54 ASTRONOMY • FEBRUARY 2022
the color chip only records
25 percent of the red light that
falls on it. Compare this with
a mono chip with a red filter
in front of it, where each pixel
records 100 percent of the red
light that falls on it.
On the other hand, with a
color chip, you are recording
all three colors simultane-
ously instead of separately.
Purists and advanced imagers
will typically go for the mono
chip because it offers moreThe Fireworks Galaxy (NGC 6946), imaged here, is a stunning face-on spiral
that straddles the border between Cepheus the King and Cygnus the Swan.
Technical details: Sixteen 20-minute subs binned 1 by 1; gain=0, offset=50;
images deBayered and combined in AstroArt; processed in Photoshop CS 6.
The Dumbbell Nebula (M27) is a stunning planetary in the constellation
Vulpecula the Fox. The author captured this view from his home in Foresthill,
California, using a Planewave CDK17 and QHY 410C camera. Technical details:
one set of eight 10-minute subs binned 1 by 1 (in RGB to help bring out the color
of the stars), gain=0, offset=50; a second set of twelve 20-minute subs binned
1 by 1 (taken with an Optolong L-eXtreme filter to capture nebulosity), gain=50,
offset=50; images deBayered and combined in AstroArt; processed as a raw
16-bit TIFF file in Photoshop CS 6.
NGC 6946M27detail and more
user control. But
I have been sur-
prised by the
performance you
can get from a
color chip if you
give it enough
exposure time.
(Remember, you
are splitting the
signal four ways!) Before we
leave this topic, I should also
point out that a color chip has
the huge benefit of avoiding
all the alignment and combi-
nation issues that come with
merging multiple color
frames taken with a mono
chip. If you want an easy life,
go with a color option like the
QHY 410C.
Another very interesting
development has happened
recently with color cameras.
Go back a few years and
narrow-band imaging was the
sole domain of mono cam-
eras. In addition to employing
red, green, and blue filters
with your mono sensor, you
could also use Hydrogen-
alpha (Hα), Oxygen-III (OIII),
and Sulfur-II (SII) filters,
which allow only light from
very narrow portions of the
spectrum to pass through.
But that all changed when
the Optolong filter company
realized that modern, highly
sensitive color cameras can
record Hα and OIII light
simultaneously when there
isn’t a standard ultraviolet and
infrared (UV-IR) rejection
filter in front of the camerachip. Optolong
built a narrow-
band filter called
the L-eXtreme
that allows only
Hα and OIII
light to come
through, blocking
out everything else. Once
deBayered, the result is
an image with the Hα
and the OIII already com-
bined, making your life even
easier. This Hα-OIII image
can then be added as another
layer to the RGB image in
Photoshop and combined
via the Lighten blend mode.
Remember, when you shoot
the RGB frame, you must add
a UV-IR rejection filter to
keep those wavelengths from
fogging up the shot.Using the 410C
QHY calls their cooled CMOS
(short for “complementary
metal-oxide-semiconductor”)
cameras COLDMOS cam-
eras. And unlike CCDs, they
have controls that emulate
DSLRs — namely offset and
gain. The offset will move
the entire range of densities
(histogram curve) away from
pure black so you will have
no clipped blacks. The gain
is very similar to the ISO of
your DSLR: It’s an ampli-
fication of the signal in the
camera. Turning up the gain
in a CMOS typically reduces
the maximum number of
photons a pixel can absorb
before it becomes saturated,
which is known as the deepThe QHY 410C is seen
here mounted to the
business end of a
PlaneWave CDK 17.