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NIR Imaging for Deep Space

Photonics Spectra
Jun 2007
Michael A. Greenwood

What is dark energy? How much is there? And what is it doing to the universe?

A growing number of scientists is hoping to answer those complex questions as preparations are made for the 2011 launch of the Joint Dark Energy Mission sponsored by NASA and the US Department of Energy. Dark energy was discovered several years ago, and some believe that it may be accelerating the expansion of the universe.


Scientists are working with the Mosir 950 spectroscopy camera designed by Intevac Inc., which offers a unique transferred-electron photocathode, for possible inclusion on upcoming missions such as the Joint Dark Energy Mission. (HVPS = high-voltage power supplies; FPGA = field-programmable gate array).

The mission is ambitious. A probe will gather data on dark energy from far-flung corners of the universe. But capturing clear NIR readings of distant supernovae (which may be emitting energy 8 to 10 billion years old, more than halfway back in time according to the big-bang theory) remains a challenge.

A recently released NIR camera — slightly smaller than an American football — may play a role. The Mosir 950 manufactured by Intevac Inc. of Santa Clara, Calif., has features that could lend it to the type of deep-space spectroscopy planned. The device incorporates a transferred-electron InGaAs/InP photocathode coupled to an electron-bombarded CCD readout. The sensor can absorb a single photon in a spectral band spanning 950 to 1650 nm (a vital gap in which elements such as water, methane and other substances can be measured).

Zero read noise

The camera is currently being modified by NASA researchers investigating its potential for deep-space readings. They are seeking to achieve zero read noise. This would result in dark rates that are orders of magnitude lower than that offered by an integrating solid-state detector. Detector background is the major factor limiting sensitivity and can critically impair results. A low-background photon counter can reduce the required observation time by factors of >3. When measuring ultrafaint objects — such as supernovae — observation times can be weeks, and this can mean the difference between success and failure.

To operate the camera in a fast-framing mode to count individual photons, the investigators are exposing the photocathode to much deeper cooling to achieve an optimal dark rate of <10–4 counts per pixel per second. It comes with –40 °C thermoelectric cooling; the researchers must achieve temperatures that are as cold as –140 °C.

The camera also is being considered for a separate research project — the study of extrasolar planets by the Terrestrial Planet Finder mission. The scientists believe that it has the potential to gather readings from atmospheres light-years away to determine whether they have anything in common with Earth. The mission, which has yet to be funded, would probe relatively near systems in the Milky Way galaxy, ~10 to 50 light-years distant. The Milky Way is about 100,000 light-years in diameter.

Scientists want to take a census of some of Earth’s neighbors and gauge their suitability for supporting life.

Accent on ApplicationsApplicationsdark energyenergyNIRSensors & Detectorsuniverse

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