New cameras to the rescue
To record planetary images, astronomers usually place high-speed digital video cameras at the focal point of their telescopes. With an intervening Barlow lens to enlarge the image, they record thousands of video frames directly onto a computer. The best of these are later processed with special software to filter out and average the blurring effects of our atmosphere. The software outputs a very low-noise master image that can be sharpened to show significantly more surface detail than any other current imaging method.
Uranus and Neptune are small and dim in visible light, and unfortunately even dimmer in the IR wavelengths needed to capture surface details. To image them successfully, though, you need to use an IR sensitive camera in combination with dedicated IR filters. Fortunately, in recent years CMOS chip manufacturers, in particular Sony, have made great advances in the IR sensitivity of their camera sensors. Sony’s chips, such as the monochrome IMX290 and even the colour IMX224, are great performers in the IR band and are incorporated into cameras made by manufacturers such as ZWO (see page 98), QHY and Altair Astro.
One benefit of imaging the ice giants in IR is that the seeing is invariably better in IR than in visible light because the longer wavelengths are less prone to the blurring effects of our atmosphere. The steadier view in the IR goes some small way to explaining why people are now having fewer problems imaging these tough targets.
Until the arrival of these new IR sensitive cameras, frame exposure times for Uranus and Neptune were 100 milliseconds (msec) or more. This made imaging very difficult because unless the seeing was rock steady these relatively long exposures would lead to movement that smeared the image. One of the key features of most of these new highly IR-sensitive CMOS cameras, however, is their very low read noise, which allows the cameras to be used at high gain. This amplification of the image makes possible much shorter exposures while maintaining image brightness, without the image degenerating into a mess of noisy columns or rows.
"One benefit of imaging ice giants in infrared is that the seeing is invariably better"