Astronomy

64 ASTRONOMY • JUNE 2018

The 128C offers full-color imaging, low noise, and ease of use.

text and images by Tony Hallas

T

here’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 cam-

eras, 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 sam-

pling and even back illumination. The result

is a new level of performance for imaging.

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 expo-

sure, 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 any-

thing 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 con-

cept to my COLDMOS exposures with

good results. Typically, I’ll set it for no

more than one-half the maximum gain and

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.