In mid-2015 Nikon introduced an “astronomical” version of its pro-level D810 camera, called the D810a, with special features to appeal to astrophotographers. What does it offer and how well does it perform?
I put a D810a to the test, using a camera on loan from Nikon Canada. To check its advantage over a “stock” camera, I shot various celestial sights taken the same nights for direct comparison with a Nikon D750.
Amateur astronomers often have their DSLR cameras modified by after-market suppliers. To compare the D810a with such a camera, I also shot with a Canon 5D MkII, long a favorite camera of astrophotographers, and modified by AstroHutech. I was interested to see how Nikon’s factory-installed “mod” would compare to an after-market modification.
Just what gets modified? And why? First, some background.
In theory, DSLR cameras shouldn’t work for long exposures of astronomical objects. In long exposures a blizzard of electronic noise from a number of sources should build up to make digital images unusable. To get around this, cameras designed specifically for astrophotography feature CCD sensors that are cooled to sub-zero temperatures, reducing or eliminating heat-induced noise. But DSLRs operate at room temperature, or at least at the temperature of the night air. Even on a cool night, they should be plagued with noise in long exposures.
They aren’t, because their hardware design and internal signal processing (Nikon’s EXPEED 4 firmware in the example of the D810a) subdues noise to manageable levels. The result is the remarkable ability to shoot subjects such as galaxies and nebulas that are so faint they need exposures of several minutes just to record anything at all.
Even so, an off-the-shelf DSLR isn’t ideal for recording colorful deep-sky targets. The main limitation comes from the cutoff filter all digital cameras employ in front of their sensors to block infra-red light (not to be confused with the anti-alias filter most cameras also employ). Digital sensors are very sensitive to infra-red, but lenses won’t focus those wavelengths properly. So letting IR through to the sensor would add an out-of-focus haze to the image.
Unfortunately for us astronomers, cutting out the IR also eliminates much of the deep red visible wavelengths emitted by photogenic “nebulas” dotted along the Milky Way. It is a case of the proverbial baby being thrown out with the bathwater.
For example, above is a stack of four 2-minute exposures taken with the D810a at ISO 800, and with the Nikkor 14-24mm lens at 14mm and f/2.8. The camera was on a motorized tracker to follow the sky. The long exposure brings out the red nebulas along the summer Milky Way around the constellation Cygnus, above centre. These are just the kinds of objects a stock camera doesn’t record well.
These nebulas (from the Latin word for “cloud”) are regions where stars are forming or dying. They are made primarily of hydrogen, the most common element in the universe. When energized, hydrogen atoms emit light only at very discrete wavelengths, among them at 656 nanometres, a visible but deep red wavelength called “hydrogen-alpha.” To record these objects, we want a sensor that can see this red “H-a” wavelength, but still be blind to unfocused infra-red light.
That’s what Nikon’s D810a does. It incorporates a special IR cutoff filter that transmits more of the good wavelengths while still cutting out the bad. The specs say it transmits four times the level of H-alpha light over a stock camera. It would take a laboratory spectrometer to confirm that claim, so I’ll take it as a given.
The tradeoff for the enhanced red sensitivity is that the color balance is compromised for normal daytime photography. But, as I discovered, the effect is negligible with the D810a, making the camera entirely suitable for all types of photography, day or night.
The D810a’s Unique Features
The D810a is Nikon’s first camera aimed at astrophotographers. It follows upon two “a” models Canon offered for a time, their 2005-vintage 20Da, and the 60Da introduced in 2012. Both were highly regarded and helped establish Canon as a leading camera brand among astrophotographers. Both were mid-priced cropped-frame DSLRs, with APS-sized sensors.
By comparison, Nikon’s D810a is a premium full-frame camera with the highest pixel count offered by any of the current Nikon DSLRs, a generous 36 megapixels. When the D810a was announced its specs immediately raised concerns among amateur astronomers. More pixels means smaller pixels, which in turn usually means higher noise. It’s no good having the right wavelengths recorded if the image is peppered with noise.
As it turns out, the fears were ungrounded. As documented below, the D810a offers noise levels almost as low as the best DSLRs on offer today, even ones with larger and fewer pixels. Nikon has done a superb job keeping noise in check while offering the benefit of higher resolution.
In addition to the hardware modification of its sensor filter, the D810a offers several unique firmware features to appeal to long-exposure photographers.
Extended Manual Exposure Times
A special M* Manual mode, shown above, extends the built-in exposure times beyond the usual 30-second limit, with additional preset exposure times of 60, 120, 180, 240, 300, 600, and 900 seconds. You can take exposures up to 15 minutes long without needing an external intervalometer.
This is a very welcome feature, one I was thankful for, as the intervalometer I use for my Nikon D750 has Nikon’s rectangular Type N3 connector. The D810 cameras use Nikon’s round Type N1 connector (shown uncovered and highlighted here), something to keep in mind should you want to operate the D810a from an external device such as a time-lapse motion controller.
As useful as the internal intervalometer was, I would have preferred to see an additional setting of 480 seconds, as the jump from 5 to 10 minutes is a big one, a full stop. I often use exposures of about 8 minutes for many deep-sky telescope targets.
Boosted Live View Brightness
The big challenge in any low-light photography is framing and focusing. Autofocus doesn’t work and the image in the optical viewfinder is far too dim and coarse for accurate focusing. We have to use Live Focus to zoom in and manually focus on a star.
With both the D810 and D810a, hitting the OK button (at lower left here) while in Live View brightens the screen to better preview a long-exposure image, and to make it easier to frame and focus at night. The boosted image is noisy, but as you can see in this actual screen shot, it does show more in nightscape scenes, especially when lit by moonlight as in this example. However, when shooting through a telescope don’t expect to see nebulas and galaxies in this boosted Live View mode. At best, some fainter stars show up better, to aid in focusing.
However, as I show above, I was pleased to discover that my standard Nikon D750 offered the same enhanced detail in Live View, but only once its Exposure Preview option was turned ON, a menu function that is well hidden! Enter Live View, hit the “i” button (not INFO), and scroll down to the next page of options at right. There you’ll find EXP (ON or OFF). If you have the camera on Bulb you won’t see this option, confusing to be sure.
The D810a doesn’t have this menu option, but on the D750 it does the same thing as the D810a’s Exposure Preview “OK” button. So this feature of the D810a, while certainly very useful, isn’t quite as unique as you might be led to believe.
If Only It Had …
Features the D810a does not offer, but should in my opinion, include a tiltable LCD screen, such as on the D750 shown above. The angled screen makes it a pleasure to use the D750 on a tripod or telescope aimed up at the sky, avoiding bending and stooping to focus or inspect images. That’s a feature much appreciated by this aging astrophotographer.
Nor does the D810 or D810a have WiFi or GPS. Neither are essential for astrophotography, but one would think these should now be standard features in all premium cameras.
Also lacking in any Nikon camera is the frame buffer Canon includes in all its full-frame DSLRs (but not in its cropped frame cameras). This undocumented and largely unknown feature allows you to shoot 4 or 5 images in quick succession even with Long Exposure Noise Reduction turned on. Each “light” frame gets stored temporarily so that when the dark frame does kick in at the end of the sequence (as shown in progress below with the D810a) that single dark frame gets subtracted internally from several images at once.
This Long Exposure dark frame subtraction eliminates hot pixels, and reduces thermal noise and amplifier glows. But on most cameras, such as all Nikons, it doubles the time it takes to record an image. But not on Canon full-frame DSLRs. Their special frame buffer provides a significant time-saver when taking sets of long exposures with LENR on. I feel it gives Canon a big advantage when shooting multi-minute shots of deep-sky objects.
Camera manufacturers — please give us at least an 8-frame buffer for LENR shots in your future “a” models.
Comparing Red Sensitivity
The main plus point of the D810a is its extended red sensitivity. To test this feature I shot 5-minute-long exposures of an H-alpha-rich region of the Milky Way, the Heart Nebula in Cassiopeia. I shot single images at ISO 1600 in quick succession with each of the three cameras, to ensure sky conditions were the same for all exposures.
I shot these through a refractor telescope with an aperture of 92mm and a focal ratio of f/4.4, on an equatorial mount that was tracking the sky. The setup is shown above, at work taking these images.
The bottom line: Nikon D810a recorded more and fainter red nebulosity than did the stock D750, but not as much as the third-party modified Canon 5D MkII.
To illustrate the difference, here are two trios of comparison images.
Set 1: Developed Raw Files
The first set shows the raw files after development in Adobe Camera Raw. I applied color correction to neutralize the sky, and boosted contrast and clarity each to 100%, as I commonly do for images of low-contrast nebulas. I set Luminance noise reduction to 40 and Color to 25, typical of such images.
Even with this basic level of processing, you can see that, compared to the stock D750, the D810a records more of the faint tendrils of the nebula, but not quite as much as with the modified Canon.
Note an artifact of DSLR images that shows up in these telescope shots. It’s one only astrophotographers see or care about: Along the bottom of the frame, and to a lesser extent along the top, there is a darkening of the image on the long edge. This is vignetting from the upraised mirror and by the walls of the mirror box itself that are shadowing the sensor. These dark edges show up only when shooting through fast but long-focal length telescopes, and as you boost the contrast. I was pleased to see that both Nikons minimized the shadowing, at least compared to the older Canon 5D MkII, where it is very pronounced. The newer Canon 6D suffers far less from this mirror box shadowing.
Set 2: Processed Final Images
The second set of comparisons, below, shows the same images, but now brought into Photoshop for even more contrast and color boost, using identical settings of Curves, Selective Color, and Brightness/Contrast applied to each image. Again, this is typical of the levels of processing needed for such objects.
But do note that these are all single images. The usual workflow for deep-sky images is to shoot, then stack, four or more exposures to further reduce noise. But with three cameras to juggle on my test nights, I was lucky just to get one exposure with each camera before clouds rolled in or sky conditions changed.
Now, with even greater enhancement, you can see the D810a pulling further ahead of the D750 in bringing out the faint Heart Nebula. But the modified Canon still does the best job – the nebula is redder and richer in faint structure.
While the D810a was not as red sensitive as the modified Canon, it certainly performs very well, and is far better than a stock camera for this type of deep-sky imaging. However, I should provide the caveat that should you opt for an after-market modified camera, your mileage may vary. The IR cutoff filters dealers insert into their modified camera have varied over the years and may not provide the same red sensitivity as my older Canon 5D MkII.
On the plus side, the benefit of the D810a’s less aggressive filter is that daytime and other nightscape images look pretty normal for color balance, without any need for custom white balance settings or huge color corrections later in processing. Any warnings from Nikon that the camera is not suitable for other photography are over-stated.
By comparison, all images with my modified 5D MkII have a huge pink cast and require lots of correction at the computer, or a custom white balance at the camera. Even then, daytime shots are never quite normal.
I found similar results when testing Canon’s 60Da and 20Da models, which offered modest levels of boosted H-a response similar to the D810a. When shooting “normal” subjects with any of these factory-modified DSLRs, their extended red end shows up mostly in sunset and sunrise scenes that record with warmer tones. For example, as these two images of the twilight sky illustrate, the D810a did a better job than the D750 in recording the pink “Belt of Venus” rimming the dark shadow of the Earth that rises in the east after sunset.
The D810a records low sun and twilight scenes with warmer, richer reds, a positive benefit I feel. Nevertheless, you wouldn’t want to use the D810a for any tasks, such as product photography, where precise color balance is critical.
While of interest primarily to deep-sky photographers who shoot through telescopes, the D810a’s extended red sensitivity will benefit nightscape photographers wanting deep, rich images of the Milky Way over landscapes. As shown in the wide-angle image of the summer Milky Way at top, the D810a will bring out more of the colorful nebulas, especially in exposures over 30 seconds taken with a tracking device that can follow the turning sky.
The perpetual concern of all astrophotographers is noise. Using extreme blow-ups of the star cluster at the heart of Heart Nebula I compare the three cameras in identical long exposures. All were taken at the same ISO speeds and ambient temperatures, and with Long Exposure Noise Reduction turned on to eliminate hot pixels and thermal noise.
I wanted to compare the cameras for their base level “shot” noise, which produces the familiar luminance and chrominance grittiness we deal with in post-processing. This noise becomes much more noticeable when we boost the contrast, as I’ve done here, and as is required for almost all astronomical images, even nightscapes.
Set 1: Deep-Sky Image
In this first set I dialed in the same levels of luminance and color noise reduction at the raw development stage, but applied no further noise reduction or filtering in Photoshop.
As might be expected, the 24-megapixel Nikon D750, with its 6-micron pixels, exhibited less noise than the 36-megapixel D810a, with its 4.9-micron pixels. The difference was about one f-stop. For example, the D810a at ISO 3200 showed a level of noise similar to what the D750 had at ISO 6400. That’s still a superb low level of noise.
By contrast, the 2008-vintage 21-megapixel Canon 5D MkII, a leader for low noise in its day, showed more noise than even the D810a, despite the 5D MkII having the largest pixels of the three cameras, at 6.4 microns across.
In addition, to its credit, dark frames taken separately with the D810a showed a very smooth, uniform black background with no odd edge glows from amplifier heat, even when boosted in brightness and contrast.
Set 2: Moonlit Nightscape Image
As a further noise test I shot the same moonlit scene, typical of nightscape images, over a range of ISO settings with all three cameras, plus with a fourth camera, a Canon 6D. All were constant 20-second exposures with a 24mm lens. Extreme closeups of the old rustic house and moonlit sky reveal the noise increasing with higher ISO, as expected. But again, at the high ISOs we use in nightscapes, the D810a outdoes the 5D MkII.
Comparing the 5D MkII to the D810a, above, shows the Canon providing what I estimate to be about 1/2-stop worse noise than the D810a. Considering the Canon 5D MkII is still valued as a low-noise camera, that’s impressive. The fact that the Nikon D810a can beat the venerable Canon dispels all the concerns that a 36-megapixel camera would be too noise-prone and compromised for astrophotography.
Now, before Canon fans get enraged, don’t take this as an indictment of Canon. Wouldn’t comparing the Nikons to a newer, contemporary Canon be more fair? Of course.
So, as shown above, I also compared the D810a to the still-current 20-megapixel Canon 6D, with its 6.5-micron pixels. I found the 6D closely matched the competitive Nikon D750 for noise in long exposures and high ISOs, putting it a similar one stop ahead of the D810a for noise performance. The comparison above shows the difference.
I present these comparisons to make this point: the edict that fewer pixels/bigger pixels = less noise doesn’t always apply. Certainly not to cameras years apart in age. With each new generation of sensors, cameras and on-board firmware, manufacturers beat down noise still further, so that today’s 36-megapixel model can match or better yesterday’s 21-megapixel leader.
This bolsters the advice I usually give to aspiring astrophotographers who ask if they should get their several-year-old DSLR modified, thinking they’d prefer to “sacrifice” an old, camera for astronomy. Since it has a lower megapixel count than a new camera, it must surely have lower noise anyway, right? No. Not if it’s five or six years old. With DSLRs, newer is better.
While there is little penalty in having the D810a’s 36 megapixels, is there a benefit? After all, those megapixels do come at a price in dollars!
Yes, I found there is a benefit. Here it is, illustrated in two more sets of comparisons.
Set 1: Stars in High-Power Telescope Views
In telescopic closeups, stars are less pixelated, with fainter stars recorded. Extreme closeups of another tiny star cluster in the Heart Nebula show how the D810a does resolve fainter stars better, with less pixelation than with the D750, and certainly the 5D MkII.
Set 2: Ground Detail in Wide-Angle Scenes
In wide-angle nightscape images, the same benefit applies. Fine details in landscapes, such as the tree branches here, are sharper and better resolved. These show a close-up of another moonlit night scene, with the D810a vs. the D750, comparing 36 vs. 24 megapixels.
The increase in resolution I noticed over the Nikon D750 comes not only from the D810a’s smaller pixels, but also from the fact that the stock D810 and its “a” variant, like Nikon’s D800e before them, have no anti-alias filter.
That said, the increase in resolution will be noticeable only when you make large prints or perform extreme crops of an image.
Still, there was a time when we thought 8 or 12 megapixels was all we would ever need for our photographs. Owners of 18- to 24-megapixel cameras (most of us today) might dismiss 36 or 50 megapixels as excessive marketing ploys. Not so. You really can see the difference in high-demand situations. Astronomical images, with their point-like stars, are a prime beneficiary.
As a final example of the D810a strutting its stuff, here’s the autumn Milky Way from Perseus (at left) to Cygnus (at right), taken with the Nikkor 14-24mm lens at 24mm and wide open at f/2.8. This is a stack of four 1-minute exposures with the camera tracking the sky, on an iOptron Sky-Tracker. It shows how the D810a can pick up the red H-alpha regions all along the Milky Way.
At a typical street price of $3,800 (US) the D810a carries about a $1,000 premium over the cost of the standard D810. That’s a major commitment for any astrophotographer, though the price is comparable to many specialized CCD astrocameras that are far less versatile.
Price aside, after a month testing the loaner Nikon D810a, I was impressed. It sets the bar high for any astronomically-modified DSLR, providing a noticeable benefit in resolution without any significant penalty in noise.
The D810a’s added features are of real value for long exposure imaging. Its extended red sensitivity is an essential feature for any photographer serious about shooting deep-sky targets, whether they be wide-angle images of the nebula-rich Milky Way, or close-ups through a telescope. And yet, its red response does not compromise the camera for all other forms of photography. But … it’s there when you need it.
I think I can say that the Nikon D810a is the best DSLR on the market for astronomical imaging. If you want the best, look no further.
About the Author
Alan Dyer lives in Alberta, Canada. He has been doing astrophotography since the dark-age days of High Speed Ektachrome film, and now with successive generations of DSLR cameras.
His website, provides several video tutorials and pages of tips and techniques for shooting the sky. He can be reached through the contact page on his website.
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