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).