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 <i>SWIFT</i> 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 <i>Swift</i>/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 <i>Swift</i> 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 coronal diagnostic spectrometer is designed to probe the solar atmosphere through the detection of spectral emission lines in the extreme ultraviolet wavelength range 15.0 - 80.0 nm. By observing the intensities of selected lines and line profiles, it is possible to derive temperature, density, flow, and abundance information for the plasmas in the solar atmosphere. Spatial resolution down to a few arcseconds and temporal resolution of seconds, allows such studies to be made within the fine-scale structure of the solar corona. Furthermore, coverage of a large wavelength band provides the capability for simultaneously observing the properties of plasma across the wide temperature ranges of the solar atmosphere. The CDS design makes use of a Wolter-Schwarzschild II telescope which simultaneously illuminates two spectrometer systems, one operating in normal incidence the other in grazing incidence. In this paper we describe the salient features of the design of the CDS instrument and discuss the performance characteristics of CDS as established through pre-delivery test and calibration activities.
The SPAN position readout device uses a charge division and measurement method to encode the coordinates of the centroid of a charge cloud and thus provides a technique for imaging with photon counting detectors of various formats; for example, microchannel plate intensifiers and gas proportional counters. Its principle of operation causes the position resolution to substantially exceed the charge measurement precision. The reduced signal to noise requirement compared with the competitive devices of comparable imaging format size enables the SPAN readout system to operate at higher input count rates. We present imaging performance results from SPAN readout systems incorporated in several detector formats. The dependence of the physical parameters of the SPAN pattern design on the detector type and geometry together with the performance trade-offs between speed and resolution for these particular detectors are discussed. The practical implementation of the SPAN readout decoding algorithm is outlined. We describe the experimental applications for which the SPAN readout system has been proposed.
Microchannel plate (MCP) detectors are often used with charge division anode readouts, such as the SPAN anode, to provide high position resolution. This paper discusses the effect on image quality, of digitization (causing fixed patterning), electronic noise, pulse height distribution (PHD) and charge cloud size. The discussion is supported by experimental data obtained from a one dimensional SPAN anode, developed for the SOHO Coronal Diagnostic Spectrometer (CDS) Grazing Incidence Spectrometer (GIS). Results from a computer model of this detector, and from a charge cloud simulation model, are also included. The SPAN anode normally has three sinusoidal electrodes with phase differences of 120 degree(s)C. An alternative configuration is to use a phase difference of 90 degree(s)C. This paper compares the advantages and disadvantages of these arrangements.
The concept for the 2D position-readout device for the SPAN photon-counting detector is presented with attention to the count rates, spatial resolution, and charge-measurement precision. The electrodes which are deposited on the planar substrate result from charge division induced by a charge cloud, the centroid position of which is encoded by the ratio of charge magnitudes. The SPAN electrode design is analyzed and theorized to permit 1000 x 1000-pixel resolution at 1 MHz. The SPAN spiral-anode six-electrode design is compared to the Vernier-anode twelve-electrode structure for encoding 2D position, and digital precision is analyzed at count rates up to 1 MHz. The SPAN readout affords resolution levels of up to 1/1000 across the entire active area at 8-bit digitization.