My first Magazine Sky & Telescope - 04.2019 | Page 66

S&T Test Report
Sony α7 III
Canon
EOS 6D MkII
p In 600% blowups of 8-minute exposures of the Andromeda Galaxy, the Sony α7 III (left) shows
just as many stars as does the Canon 6D MkII (right). The α7 III does not suffer from the “star eat-
ing” problem that plagued earlier versions of the fi rmware. No noise reduction or sharpening was
applied to the raw images in processing with Adobe Camera Raw.
Star Colors
What the Sony does exhibit down at
the pixel level are stars that seem overly
colorful. In particular, many faint
stars look green, but in reality there
are no green stars! I also see this effect
on some images taken with my Nikon
D750, which also uses a Sony sensor,
but never on Canon images.
In a Bayer-array sensor used by most
consumer cameras, each pixel has either
a red, green, or blue filter in front of
it, but there are twice as many green-
filtered pixels as red or blue ones. So are
the green stars an artifact of pixel-level
smoothing being applied by the camera?
Or is the effect caused by the anti-alias
filter in front of the sensor? Or might
it be an artifact of the “de-Bayering”
of the data from the Sony sensor? The
“green star” effect is present whether or
not I used Long Exposure Noise Reduc-
tion, but it does vary depending on the
software used, suggesting it is a de-
Bayering effect.
For example, I see it in when opening
the Sony’s raw files with Adobe Camera
Raw, Affinity Photo, Capture One 11,
DxO PhotoLab 2, and even Sony’s own
Image Edit raw developer software.
The open source Raw Therapee
software, which offers a choice of de-
Bayering routines, also shows green
and overly colorful stars with most
of its choices, but not all. Using the
IGV or LMMSE options recommended
for high-ISO images produces more
neutral stars but also tends to wipe out
wanted star colors. The same was true
of ON1 PhotoRaw 2019, a new pro-
gram vying to replace Adobe’s Creative
Cloud suite.
The new RAW module in PixInsight
1.8.6 worked well, with its default
VNG de-Bayer routine producing neu-
tral star colors and no strange pixel-
level artifacts.
Long Exposure Noise
Reduction
Unlike CCD cameras, DSLRs and mir-
rorless cameras are not temperature-
regulated. As such, for the most accu-
rate subtraction of dark frames I prefer
to use the camera’s Long Exposure
Noise Reduction (LENR) option, which
forces the camera to take and internally
subtract a dark frame for each light
frame taken.
Sony’s LENR worked well, elimi-
nating most of the hot pixels from
thermal noise, but not all. On warm
nights, Sony’s routine still left some hot
pixels. Most noticeable in the shadows
of nightscape images, these required
manual cleanup.
Red Sensitivity
The Sony α7 III I tested was an off-
the-shelf stock unit. As such, sensitiv-
ity to the deep-red hydrogen-alpha
wavelength, required for picking up
faint nebulosity, was no better, but
no worse, than my stock Canon and
Nikon DSLRs.
Image
Edit
64
Adobe
Camera
Raw
DxO
PhotoLab
A PR I L 2 019 • SK Y & TELESCOPE
Raw
Therapee
VNG4
Raw
Therapee
IGV
PixInsight
VNG
t Depending on the software used to develop
Sony’s RAW fi les, stars tend to show overly
abundant colors. This compares 500% blow-
ups of M31 with, from left to right, Sony’s own
Image Edit, Adobe Camera Raw, DxO Photo-
Lab, Raw Therapee with its VNG4 (colorful) and
IGV (neutral) de-Bayer routines, and with VNG
mode in PixInsight.