Rocket-borne instrument with a high-resolution microchannel plate detector for planetary UV spectroscopy

Comparison of the imaging characteristics of curved-channel and straight-channel microchannel plates

We have measured and compared the spatial fidelity of two types of microchannel plates over roughly half of their active area. Measurements of the spatial fidelity of curved-channel microchannel plates confirm earlier reports of large (>25 microm), irregular position offsets between the front and the back of the microchannel plates. Straight-pore microchannel plates used in a chevron configuration, on the other hand, showed almost no such position offsets (<4 microm).

Photometric calibration of the first three spectroscopic orders of an extreme-ultraviolet spectrometer by use of synchrotron radiation

We report here a technique for determining the quantum throughput of a high-resolution, extreme-UV spectrometer by use of the National Institute of Standards and Technology's Synchroton Ultraviolet Radiation Facility at Gaithersburg, Md. (SURF-II). Observations were obtained at three different synchrotron operating energies with and without a magnesium fluoride filter and a tin-germanium filter. This calibration permits us to extract system efficiencies for three overlapping spectroscopic orders with uncertainties of 10-30%. The uncertainties quoted in this preliminary experiment were limited by the available time, and we propose that with minor refinements a significant improvement could be realized.

Light-trap design using multiple reflections and solid-angle attenuation: application to a spaceborne electron spectrometer

The design and performance of a new light trap for a spaceborne electron spectrometer are described. The light trap has a measured photon-rejection ratio of 2 x 10(-11), allowing only one in 5 x 10(10) incident photons to reach the sensitive area of the instrument. This rejection is more than sufficient because the ambient ultraviolet in Earth orbit requires a rejection no better than 10(-8) to maintain the photon interference to less than 10 count/s. The light trap uses triple reflections to keep most of the light passing through the entrance slit away from the sensitive area of the spectrometer. However, because of electron-optic requirements, the edge of one of the metallic electrodes falls within the field of view of the sensitive area, allowing double-reflection photon paths to reach the sensitive area. Assuming diffuse reflectance r, the author shows that the photon rejection can be written as epsilon = G(2)r(2) + G(3)r(3) with G(3) approximately 10 times larger than G(2). Both coefficients depend only on the internal surface geometry; G(2) represents the electrode edge reflections (second-order) and G(3) represents the triple-reflection (third-order) paths. As shown by the analysis and measurements taken at two different values of r, the rejection is controlled by triple reflections if r > 0.08. It is shown that the average reflectance of all the internal surfaces must be less than 0.006, which is consistent with the data on the black coating applied to all surfaces. The analysis makes it possible to compare the photon contributions of each of the internal reflecting areas and to estimate the effective scattering width of the metallic electrode edge.

Position offsets in curved-channel microchannel plate detectors

Microchannel plate detectors are widely used for ultraviolet (UV) and extreme-ultraviolet (EUV) observations. Curved-channel microchannel plates, or C plates, provide an electron gain of approximately 10(6) in a single plate because the curved channels prevent ion feedback. However, offsets between input and output in curved-channel microchannel plates produce slight distortions in the spatial or spectral scale of thedetector, complicating the use of such detectors for high-resolution imaging and spectroscopy. We have examined the image distortion caused by nonconstant curvature of the channels by (a) observing spectral nonlinearities in an EUV spectrometer, (b) measuring the positions of several plugged channels of a typical plate and comparing the positions of the channels on both the input and the output sides, and (c) mapping input versus output of typical plates, using a mechanically scanned spot of UV light. We find that a typical C plate with 25-microm-diameter pores exhibits +/-25-microm image distortion across the plate. Calibration of this image distortion as reflected in the spectral nonlinearity of our EUV spectrograph improves the spectral line positions by a factor of 4, permitting solar-emission-line Doppler shift determination of +/- 2 microm (+/- 1 km/s).

Hex-square moire patterns in imagers using microchannel plates

In electronic imaging detectors using microchannel plates, the mismatch between the pixels on a square mesh and the microchannels on a hexagonal mesh produces moire image defects. Theoretical statistical estimates of the sizes of the microposition offsets and the flat field intensity errors are calculated, showing the trade-off between resolution and position accuracy. A distinction is made between moments of spot images and moments of the single pixel response functions. As the resolution between the hex and square meshes is improved, the detector resolution is improved but at the expense of an ~10% moire pattern. These moire patterns will not be properly corrected by dividing by the flat field image.