Jeffrey S. Morgan

High resolution decoding of Multi-Anode Microchannel Array detectors

The Multi-Anode Microchannel Array (MAMA) is a photon counting detector which utilizes a photocathode for photon to electron conversion, a microchannel plate (MCP) for signal amplification and a proximity focused anode array for position sensitivity. The detector electronics decode the position of an event through coincidence discrimination. The decoding algorithm which associates a given event with the appropriate pixel is determined by the geometry of the array. A new algorithm incorporated into a CMOS Application Specific Integrated Circuit (ASIC) decoder which improves the pixel spatial resolution is described. The new algorithm does not degrade the detector throughput and does not require any modifications to the detector tube. The standard MAMA detector has a pixel size of 25 x 25 square microns, but with the new decoder circuit the pixel size is reduced to 12.5 x 12.5 square microns. We have built the first set of decode electronics utilizing the new ASIC chips and report here on the first imaging tests of this system.

Imaging extreme ultraviolet spectrometer employing a single toroidal diffraction grating: the initial evaluation.

A high-efficiency extreme ultraviolet (EUV) imaging spectrometer has been constructed and tested. The spectrometer employs a concave toroidal grating illuminated at normal incidence in a Rowland circle mounting and has only one reflecting surface. The toroidal grating has been fabricated by a new technique employing an elastically deformable submaster grating which is replicated in a spherical form and then mechanically distorted to produce the desired aspect ratio of the toroidal surface for stigmatic imaging over the selected wavelength range. The fixed toroidal grating used in the spectrometer is then replicated from this surface. Photographic tests and initial photoelectric tests with a 2-D pulse-counting detector system have verified the image quality of the toroidal grating at wavelengths near 600 A. The results of these initial tests are described in detail, and the basic designs of two instruments which could employ the imaging spectrometer for astrophysical investigations in space are briefly described, namely, a high-resolution EUV spectroheliometer for studies of the solar chromosphere, transition region, and corona and an EUV spectroscopic telescope for studies of nonsolar objects.

MAMA detector systems - A status report

A number of different MAMA detector systems are now in use at both ultraviolet and visible wavelengths. MAMA detectors with formats of 256 x 1024 pixels and with pixel dimensions of 25 x 25 microns2 are being used in the laboratory and at ground-based telescopes and an ultraviolet version has recently been flown on a Black Brant sounding rocket. Third-generation (224 x 960)-pixel MAMA detectors with 25 x 25 microns2 pixels are also being used in the laboratory and at ground-based telescopes and a (224 x 960)-pixel detector with 14 x 14 microns2 pixels is currently under test in the laboratory. Third-generation MAMA detectors with formats of 360 x 1024 pixels are under development for use at extreme ultraviolet (EUV) wavelengths on the European Space Agency/NASA Solar and Heliospheric Observatory (SOHO) mission and detectors with 1024 x 1024 and 2048 x 2048 pixels are under development for use at ultraviolet and far ultraviolet (FUV) wavelengths on the NASA Goddard Space Flight Centers Hubble Space Telescope Imaging Spectrograph (STIS). Both of these detectors have pixel dimensions of 25 x 25 microns2. This paper describes the configurations, modes-of-operation and some of the latest performance data for the different detector systems.

Imaging MAMA detector systems

Imaging multianode microchannel array (MAMA) detector systems with 1024 x 1024 pixel formats have been produced for visible and UV wavelengths; the UV types employ solar blind photocathodes whose detective quantum efficiencies are significantly higher than those of currently available CCDs operating at far-UV and EUV wavelengths. Attention is presently given to the configurations and performance capabilities of state-of-the-art MAMA detectors, with a view to the development requirements of the hybrid electronic circuits needed for forthcoming spacecraft-sensor applications. Gain, dark noise, uniformity, and dynamic range performance data are presented for the curved-channel chevron, Z-plate, and helical-channel high gain microchannel plate configurations that are currently under evaluation with MAMA detector systems.

High time-resolution imaging with the MAMA detector systems

Current uses of the MAMA detector which utilize the photon time-tagging capabilities of these detectors are reported. These applications currently include image stabilization by means of post-processing corrections of platform drift and speckle interferometry. The initial results of a sounding rocket experiment to obtain UV images of NGC 6240 and results from speckle interferometry of Neptunes moon Triton are presented.

Position Sensitivity Of MAMA Detectors

We report on the results of laboratory and telescopic measurements of the position sensitivity of a visible MAMA. detector utilizing a coarse-fine array. We also report on the photometric accuracy of this detector under the condi-tion of point source illumination. The detector measured uses a bialkalai photocathode and has 25 μm2 pixels in a 256 x 1024 format. It was illuminated with a 3.5 pm spot of light (FWHM) which was moved across the detector in 1 μm increments. It is found that computed centroid positions are accurate across the entire array to within 0.04 pixels. For the extreme case of illumination with a delta function source, this detector experienced photometric errors of 10-15%. Both the position sensitivity and the photometric accuracy with point source illumination are limited in this detector by errors in the flat field corrections caused by the presence of cross-coupling between adjacent pixels. Initial measurements with a MAMA detector utilizing a fine-fine array indicate that this cross-coupling has been eliminated in the most recent versions of the detector.

Fabrication of toroidal and coma-corrected toroidal diffraction gratings from spherical master gratings using elastically deformable substrates: a progress report

A technique has been developed which permits toroidal, and coma-corrected toroidal, diffraction gratings to be replicated from spherical master gratings by the use of elastically-deformable substrates. Toroidal gratings correct for astigmatism and, thus, make it possible to construct stigmatic spectrometers that employ a single reflective diffraction grating. These spectrometers are particularly useful for the extreme-ultraviolet (EUV) wavelength range, where reflection coefficients are low, since the single optical surface provides for dispersion, focusing, and astigmatism correction. The fabrication procedures for the pure toroidal, and coma-corrected toroidal, gratings are described, and initial test results are presented. The use of the toroidal gratings in a high-resolution sounding-rocket EUV spectroheliometer, and in both the coronal diagnostics spectrometer and the ultraviolet coronagraph spectrometer on the ESA/NASA solar and heliospheric observatory mission, is described briefly, and the use of this technique for the fabrication of a coma-corrected toroidal grating for the prime Rowland spectrograph of the FUSE/Lyman mission is briefly discussed.

Performance Characteristics Of A Curved-Channel Microchannel Plate With A Curved Input Face And A Plane Output Face

The performance of a high-gain curved-channel microchannel plate (C2MCP) with a spherical concave input face and a plane output face has been evaluated in the laboratory. This format allows the input face of the MCP to match a curved focal surface, such as in a Rowland circle spectrometer mounting, and, at the same time, permits the use of a high-resolution plane readout array in proximity focus with the output face. The MCP was evaluated in a discrete-anode Multi-Anode Microchannel Array (MAMA) detector system. The MCP tested had channel diameters of 12 gm, a rectangular active area of 9 x 27 mm2, and a front face radius-of-curvature of 250 mm. The length-to-diameter (LID) ratio of the channels varied from 136:1 at the edges of the active area to 106:1 at the center. The variation of the LID ratio across the active area of the MCP allowed the relationship between the saturated modal gain of the pulse-height distribution and the LID ratio to be examined from modal gain measurements. The saturated ,modal gain was found to be inversely proportional to the LID ratio and directly proportional to the applied MCP voltage. The measured performance characteristics are described and compared with gain models based upon the geometric parameters of the MCP.

Design and test of a high-resolution EUV spectroheliometer

The HiRES High-Resolution EUV Spectroheliometer is a sounding rocket instrument yielding very high spatial, spectral, and temporal resolution images of the solar outer atmosphere, on the basis of a 45-cm Gregorian telescope feeding a normal-incidence stigmatic EUV spectrometer with imaging multianode microchannel-array detector system, as well as an IR spectrometer with imaging CCD detector system. Attention is given to the expected performance of this system, including the effects of vibrational misalignments due to the sounding rocket flight environment.

High-resolution stigmatic EUV spectroheliometer for studies of the fine scale structure of the solar chromosphere, transition region, and corona

We describe the design of a high-resolution stigmatic extreme-ultraviolet (EUV) spectroheliometer, configured for flight on a Black Brant sounding rocket, which consists of a 45-cm Gregory telescope coupled to a spectrometer employing a single toroidal diffraction grating in a normal-incidence Rowland circle mounting and an imaging pulse-counting Multi-Anode Microchannel Array (MAMA) detector system. The toroidal diffraction grating is fabricated by a new technique employing an elastically-deformable sub-master grating which is replicated in a spherical form and then mechanically distorted to produce the desired aspect ratio of the toroidal surface for stigmatic imaging over the selected wavelength range. The spectroheliometer will produce spatially-resolved spectra of the chromosphere, transition-region and corona with an angular resolution of 0.4 arc sec or better, a spectral resolution AII of about 1O in first order, and a temporal resolution of the order of seconds. Because of the geometric fidelity of the MAMA detector system, the speciroheliometer will be able to determine Doppler shifts to a resolution of at least 2 mA at wavelengths near 600 A (-1.0 km s1), depending on the level of the accumulated signal. The unique characteristics of the spectroheliometer will be used in combination with plasma-diagnostic techniques to study the temperature, density and velocity structures of specific features in the solar outer atmosphere.