My first Magazine Sky & Telescope - 04.2019

If you wish to go after nebulae, con- sider getting your camera modiﬁ ed by having its infrared cutoff ﬁ lter replaced with one that lets through more of the deep red wavelengths. Note, though, that Spencer’s Camera ( spencerscam- era.com ), which modiﬁ es many models of cameras, speciﬁ cally warns against modifying the α7 III due to the light- leak issue discussed next. Sony α7 III Canon EOS 6D MkII Modiﬁ ed 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 “ampliﬁ er 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 shut- ter and is apparent only in multi-min- ute 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 awk- ward 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 deep- sky 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 ﬂ at ﬁ elds and other local corrections when processing, with a mirrorless camera we should not have to deal with any vignetting from intru- sions 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 fi lter- modifi ed 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 40- second 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 track- ers and camera lenses, the Sony can produce great wide-ﬁ eld 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 ﬁ rst choice. While it offers low noise and easy focusing, I ﬁ nd the Sony’s edge glows and sensor shadow- ing serious impedi- ments 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 lead- ing 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 break- through 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 fl aws 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 65

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