The Ionospheric Connection Explorer Mission: Mission Goals and Design.

The Ionospheric Connection Explorer, or ICON, is a new NASA Explorer mission that will explore the boundary between Earth and space to understand the physical connection between our world and our space environment. This connection is made in the ionosphere, which has long been known to exhibit variability associated with the sun and solar wind. However, it has been recognized in the 21st century that equally significant changes in ionospheric conditions are apparently associated with energy and momentum propagating upward from our own atmosphere. ICON's goal is to weigh the competing impacts of these two drivers as they influence our space environment. Here we describe the specific science objectives that address this goal, as well as the means by which they will be achieved. The instruments selected, the overall performance requirements of the science payload and the operational requirements are also described. ICON's development began in 2013 and the mission is on track for launch in 2017. ICON is developed and managed by the Space Sciences Laboratory at the University of California, Berkeley, with key contributions from several partner institutions.

The Far Ultra-Violet imager on the ICON mission.

ICON Far UltraViolet (FUV) imager contributes to the ICON science objectives by providing remote sensing measurements of the daytime and nighttime atmosphere/ionosphere. During sunlit atmospheric conditions, ICON FUV images the limb altitude profile in the shortwave (SW) band at 135.6 nm and the longwave (LW) band at 157 nm perpendicular to the satellite motion to retrieve the atmospheric O/N2 ratio. In conditions of atmospheric darkness, ICON FUV measures the 135.6 nm recombination emission of O+ ions used to compute the nighttime ionospheric altitude distribution. ICON Far UltraViolet (FUV) imager is a CzernyTurner design Spectrographic Imager with two exit slits and corresponding back imager cameras that produce two independent images in separate wavelength bands on two detectors. All observations will be processed as limb altitude profiles. In addition, the ionospheric 135.6 nm data will be processed as longitude and latitude spatial maps to obtain images of ion distributions around regions of equatorial spread F. The ICON FUV optic axis is pointed 20 degrees below local horizontal and has a steering mirror that allows the field of view to be steered up to 30 degrees forward and aft, to keep the local magnetic meridian in the field of view. The detectors are micro channel plate (MCP) intensified FUV tubes with the phosphor fiber-optically coupled to Charge Coupled Devices (CCDs). The dual stack MCP-s amplify the photoelectron signals to dominate the CCD noise and the rapidly scanned frames are co-added to digitally create 12-second integrated images. Digital on-board signal processing is used to compensate for geometric distortion and satellite motion and to achieve data compression. The instrument was originally aligned in visible light by using a special grating and visible cameras. Final alignment, functional and environmental testing and calibration were performed in a large vacuum chamber with a UV source. The test and calibration program showed that ICON FUV meets its design requirements and is ready to be launched on the ICON spacecraft.

Single-quantum dot imaging with a photon counting camera.

The expanding spectrum of applications of single-molecule fluorescence imaging ranges from fundamental in vitro studies of biomolecular activity to tracking of receptors in live cells. The success of these assays has relied on progress in organic and non-organic fluorescent probe developments as well as improvements in the sensitivity of light detectors. We describe a new type of detector developed with the specific goal of ultra-sensitive single-molecule imaging. It is a wide-field, photon-counting detector providing high temporal and high spatial resolution information for each incoming photon. It can be used as a standard low-light level camera, but also allows access to a lot more information, such as fluorescence lifetime and spatio-temporal correlations. We illustrate the single-molecule imaging performance of our current prototype using quantum dots and discuss on-going and future developments of this detector.

Detectors for single-molecule fluorescence imaging and spectroscopy.

Single-molecule observation, characterization and manipulation techniques have recently come to the forefront of several research domains spanning chemistry, biology and physics. Due to the exquisite sensitivity, specificity, and unmasking of ensemble averaging, single-molecule fluorescence imaging and spectroscopy have become, in a short period of time, important tools in cell biology, biochemistry and biophysics. These methods led to new ways of thinking about biological processes such as viral infection, receptor diffusion and oligomerization, cellular signaling, protein-protein or protein-nucleic acid interactions, and molecular machines. Such achievements require a combination of several factors to be met, among which detector sensitivity and bandwidth are crucial. We examine here the needed performance of photodetectors used in these types of experiments, the current state of the art for different categories of detectors, and actual and future developments of single-photon counting detectors for single-molecule imaging and spectroscopy.

Fluorescence lifetime microscopy with a time- and space-resolved single-photon counting detector.

We have recently developed a wide-field photon-counting detector (the H33D detector) having high-temporal and high-spatial resolutions and capable of recording up to 500,000 photons per sec. Its temporal performance has been previously characterized using solutions of fluorescent materials with different lifetimes, and its spatial resolution using sub-diffraction objects (beads and quantum dots). Here we show its application to fluorescence lifetime imaging of live cells and compare its performance to a scanning confocal TCSPC approach. With the expected improvements in photocathode sensitivity and increase in detector throughput, this technology appears as a promising alternative to the current lifetime imaging solutions.

A space- and time-resolved single photon counting detector for fluorescence microscopy and spectroscopy.

We have recently developed a wide-field photon-counting detector having high-temporal and high-spatial resolutions and capable of high-throughput (the H33D detector). Its design is based on a 25 mm diameter multi-alkali photocathode producing one photo electron per detected photon, which are then multiplied up to 107 times by a 3-microchannel plate stack. The resulting electron cloud is proximity focused on a cross delay line anode, which allows determining the incident photon position with high accuracy. The imaging and fluorescence lifetime measurement performances of the H33D detector installed on a standard epifluorescence microscope will be presented. We compare them to those of standard single-molecule detectors such as single-photon avalanche photodiode (SPAD) or electron-multiplying camera using model samples (fluorescent beads, quantum dots and live cells). Finally, we discuss the design and applications of future generation of H33D detectors for single-molecule imaging and high-throughput study of biomolecular interactions.

Photon-Counting H33D Detector for Biological Fluorescence Imaging.

We have developed a photon-counting High-temporal and High-spatial resolution, High-throughput 3-Dimensional detector (H33D) for biological imaging of fluorescent samples. The design is based on a 25 mm diameter S20 photocathode followed by a 3-microchannel plate stack, and a cross delay line anode. We describe the bench performance of the H33D detector, as well as preliminary imaging results obtained with fluorescent beads, quantum dots and live cells and discuss applications of future generation detectors for single-molecule imaging and high-throughput study of biomolecular interactions.

Electronic and optical moi? Interference with microchannel plates: artifacts and benefits.

The spatial resolution of position-sensitive detectors that use stacks of microchannel plates (MCP's) with high-resolution anodes can be better than 20-microm FWHM [Proc. SPIE 3114, 283-294 (1997)]. At this level of accuracy, channel misalignments of the MCP's in the stack can cause observable moiré interference patterns. We show that the flat-field detector response can have moiré beat pattern modulations of as great as approximately 27% with periods from as small as a few channel diameters to as great as the size of a MCP multifiber. These modulations, however, may be essentially eliminated by rotation of the MCP's or by a mismatch of the channel sizes. We also discuss how the modulation phenomena can be a useful tool for mapping the metric nonlinearities of MCP detector readout systems. Employing the optical moiré effect, we demonstrate a simple, but effective, technique for evaluation of geometrical deformations simultaneously over a large MCP area. For a typical MCP, with a 60-channel-wide multifiber, we can obtain accuracies of 1.2 mrad for multifiber rotations and twists and 35/(L/p) mrad for channel-long axis distortions (where L/p is MCP thickness to interchannel distance ratio). This technique may be used for the development of MCP x-ray optics, which impose tight limitations on geometrical distortions, which in turn are not otherwise easily measurable with high accuracy.

Highly absorptive coating for the vacuum ultraviolet range.

We have developed new, highly absorptive coatings for the vacuum UV wavelength range. These coatings display two distinct granularity scales: large structures of a 10-100-microm scale form efficient light traps, upon which are superimposed structures of a submicrometer scale. We present results for the total hemispherical reflectivity at normal incidence for 121.6 nm and at a grazing angle incidence for 17.1, 30.4, 58.4, and 121.6 nm. These measurements were made for the new coatings as well as for various coatings in common use. Absorption of the new coatings is in some cases higher than for the best-known coatings and, in contrast to the latter, they are mechanically robust.

Extreme ultraviolet quantum detection efficiency of rubidium bromide opaque photocathodes.

We present measurements of the quantum detection efficiency (QDE) of rubidium bromide opaque photo-cathodes over the 44-1560-A wavelength range. We achieved QDEs of >60% at lambda = 68 A, and>40% at lambda approximately 920 A, for RbBr photocathode layers applied to the surface of microchannel plates (MCPs). The photoelectric threshold is observed at lambda approximately 1560 A, and there is a broad ( approximately 100-A) QDE minimum centered at lambda approximately 775 A which correlates with 2x the band gap energy for RbBr. The QDE is characterized by four peaks centered at lambda approximately 68 A, lambda approximately 400 A, lambda approximately 600 A, and approximately 1050 A. The QDE peaks at lambda approximately 400 A, approximately 600 A, and approximately 1050 A correspond with emission of 3, 2, and 1 photoelectrons, respectively. The QDE at the lambda approximately 68-A peak is associated with a d-f resonant absorption feature of RbBr. QDE contributions of the photocathode material inside the channels, and on the interchannel web, have been determined. Measurements of the angular variation of the QDE from 0 degrees to 35 degrees to the channel axis are also presented. We describe a simple QDE model and show that its predictions are in accord with the QDE measurements. Preliminary assessment of the stability of RbBr indicates that no QDE degradation occurs after limited exposure (20 h) to air at low humidity (<30%). Examination of the photocathode structure with an electron microscope reveals a rough surface with a scale of the order of 0.5 microm.

Soft x-ray and extreme ultraviolet quantum detection efficiency of potassium bromide photocathode layers on microchannel plates.

The quantum detection efficiency (QDE) of potassium bromide, applied directly to the surface of a microchannel plate (MCP), has been measured over the wavelength range from 44 to 1216 A. We present the first measurements for the QDE of KBr between 44 and 256 A. These show that there is a high QDE peak (~70%) centered at ~70 A. The results at wavelengths above 256 A agree with our previous study. Investigation of the angular dependence of the QDE indicates that maximum efficiencies are achieved for graze angles

Soft x-ray and extreme ultraviolet quantum detection efficiency of potassium chloride photocathode layers on microchannel plates.

We present measurements of the quantum detectio efficiency (QDE) of potassium chloride, applied directly to the surface of a microchannel plate (MCP), over the 44-1460-A wavelength range. The contributions of the photocathode material in the channels, and on the interchannel web, to the QDE have been determined. Two broad peaks in the QDE centered at lambda congruent with 500 A and lambda congruent with 900 A are apparent, the former with ~40% peak QDE and the latter with ~30% peak QDE. The photoelectric threshold is observed at lambda ~1400 A, and there is a narrow QDE minimum at lambda~ 670 A, which correlates with 2xthe band gap energy for KC1. The angular variation of the QDE from 0 to 35 degrees to the channel axis has also been examined. We describe a simple QDE model and show that its predictions are in accord with our QDE measurements. Assessment of the stability of KC1 shows that there was no significant degradation of the QDE at wavelengths of

Ultraviolet quantum detection efficiency of potassium bromide as an opaque photocathode applied to microchannel plates.

We have measured the quantum detection efficiency (QDE) of potassium bromide as a photocathode applied directly to the surface of a microchannel plate over the 250-1600A wavelength range. The contributions of the photocathode material in the channels and on the interchannel web to the QDE have been determined. Two broad peaks in the QDE centered at ~450 and ~1050 A are apparent, the former with ~50% peak QDE and the latter with ~40% peak QDE. The photoelectric threshold is observed at ~1600 A, and there is a narrow QDE minimum at ~750 A which correlates with 2x the band gap energy for KBr. The angular variation of the QDE from 0 to 40 degrees to the channel axis has also been examined. The stability of KBr with time is shown to be good with no significant degradation of QDE at wavelengths below 1216 A over a 15-day period in air.