Astronomy - USA (2022-02)

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For 20 years, I have been
using charge-coupled device
(CCD) cameras, and I cur-
rently own the top-of-the-line
SBIG STX-16803. But while
studying two images I recent-
ly made using the latest QHY
410C CMOS camera, I had to
wonder: Is CCD dead?
For years, I lectured about
the asymptotic boundary of
noise in CCD images. In a
basic sense, this means that
no matter how many frames
you take to increase your
signal-to-noise ratio for a
cleaner image, you will always
run into a wall of noise when
you stretch your image to
bring out deep shadows. But
with QHY’s new CMOS cam-
era, this troublesome wall of
noise is nonexistent.
The QHY 410C is a one-
shot color camera that utilizes
the back-illuminated Sony
IMX410 CMOS chip found
in high-end cameras like the
Nikon Z6 and the Sony A7
III. But the 410C has taken
the full-frame (35 millimeter)
24-megapixel chip and
mounted it in a camera with
regulated cooling and zero
amplifier glow, helping drive
the noise to such a low level.
There also aren’t any
noticeable cosmic ray hits,
despite high quantum effi-
ciency. (Quantum efficiency is
the percentage of photons that
are converted into recorded
data.) With my SBIG STX-
16803 CCD, quantum effi-
ciency is around 60 percent;
with a back-illuminated
CMOS, it’s about 80 percent.
The commercial availabil-
ity of these back-illuminated
chips has been a big game-
changer in photography. In
a normal, front-illuminated
chip, all the supporting elec-
tronics surround each light
receptor (pixel). Thus, some
of the area that receives light
does not record it. In a back-
illuminated chip, the

supporting electronics are on

the back of the chip, allowing

100 percent of the light recep-

tors to absorb and record

light. This results in a chip

with much higher sensitivity,

which is ideal for astronomy

and astrophotography.

Color vs. mono

CMOS cameras come in

two basic f lavors: color and

mono. A color camera splits

the light into red, green, and

blue (RGB) colors using a

Bayer matrix. This typically

consists of a microscopic

array of colored filters —

assembled in a pattern of

two green, one blue, and

one red — placed over the

light receptors. Each of these

clusters equals one color

pixel. There are millions of

clusters, so software must

know the exact pattern used

to create the colored pixels

from the matrix through a

process called deBayering.

This is done automatically in

DSLR and phone cameras.

But for astrophotography, the

raw Bayered image must be

deBayered using computer

software like Astro Art.

This leaves you with a raw

16-bit colored image that you

can then stretch like a CCD

image. Some photographers

are concerned that color cam-

eras have less resolution than

mono cameras. But in prac-

tice, I have found that the lim-

iting factors are the seeing,

the guiding, and the optics,

not the type of camera.

Before deBayering, all you

see on your monitor is little

black and white squares,

which brings us to mono

CMOS cameras. You can

order CMOS chips that do not

have a Bayer matrix installed.

These mono chips are meant

for use with color filters

placed in front of them.

Separate red, green, and blue

exposures are made with the

mono chip, then these frames

are combined using software

to create color.

So, what’s the difference?

With a mono chip, you’re

using all the pixels on the chip

to capture each particular

color of light. As a result, you

will reveal more detail and

possibly achieve a higher sig-

nal-to-noise ratio. But the

same chip in a color model is

split into four receptors per

color pixel. For example, with

only one red receptor per pixel,

The Iris Nebula (NGC 7023) snaps into view in this shot taken with a PlaneWave CDK 17-inch telescope and QHY 410C

camera from the author’s California home last fall. Technical details: Six 15-minute subs binned 1 by 1; gain=0, offset=50;

images deBayered and combined in AstroArt to a raw 16-bit TIFF file; stretched and processed in Adobe Photoshop CS6.

NGC 7023