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.
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.
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.
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.
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.
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.
