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Given the huge interest in astrophotography these days -- then again, maybe it's just us -- I'm surprised at how few cameras have been retrofitted for it in the past few years. The only current model from a major manufacturer thus far has been the Canon 60Da, and it's a few years old. Now Nikon has modified its flagship D810 the same way, with an IR cut filter, to optimize it for astrophotography.
Most image sensors are highly sensitive to infrared wavelengths, so camera manufacturers use a filter to block them in order to prevent overheating and overwhelming visible light reception. But there's a lot of infrared and ultraviolet -- i.e., invisible -- radiation in space, much of which is in the near-infrared spectrum at about 656nm (IR starts at about 700nm). Namely, the wavelength of the energy released when an electron drops from one energy level to a lower level in a hydrogen atom, which has a specific spectral characteristic at exactly 656.281nm (dubbed hydrogen alpha).
The IR cut filter allows that small band of wavelengths through, allowing the sensor to capture the light from distant astronomical bodies emitting within the H-alpha spectrum, such as cooling stars and nebulae. (The University of Chicago's Multiwavelength Astronomy site has great visuals and explanations.)
Other customized features in the D810A include a long-exposure manual mode of up to 15 minutes, optimized ISO sensitivity from ISO 200 up (the D810 goes down to ISO 32), plus an exposure simulation in Live View for shutter speeds greater than 30 seconds. Nikon will also release an updated version of its Capture NX-D software with optimized noise reduction as well.
Nikon has not yet released pricing or availability information for the US, but it'll go on sale at the end of May in the UK for £3,000, which roughly converts to around $4,500 or AU$5,900.