Sky & Telescope - April 2019 - 65

If you wish to go after nebulae, consider getting your camera modiﬁed by
having its infrared cutoff ﬁlter replaced
with one that lets through more of the
deep red wavelengths. Note, though,
that Spencer's Camera (spencerscamera.com), which modiﬁes many models
of cameras, speciﬁcally warns against
modifying the α7 III due to the lightleak issue discussed next.

Sony α7 III

Canon
EOS 6D MkII

Modiﬁed Canon
EOS 5D MkII

Edge Glow
The most disconcerting trait of the α7
III for deep-sky imaging is the presence
of a purple glow along the left edge of
the frame. It's always in the same place
and of the same shape regardless of
whether LENR is used or not. If this was
a thermal "ampliﬁer glow" from warm
electronics, subtracting a dark frame
should eliminate it. It does not.
The purple glow comes from an
infrared-emitting sensor near the shutter and is apparent only in multi-minute exposures of deep-sky objects. The
large degree of contrast enhancement
required in deep-sky image processing
makes the glow visible. Counteracting
it requires painting in masked local
adjustments during processing, an awkward workaround.

Sensor Shadowing
With DSLRs the raised mirror can cast
a subtle shadow along the bottom edge
of the frame, which, like edge glows,
becomes visible only under typical deepsky processing. With no mirror, I would
have expected the Sony's image to be
free of any such edge shadows, but not
so. A metal mask in front of the sensor
has two tabs that intrude into the light
path, creating two dark bands on either
side of the top edge of the frame.
While these shadows can be dealt
with using ﬂat ﬁelds and other local
corrections when processing, with a
mirrorless camera we should not have
to deal with any vignetting from intrusions around the sensor.

p Same-night shots of IC 1396 in Cepheus, here with minimal processing, show the Sony α7 III
nearly identical to a stock Canon 6D MkII for red sensitivity. Neither can match the author's filtermodified Canon 5D MkII for its ability to record Hα nebulosity.

video work, though at a higher price.
For nightscape still and time-lapse
photography, with typical 4- to 40second exposures, the Sony has become
one of my favorite cameras. Its bright
live view makes it a joy to frame and
focus night scenes. For long-exposure
photography with simple sky trackers and camera lenses, the Sony can
produce great wide-ﬁeld images of
constellations and the Milky Way. Users
needn't fret over stars being "eaten."
However, for the demands of deep
sky close-ups through telescopes, the
Sony α7 III would not be my ﬁrst choice.
While it offers low noise and easy
focusing, I ﬁnd the
Sony's edge glows
and sensor shadowing serious impediments to getting
clean images of

deep-sky targets. DSLRs from Canon
and Nikon still do a better job. How
their own full-frame mirrorless cameras
introduced in late 2018 will perform for
deep-sky imaging remains to be seen.
Sony has set a high standard in leading the mirrorless revolution. But for
deep-sky astrophotography the Sony α7
III leaves performance shortfalls that
competitors can beat in the new and
expanding mirrorless market.
■ Contributing Editor ALAN DYER has
used DSLRs since 2004 with the breakthrough Canon Rebel 300D. Visit his
blog at amazingsky.net.

Recommendations
For real-time 4K videos of aurorae, the
α7 III is superb. Only Sony's α7s cameras
will outperform the α7 III for low-light

p This single image, modestly contrast-enhanced, displays the Sony α7 III's two main flaws for
deep-sky imaging: the purple edge glow at left from an internal light source, and the dark shadows
at top (and also slightly at bottom) caused by vignetting from a mask in front of the sensor.
sk yandtele scope.com * A PR I L 2 019

http://www.spencerscamera.com http://www.spencerscamera.com http://www.amazingsky.net http://www.skyandtelescope.com

Sky & Telescope - April 2019

Table of Contents for the Digital Edition of Sky & Telescope - April 2019