QHY8 Pro CCD camera
What makes the QHY8 Pro interesting is its price tag under £1,800: as large-format, cooled astronomical CCD cameras go, that’s something of a bargain. It’s a one-shot colour CCD capable of taking full-colour images without having to combine three separate colour channels, with impressive image size and very low noise.
At its heart is a 6-megapixel, 28.4mm Sony Super HAD imaging chip, the same sensor found in several mainstream DSLRs and at least one other highly regarded astronomical CCD camera. The camera has a small circular profile, making it ideal for use with a HyperStar lens unit. This is an add-on corrector lens that replaces the secondary mirror of a Schmidt-Cassegrain telescope to create a very fast focal ratio, wide-field scope that can take highly detailed images.
The camera connects to a telescope with a female T-thread at the front of the 2-inch barrel. Behind this is the large sensor, protected by an infrared blocking filter. At the rear of the camera are ports for power, autoguiding and USB 2.0. Power is supplied externally by a DC-201 adaptor that needs a 12V/4A feed either from a battery or an optional mains transformer.
Two control applications come with the camera, EZCap and CCDCap, but you can also use any other capture program as long as it has ASCOM driver support. The camera is very simple to control, but it’s important that two values called gain and offset are correctly set in the software before you begin imaging. Gain is similar to ISO in a DSLR camera, while offset needs to be set just above the minimum signal the camera can detect. This means images match the QHY8 Pro’s full dynamic range.
The camera has a controllable two-stage Peltier cooling system with a high-speed fan to disperse excess internal heat. With the software, we brought the sensor down to 45°C below ambient temperature in just a few minutes. The cooling system is so efficient that the chip can frost over if you’re not careful. Sensor temperature and cooling system power can be tuned with the software, which can also record each image’s sensor temperature.
We tested the camera on a number of faint summer deep-sky objects. The Witch’s Broom Nebula came out beautifully, as did the faint wisps of gas clouds surrounding the star Gamma Cygni. This is thanks to the QHY8 Pro sensor’s excellent ability to record incoming photons. A measure of this is called quantum efficiency (QE); the QHY8 Pro’s QE peaks at 60 per cent for green light, dropping off to 50 per cent for blue and red. These values sit between the QE of a typical DSLR and that of a top-end mono CCD, which may have QEs of around 80 per cent.
Each pixel in the sensor is covered by either a red, green or blue filter, arranged in a pattern called a Bayer matrix. The sensor in the camera is actually monochromatic, but you can reconstruct a full colour image by passing its Bayer-filtered output through a de-Bayering program. Neither of the two control apps support this function, but the camera ships with an image processing program called Nebulosity that does; the full version of this costs £40.
Each pixel measures 7.8 microns square and can be grouped together as 2x2 or 4x4 blocks, a technique known as binning. This increases sensor sensitivity, but colour information is lost. The camera outputs 16-bit images showing excellent dynamic range at lower gain levels. A full-size unbinned image took around 14 seconds to download from the chip, although a high-speed mode can speed this up. It’s useful to use the high-speed mode for previewing and focusing, but turn it off for image captures as it introduces additional noise.
The QHY8 Pro is an excellent low-noise, cooled CCD camera. It’s worth considering alongside the Starlight Xpress SXV-M25C or SBIG ST-4000XCM colour CCDs, especially as it’s about half the price.
Find out where to buy your equipment with our Retailer Guide.
Keeping the noise down
The one feature that really stood out in the QHY8 Pro’s images was the incredibly low noise. This is thanks to the camera’s impressive in-built cooling system, especially noticeable during our imaging tests since they took place over the summer months when the nights were relatively warm.
Attempting the same sort of imaging with a conventional uncooled DSLR camera is an open invitation for thermal noise to randomly stomp unwanted artefacts across your shot. Stacking helps, but with the QHY8 Pro’s low noise profiles, this is far less of an issue. You’ll still need to calibrate your images for best results, especially with bias and flat frames. These are particularly important if your scope doesn’t evenly illuminate the full area of the QHY8 Pro’s sensor.
For £1,799 you have a device that offers the impressive image size of a DSLR, the convenience of one-shot colour and the low noise of a cooled astro CCD camera in one.
This review appeared in the August 2010 issue of Sky at Night Magazine