for Nightscapes & Astrophotography
by Richard S. Wright, Jr.
There is no question that DSLR’s have changed modern photography— especially capturing nightscapes & astrophotography— for the better. One of the important ways that digital technology has advanced over film is in the realm of low light photography. Modern digital sensors are not only more sensitive to light than chemical emulsions of film, but they behave linearly in response to the influx of photons. What this means is that faint signal is better captured, and photographic detail continues to be captured where film would “flatten out” for a long exposure of a dimly lit scene. While this has practical uses for everyday or vacation photography, such as capturing floating heads in theme park rides
(Figure 2 was shot with a handheld Canon 5D Mark III in the Haunted Mansion ride at Walt Disney World), it also has opened up nightscape and deep sky astrophotography to scores of everyday photographers. I work for a company that serves this market, and the driving growth right now in astrophotography is coming from DSLR owners. In fact, many people are getting into astrophotography by first purchasing a DSLR. Serious and professional astronomy is dominated by cooled monochrome CCD cameras that create color images by combining multiple exposures taken with different filters. Many amateurs and enthusiasts go this route as well, but most start with a DSLR. In fact, many stay with DSLR based astrophotography because of its many advantages and it’s affordability. A DSLR can be used for multiple purposes; daytime birding, birthday parties, weddings, nightscape images, and even deep sky astrophotography when coupled with an astrograph (fancy name we use for telescopes that are used for imaging as opposed to visual viewing of the sky)
Undoubtedly the best known organization for nightscape photography is TWAN (The World at Night(1). You owe it to yourself to visit their web site… regularly! Many new books and guides are cropping up too taking advantage of this growing photographic specialty, such as Alan Dyer’s iBook title NIGHTScapes and TIMELapses(2), or the eBook Nightscape by David Kingham(3). Or if you just looking for eye candy, or top-notch coffee table material, check out Rogelio Bernal Andreo’s web site at www.deepskycolors.com. It’s amazing what you can do today with nothing more than a tripod and a fast lens.
So, if DSLR’s are so great for nightscapes and deep sky photography, what’s this business about “Modified DSLR’s”? Despite their versatility, DSLR’s were not designed for taking pictures of objects in the night sky; they are intended for daytime or even nighttime terrestrial photography. The imaging chip in a DSLR is very sensitive to light; in fact, it is very sensitive to wavelengths of light that your eye cannot see readily. Because of this wide sensitivity, DSLR manufacturers put filters in front of the imaging chip that filter out UV and infrared light that would swamp the signal from visible light. There are enthusiasts (and service providers) who will remove these filters completely, allowing infrared photography(4) for example. Nighttime security cameras also capitalize on this ability.
Now… here’s the thing for astrophotography: the universe (and the night sky) is filled with stars, and dust, and gas. The most common gas is Hydrogen, and there are huge clouds in the universe of hydrogen gas, both where stars are being formed, and where stars have died in super nova explosions. When this gas is ionized by nearby stars, it emits a very specific wavelength of visible light, a reddish glow that we call Hydrogen Alpha, or Ha for short. The wavelength of this light (656 nm) is very close to infrared, so close in fact, that most DSLR’s actually prevent a good deal of this light from reaching the chip. While this wavelength is visible to the human eye, it is not especially sensitive to it (we are actually most sensitive to green light!). Good thing too because sunlight is flooded with Ha light too, and without your IR cutoff filter, your DSLR images would be very reddish.
A “modified” DSLR is one where this manufacturer supplied filter is removed, and replaced with a new filter that still blocks the infrared light, but allows more of the Ha light to come through. This is becoming a very popular customization, and there is even a Yahoo group dedicated to this topic (search for “DSLRModifications” on yahoogroups.com). Gary Honis (5) supplies information for free on the web on how to perform this modification yourself, and for a fee will perform the modification for you, an option I chose some years ago with my Canon T1i… I’m not especially mechanically inclined.
For tripod sky shots, a modified camera will pick up more of these Ha emissions. For example, look for the small pinkish features in the Milky Way image in Figure 5. These are bright nebula that would otherwise disappear into the background stars in an unmodified DSLR. This image was taken with a 20 second exposure on a tripod with an f/1.4 50mm lens. Taking this a bit further, a more advanced technique is to take multiple long exposures and “stack” them, increasing the signal to noise ratio on long exposure images. Figure 6 shows such an image taken with a Canon 60Da and a 200mm lens on a tracking platform. Individual images were 5 minutes long.
This image contains two popular nebula targets, the North American Nebula and the Pelican Nebula, so named for their distinctive shapes. These objects are quite large, but too dim to be seen with the unaided eye (we are talking several full moon widths here – it is only a 200mm lens after all!).
Finally, for those really intent on pursuing astrophotography to the limit, a dedicated astrograph (a telescope intended for imaging) can be used with a high quality mount that will track objects across the sky. Long exposures though such an instrument will yield results like that shown in Figure 7, which was taken with the authors modified Canon 5D Mark II, and an f/3 astrograph from Officina Stellare.(6)
What about regular photography?
When I first started doing astrophotography with a DSLR, I resisted for a couple of years going the modified route. I could really only afford one DSLR, and I also like taking pictures of normal things… you know, birds, my kids, etc. A modified DSLR takes normal daytime images with a reddish cast, and you have to fix it either by shooting raw and fixing the white balance in post, or by shooting a gray card all the time and using a custom white balance. As it turns out, this was not as big a deal as I thought it was, and I later regretted waiting so long to have my DSLR (a Canon T1i at the time) modified.
Rather than go the gray card route, I just always shot in raw and fixed the white balance in Adobe Raw. In more recent years, Astronomik, a German company, has introduced a series of clip-in filters that will restore Canon cameras (Canon is the most popular camera for astrophotography for reasons a bit beyond the scope of this article) to their original filtered state(7). Even more exciting is Canon has actually introduced a specific DSLR model, the 60Da aimed at astrophotographers (the 20Da was also aimed at Astrophotography, but was discontinued some years ago).
Just right for nightscapes and astrophotography, the 60Da has a cropped APS-C sized sensor, and a filter that blocks UV and IR light, but allows Ha light through. Canon has built compensation for this extra red sensitivity into the cameras white balance firmware so you can use the camera for traditional daytime shots and for astrophotography without having to resort to custom white balances or raw processing (just in case you just want to shoot jpeg after all). The image in Figure 7 was taken with this camera, and a 200mm Canon f/2.8 L II lens (stopped down to f/4). In addition, the 60Da comes with the standard Canon warranty. I remember the chill up my spine the day after I purchased a brand new 5D Mark II, when I sent it off to have a warranty voiding operation performed on it… a risk that should not be taken lightly. Anyone serious about night sky photography should consider camera modification, and/or purchasing a DSLR “premodified” such as a 60Da. You might even consider purchasing an older used DSLR on eBay and having it modified if you want to keep you main camera pristine. Nightscape photography is the gateway to “Astrophotography”; after all it consists of scenes of both terrestrial and celestial elements. Using the provided references here, any competent photographer can expand their repertoire with nothing more than a tripod and some fast glass. An intervalometer, and a tracking platform might be next, and who knows, full-blown telescope mounts with astrographs may be in your future.
About the Author
Richard S. Wright Jr. is a Senior Software Developer for Software Bisque (www.bisque.com), a leader in astronomy hardware and software products for professional and serious amateur astronomers. His astrophotography blog can be found at www.eveningshow.com/AccidentalAstro, and his image gallery at www.eveningshow.com
4 For a guide to filter-less and infrared photography visit www.lifepixel.com