addition, new technologies such as CMOS (Complementary Metal Oxide Semiconductors) and STJs (Superconductiong Tunnel Junctions) are being developed and the future of astronomical detectors looks very exciting, especially in an era of giant telescopes performing at their diffraction-limit. This revised version was published online in July 2006 with corrections to the Cover Date.     

Radiation shielding study for the joint European X-ray Telescope focal plane Detectors

The requirements, philosophy and implementation of inorbit radiation shielding for the Charge Coupled Devices (CCDs) on-board the Joint European X-ray Telescope (JET-X) are described in detail. Relevant trade-offs between displacement damage, spectral degradation, instrument mass and mission lifetime are examined and a maximum permissible fluence at the CCDs derived. The calculations show that for the ambient JET-X radiation environment no benefit is obtained by increasing the shield thickness above 30 mm of aluminum due to the local production of cascade nucleons. However, a large flare of the August 1972 type will exceed the required maximum fluence by a factor of 2. In order to survive such a flare, a thicker shield is required. Because of mass constraints, JET-X will fly a composite shield composed of 20 mm of aluminum on the outside and 5 mm of tungsten on the inside. Such a shield is designed to ensure that the degradation in the CCD FWHM energy resolution is no more than 40% around the Fe line over the nominal two year mission lifetime (a factor of 2 x the intrinsic line broadening). The predicted degradation in energy resolution and the efficacy of the shield design has been recently verified by experiment (Owens et al., Nucl. Instr. and Meth., A361 (1995) 602).     

Phase Resolved High Speed Photometry and Spectroscopy of Pulsars

We describe the implementation of two High Time Resolution modes for ESO's new generation CCD controller FIERA. These new modes have been used to perform phase-resolved high speed photometry and spectroscopy of pulsars with the FORS instruments at the VLT.