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Astronomy tests QHYCCD’s new astrocamera

The 128C offers full-color imaging, low noise, and ease of use.
ASYER0618_01
QHYCCD’s 128C contains a full-format (36 mm by 24 mm) cooled CMOS chip. The “purple” filter is the ultraviolet/infrared-blocking filter necessary for the camera to produce clean RGB images.
Tony Hallas
There’s a new kid in town, and his name is COLDMOS. At the 2017 Advanced Imaging Conference in San Jose, California, I couldn’t help noticing some shiny new cameras without filter wheels. I went in for a closer look.

The cameras had various CMOS color chips in them just like many DSLR cameras, but they were cooled like the chip in a CCD camera. They also allowed you to download a RAW file as a FITS file, a big advantage for advanced imagers. In some of these cameras, the amplifier noise was less than expected.

Would I be interested in trying one out? I was curious, so I said yes.

What’s in a name?
The QHY 128C uses Sony’s IMX128 chip that Nikon uses in its D750 SLR. This is a full-frame 35-millimeter chip with 6-micron pixels that can record a 24-megapixel image in 14 bits. The QHY camera features a 128-megabyte image buffer and USB 3.0 connectivity for fast and smooth downloads.

QHYCCD, the camera’s manufacturer, named the camera COLDMOS because the CMOS chip is cooled during use, and to differentiate it from other types of cameras that use CMOS chips. Over the last several years, Sony has been moving the CMOS chip design forward. These new devices feature a lot of advanced technology found in CCD chips, like double-correlated sampling and even back illumination. The result is a new level of performance for imaging.
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This image of the Orion Nebula (M42) took only one hour of exposures to create. The author described a time that short to produce a shot of this quality as “crazy!”
Tony Hallas
Enter the matrix
Let’s take a quick look at how CMOS works. Behind all digital recording devices is a photosensitive chip. All of these chips are monochrome in the sense that they do not differentiate colors. To get color, manufacturers add a microscopic grid of red, green, and blue filters on top of the chip. This is called the Bayer matrix, and typically it’s composed of one red, one blue, and two green squares for each unit of color.

Looking at a raw result from an exposure, you would only see various shades of gray and lots of little squares. The magic comes via software that combines each color unit into a single point of true RGB color. In a DSLR, this happens internally, and you see the color image immediately. With a COLDMOS camera, you need to perform the combination yourself.

The 128C differs from CCD cameras in another huge way: You can adjust the gain (or ISO) of the chip. If you increase the gain, you can take many short exposures and combine them. Although the noise does not increase significantly, you lose deep-well capacity. This means that anything bright will wash out, and there will be no data there. The core of the Orion Nebula (M42), for example, would be a pure white blob showing no detail.

Experiments that I’ve done with a DSLR indicate that the CMOS chip performs best when I capture longer exposures at lower ISO settings. I have applied the same concept to my COLDMOS exposures with good results. Typically, I’ll set it for no more than one-half the maximum gain and exposures between 10 and 20 minutes, depending on the brightness of the target and focal ratio of the imaging device.

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To create this wide-field image of the region around M78 in Orion, the author connected the 128C to a Stellarvue SVQ-100 refractor. He set the gain at 2,000 and combined eight 15-minute exposures.
Tony Hallas
Let’s compare
Now it’s time to answer the question that everyone asks: Is this as good as a CCD camera? No, but it is close. The main reason is the CMOS system: Incoming light has to be split among four separate receptors for every point of RGB. Furthermore, you’re limited to the specific colors that the filters of the Bayer matrix give you.

In a CCD system, you image through filters made specifically for astrophotography, so each color fully covers the chip. There’s no splitting up the light. Using individual filters in front of a monochromatic chip is laborious and time consuming by comparison, but the end result benefits from each color getting full coverage. Additionally, you can expose the luminance separately from the color data, greatly enhancing the depth of detail.
This brings up another important difference. With the COLDMOS camera, the acquisition of a color image is instantaneous; with a CCD, it is sequential. If you are trying to take a color photograph of something moving fast, like meteors or satellites, the COLDMOS camera works great. The CCD does not. You also can raise the gain of the COLDMOS to record faint objects.
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The author used this setup for all his shots, including both of the pictures he shot for this story.
Tony Hallas

Although it is possible to add narrowband filters in front of a COLDMOS camera, remember that there is only one receptor out of the four that will record, for example, Hydrogen-alpha (Hα) light. The matrix’s red filter allows the Hα to go through, but the green and blue filters will block it. So, you are getting only one-quarter the resolution and sensitivity that you would get with a CCD camera where 100 percent of the Hα light is recorded. In other words, don’t do that.

table

We have a verdict
The 128C camera is easy to use. There’s no filter wheel, no complicated registration, and no combining of colors. It’s all done for you via the Bayer matrix. And dare I say it? This camera was also fun to use. (But you do need to know how to stretch and enhance raw data to get the results you see in this article.) The camera has limitations, of course, but if all you want to do is take some good color images of the night sky, you can do a lot with it.

In line with this, the COLDMOS camera works well with a large variety of optical devices, from camera lenses and refractors to long-focal-length telescopes. The gain of the camera can be adjusted to the f-ratio of your imaging device and the nature of your target.

Furthermore, the camera is well suited to situations where you have only one night available for imaging. For example, each photo in this article was made with two or fewer hours of total exposure time.

QHYCCD’s 128C COLDMOS camera brings a new perspective to astrophotography. CMOS technology continues to evolve, and this camera uses it in a new way to image the night sky.
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