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Chapter 1: Instrument Overview
Introduction
This chapter provides a somewhat detailed description of the GALEX instrument and detectors. Users not interested in these details, can skip to the table at the end of this chapter which summarizes the performance and capabilities of GALEX. Additional information about the instruments, their performance, and calibration can be found in these two publications: Morrissey et al. (2005, ApJ, 619, L7) and Morrissey et al. (2007, ApJS, 173, 682).
The instrument is a 50 cm Ritchey-Chretien telescope, with a selectable imaging window or objective grism, feeding a pair of detectors simultaneously via a multilayer dichroic beamsplitter. A view of GALEX on the ground is shown in Figure 1 while a schematic cross section of the instrument is shown in Figure 2.
Optics
The adopted design meets the challenging requirements of providing moderate-resolution optics with a high throughput and large field of view in four different optical paths (two UV simultaneous channels, with imagery and slitless spectroscopy modes), while keeping the instrument compact and simple to build and adjust.
As needed for a SMEX program, special attention was paid to keep the concept tolerant to component positioning, especially for the moveable parts. The slightly modified Ritchey-Chrétien telescope is astigmatism-corrected by a low power fused silica aspheric window in the converging beam. This aspheric window bears a multilayer dichroic coating to separate the FUV (reflected) and NUV (transmitted) light. Its reflecting entrance side corrects the FUV channel, whereas the exit side cancels the entrance side effects and brings in the required amount of correction for the NUV channel. Allowance has been made for a small wedge on this aspheric window to compensate for the coma it induces in the NUV convergent beam.
A selectable CaF2 75 g/mm grism (Figure 3) provides slitless spectroscopy over the whole GALEX FOV. Its wedge angle is adjusted to correct for the coma induced in a converging beam, simultaneously for orders 2 and 1 spectra of the FUV and NUV channels respectively. It turns out that the resulting deviation is low, a primary advantage to allow switching between imagery and slitless spectroscopy. This switching is performed by exchanging the grism with a low power plano-convex CaF2 imaging window. These two components are mounted on a rotating wheel that also provides an opaque position, and is the only moveable part of the telescope.
The grism and the imaging window positional tolerances are loose because they are dioptric components with only a low power. This low power is optimized to correct for the axial chromatism of all the transmissive elements. The field curvature is cancelled by the power in the detector windows, and the detectors are tilted to the best plane. A remarkable feature is that a unique blaze angle on the grism facets is found to provide a well centered efficiency for each channel, owing to the CaF2 index variation with wavelength. There is no in-flight refocus capability.
Three of the GALEX back focal assembly optics are coated with multilayer filters designed to enhance the in-band throughput and off-band rejection of the GALEX instrument. The dichroic coating applied to the entrance face of the fused-silica aspheric corrector plate separates the FUV (reflection) and NUV (transmission) optical paths. This all-dielectric dichroic coating provides a significant improvement over conventional 40%-40% UV beamsplitter coatings, with a mean reflectance of 61% over the 1400-1700Å band and a mean transmittance of 83% over the 1800-2750Å band. A transmissive blue-edge filter coated on MgF2 provides 10% rejection of the OI 1304 airglow line for the FUV channel. A reflective broad-band red-blocking filter on the NUV folding mirror has an edge at 2800Å. This edge yields an additional factor of 10-20 rejection of the NUV zodiacal light background above and beyond the natural CsTe detector photocathode cut-off. This combination of the detector cut-off and the filter means that the a so-called "red leak" is not present for NUV GALEX data.
The CaF2 imaging window and grism were provided by our French partners at the Laboratoire d’Astrophysique de Marseille. The instrument integration, central processor, thermal control, test and project management were all provided by the Jet Propulsion Laboratory (Pasadena, CA). A summary of instrument optical specifications is presented in Table 1.
Detectors
The detectors are each sealed tubes containing a “Z” stack of three 75 mm diameter (65 mm active diameter) microchannel plates (MCPs) and a 2-dimensional delay-line anode readout. They were fabricated by the Space Sciences Laboratory, University of California at Berkeley in collaboration with the California Institute of Technology. The two are basically identical, differing primarily in cathode choice and location as shown in the schematic diagram of Figure 5.
The FUV detector has a CsI cathode on the front MCP, several millimeters below a QE-enhancing grid of wires that is on the inside of the MgF2 tube window. The NUV detector has a Cs2Te cathode deposited over a thin metal layer on the inside of its fused silica window. Because the NUV cathode is proximity focused, it requires the window to be in close proximity to the front MCP. In this case, the NUV window/cathode is 300 µm from the plate, compared to about 6 mm in the FUV window/grid configuration. NUV resolution (5.3" ) is degraded by about 20% compared to FUV (4.2") due to the proximity focusing. Individual photons incident at the cathode set off an isolated avalanche of current inside parallel 10-12 micron diameter microchannel plate (MCP) pores as they accelerate through a few-keV field. The tubes are run between 5200 V (NUV) and 6200 V (FUV), which is temperature-dependent since the resistance of the MCPs varies significantly over the 0 – 30C operating range. In flight, the detectors have only been operated close to 20C in order to fix their calibration. The charge cloud at the rear of the MCP stack, which is approximately 10^7 electrons in size, is deposited on a double-layer anode (Figure 6), divided, and finally measured at each of four outputs. The layers of the anode form a pair of orthogonal delay lines. See Siegmund et al. (1999, SPIE, 3765, 429) for more details about the GALEX detectors.
Performance Parameters
Timing differences measured for the charge pulses at each corner of the anode are proportional to the position of the initiating photon event. For faint UV observations, the combination of low background (zero read noise) and high red rejection that these detectors possess represents a favorable trade-off against the superior QE and field flatness of more conventional CCD detectors. Furthermore, MCP detectors do not require cooling, an important consideration in the contamination-sensitive UV band. In order to improve photometric performance, observations are dithered in a spiral pattern approximately 10' in diameter (at a rate of approximately 0.5 revolutions per minute) that smooth out small scale variations in the flat field and reduce bright source fatigue on the MCPs.
