The Ultraviolet and Optical telescope (UVOT) on board the SWIFT observatory, plays an important part in the quest to understand gamma-ray bursts. As its name suggests, the UVOT obtains ultraviolet and optical data at high time resolution, with 7 broad band filters and 2 low resolution grisms. This paper forms the second of a pair of papers presenting the initial on-board calibration of the UVOT. The first one (Part 1) deals with distortion, large and small scale sensitivity variations and the telescope point spread function. In this paper we cover the following topics: the photometry of the broadband filters including colour transformations and linearity; the wavelength calibration and sensitivities of the grisms; time resolution and red leak.
The Ultraviolet and Optical telescope (UVOT) is one of the three instruments on board of the SWIFT observatory. UVOT is on the cutting edge of our ability to observe and eventually help scientists to understand gamma-ray bursts. As any space-based telescope it requires both pre-flight and on-orbit calibrations. This paper is the first of a pair of papers presenting the initial on-board calibration of the UVOT. In particular, we'll discuss distortion, large and small scale sensitivity variations and the telescope point spread function.
The Swift Gamma Ray Burst Explorer, chosen in October 1999 as NASA's next MIDEX mission, is now scheduled for launch in October 2004. SWIFT carries three complementary instruments. The Burst Alert Telescope (BAT) identifies gamma-ray bursts (GRBs) and determines their location on the sky to within a few arc-minutes. Rapid slew by the fast-acting SWIFT spacecraft points the two narrow field instruments, an X-ray Telescope (XRT) and an Ultraviolet/Optical Telescope (UVOT), to within the BAT error circle within 70 seconds of a BAT detection. The XRT can determine burst locations to within 5 arc-seconds and measure X-ray spectra and photon flux, whilst the UVOT has a sensitivity down to 24th magnitude and sub arc-second positional accuracy in the optical/uv band. The three instruments combine to make a powerful multi-wavelength observatory with the capability for rapid determination of GRB positions to arc-second accuracy within a minute or so of their discovery, and the ability to measure light-curves and red-shifts of the bursts and after-glows. The paper summarises the mission's readiness for October's launch and operations.
The Swift/UVOT is a 30-cm aperture imaging telescope that is sensitive to photons in the wavelength range 170nm-600nm and is designed to provide near-ultraviolet and optical measurements of γ-ray bursts and other targets that the Swift observatory observes. The performance of the telescope and its photon counting detectors has been assessed in a series of calibration measurements made under vacuum conditions in a test facility at the Goddard Space Flight Center. We describe some of the results of this campaign, including measurements of the instrument throughput, image quality and distortion, and linearity of response. We also describe the spectroscopic capability of the instrument, which is enabled by the use of two grisms operating in the UV and optical bands respectively. The results from the ground calibration activities will form the basis for establishing the full calibration matrix of the instrument once on orbit.
The UV/optical telescope (UVOT) is one of three instruments flying aboard the Swift Gamma-ray Observatory. It is designed to capture the early (~1 minute) UV and optical photons from the afterglow of gamma-ray bursts as well as long term observations of these afterglows. This is accomplished through the use of UV and optical broadband filters and grisms. The UVOT has a modified Ritchey-Chretien design with micro-channel plate intensified charged-coupled device detectors that provide sub-arcsecond imaging. Unlike most UV/optical telescopes the UVOT can operate in a photon-counting mode as well as an imaging mode. We discuss some of the science to be pursued by the UVOT and the overall design of the instrument.
The Swift MIDEX mission is the first-of-its-kind observatory for multi-wavelength transient astronomy. The goal of the mission is to ascertain the origin of gamma-ray bursts and to utilize these bursts to probe the early universe. The Ultra- Violet/Optical Telescope (UVOT) is one of three telescopes flying aboard Swift. The UVOT is a working 'copy' of the Optical Monitor on the X-ray Multi-mirror Mission (XMM- Newton). It is a Ritchey-Chretien telescope with microchannel plate intensified charged-coupled devices (MICs) that provide sub-arcsecond imaging. These MICs are photon counting devices, capable of detecting very low signal levels. When flown above the atmosphere, the UVOT will have the equivalent sensitivity of a 4 m telescope on the ground, reaching a limiting magnitude of 24 for a 1000 second observation in the white light filter. A rotating filter wheel contains sensitive photometric broadband UV and visual filters for determining photometric redshifts. The filter wheel also contains UV and visual grisms for performing low-resolution spectroscopy.
The optical/UV monitor (OM) on the ESA x-ray cornerstone mission XMM is designed to provide simultaneous optical and UV coverage of x-ray targets viewed by the observatory. The instrument consists of a 30 cm modified Ritchey-Chretien telescope. This feeds a compact photon counting detector operating in the blue part of the optical spectrum and the UV (1600 - 6000 angstrom). The OM has a square field of view of approximately 24 arcmin along the diagonal, and will cover the central region of the field of view of the EPIC x- ray cameras where the x-ray image quality is best. Because of the low sky background in space, the sensitivity of the OM for detecting stars will be comparable to that of a 4-m telescope at the Earth's surface; it should detect a B equals 24th magnitude star in a 1000 s observation using unfiltered light. The pixel size of the detector corresponds to 0.5 arc seconds on the sky in normal operation. In front of each of two redundant detectors are filter wheels containing broad band filters. The filter wheels also contain Grisms for low resolution spectroscopy of brighter sources (lambda/Delta lambda 200) and a 4x field expander which will allow high spatial resolution images of the field center to be taken in optical light.
A multi-national consortium of research groups are developing the XMM (x-ray multi-mirror mission) optical monitor to provide a capability for optical identification and photometry of x-ray sources observed by the XMM observatory. This will be the first multi-wavelength facility dedicated to monitoring the variability of diverse sources from the optical through to x-ray wavelengths. Here we describe the system design and discuss progress in the breadboard phase of the development program.
The Optical/UV Monitor Telescope (XMM-OM) on the ESA X-ray Cornerstone mission XMM is designed to provide simultaneous optical and UV coverage of all sources viewed by the observatory in the X-ray band. The instrument consists of a 30 cm Ritchey-Chretien telescope. This feeds a compact photon counting detector that operates in the blue part of the optical spectrum and the UV (1600 - 5500 angstroms), and simultaneously a cooled CCD detector which registers the red light (5500 - 10000 angstroms). The XMM-OM will have a field of view of approximately 25 arcmin diameter, matching that of the X-ray cameras on XMM, and a spatial pixel size in normal operation of 1 arcsec in the blue, and about 1.8 arcsec in the red. Because of the low sky background in space, the sensitivity of the XMM- OM for detecting stars will be comparable to that of a 4-m telescope at the Earth's surface, and it should detect a B equals 24th magnitude star with a photon counting detector in a 1000 s observation using unfiltered light.
The compact photon-counting detector SPAN is described which offers 25-micron spatial resolution and a 25-mm imaging diam. The SPAN detector incorporates position readout within a vacuum-sealed optical-intensifier tube, and a photocathode is used to sense the images with high-blue and near-UV sensitivity. A microchannel-plate intensifier generates an electron cloud that is measured with a position-sensitive readout developed for this application. The position-sensitive readout is a conductive device that, in the context of the SPAN, permits a high count rate and spatial resolution greater than the charge-measurement precision of each electrode. Preliminary photon counting is demonstrated, and the results suggest that the SPAN has a resolution of better than 1/1000 and effective linearity with 8-bit digitization.