VITAL STATS
• • Price £299.80 Sensor Sony Exmor IMX224 1/3-inch CMOS, 3.75µm pixels, 4.8x3.6mm chip size Resolution
1.2 megapixels (1304x976) Frame rate Up to 64fps (12-bit) or 150fps (10-bit) Extras 1.25-inch adaptor, USB 3.0 cable, autoguiding cable, T- to C-mount adaptor, 2.1mm CCTV lens Dimensions 62mm diameter, 36mm depth Weight 108g Supplier 365Astronomy www.365astronomy.com Tel 020 3384 5187
ZWO’s ASI224 high frame rate colour camera is a device with a split personality thanks to its impressive infrared sensitivity. From the outside, there’s little that makes it stand out from the other cameras in the ZWO range: its red, cylindrical body giving little away about the capabilities of the technology inside.
The ASI224 uses a Sony IMX224 sensor. As with most colour sensors, this is a monochrome chip overlaid with a Bayer matrix – a repeating 2x2 pattern of coloured filters. The pattern consists of one red, one blue and two green filters, with one filter per pixel. The resulting greyscale image has to be ‘deBayered’ to restore the colour, a process performed either at the point of capture or during the registration and stacking phase, depending on the software you use.
For our tests, we used the freeware FireCapture control program for capture and AutoStakkert for registration and stacking. We deferred deBayering to AutoStakkert to maintain as high a frame rate as possible via FireCapture. ZWO supplies the drivers necessary to get the camera working, but we needed to make sure we were using the latest version of FireCapture to get everything working properly.
The IMX224 sensor has excellent infrared sensitivity. Leakage through the Bayer matrix filters is similar from 840nm, meaning that all pixels work at the same sensitivity irrespective of the colour filter they’re fitted with. This means that the camera can be used for true infrared imaging when a suitable infrared-pass filter is fitted. This is great for imaging objects exhibiting detail in infrared, for example the bright planets. It’s especially useful for Jupiter and Saturn when using speciality filters, such as methane (CH4), which is centred on 889nm.
Probing the ice giants
The ASI224 is especially good on the ice giants, Uranus and Neptune. The camera’s infrared sensitivity means it’s possible to record the tiny discs presented by these distant worlds at reasonable frame rates. Subsequent processing can then pull out faint bands and bright spots. Using a 14-inch scope at f/28, we recorded Uranus