It's easy to take high-quality images of the lunar disk. You can shoot the Moon!
The digital imaging revolution has taken astronomy by storm. Spectacular images recorded by amateurs using digital single-lens reflex (DSLR) cameras, specialized planetary cameras, and large format CCDs dominate the pages of this magazine. But the imaging devices you most likely to own are smartphones, tablets, and compact “point-and-shoot” cameras. Surprisingly, these simple devices are capable of producing pretty, even stunning images of our nearest neighbor, the Moon. It’s not quite as easy as just walking up to the eyepiece and snapping away — but that’s the general idea. So here are some tried-and-true tips for basic lunar imaging.
Whether you have a smartphone or tablet (which usually have small, fixed-focus lenses) or a much more versatile compact camera or DSLR, all employ a similar set of operational modes. Some have a “manual” setting, and that’s preferable to the “automatic” mode, which tends to overexpose lunar images. Apps like NightCap Pro can add these features to your smartphone. Using the manual setting, you can adjust the shutter speed, exposure, aperture, and ISO (or at least your phone’s brightness setting) to control the amount of light reaching the camera’s detector. A fast shutter speed helps minimize blurriness due to atmospheric turbulence, wind, a nontracking mount, or an unsteady hand holding the camera up to the eyepiece.
Perhaps the most important setting is ISO. Basically, the higher the ISO value, the greater the detector’s sensitivity to light. Typical smartphones or compact cameras have an ISO range of 100 to 1600, while new “low-light” models can reach 6400, 12,800, or higher. The tradeoff is that higher ISOs add more noise or “graininess” in the image. Try using 200 or 400, at least to start, for the greatest dynamic range and lowest image noise.
The beauty of lunar imaging is that even a small 60-mm refractor or 4-inch reflector can produce stunning images. Aperture isn’t a major factor, as the camera is using the telescope as a giant telephoto lens. This optical arrangement — telescope, eyepiece, and a camera with an attached lens — is called afocal photography.
While unguided telescopes can produce nice results, a telescope that’s tracking the Moon will generally produce better, less blurry images. If you have a steady hand, then just center the camera’s lens over the eyepiece and use the self-timer function to secure decent images.
However, to get the sharpest images and best resolution, some mechanical help will come in handy. Almost indispensable for basic lunar imaging is a good photo tripod. Most cameras have a ¼-20 threaded hole for attaching it, or you can purchase an inexpensive tripod adapter for your smartphone. (In fact, most “selfie sticks” employ a simple yet effective tripod adapter.)
If tripods aren’t your style, then get a smartphone or compact-camera adapter that clamps onto the focusing tube and positions the camera lens directly over the eyepiece.
Technology offers alternatives to using the self-timer approach. Perhaps your camera can be used with a mechanical or electronic remote release. With some cameras a wireless Bluetooth controller can “trip” the shutter from up to 30 feet away.
Now Let’s Go Image!
It’s time to put your equipment and technique to practice by doing some lunar imaging. Before you head outside, however, make sure the camera lens is clean and that the largest possible image file is selected. Make sure your battery is charged — and turn off the flash!
Point your telescope at the Moon, focus the eyepiece, and then position the camera lens directly over the eyepiece. Make sure it’s pointing straight in, not tilted, to minimize distortions. Now use the telescope’s focuser to produce a crisp image onto the camera’s display. Go with low-power eyepieces, which tend to have larger field lenses and good eye relief. This makes centering the Moon easier and reduces vignetting (reduced image brightness) around the frame’s periphery.
If possible, try not to use the automatic-exposure mode, as this tends to under- or overexpose the image. (That said, it’s easier to bring out detail in an underexposed image via computer processing.)
A technique called bracketing works particularly well with lunar imaging. By shooting many images over a wide range of exposures and ISOs, you can accommodate the huge brightness range of lunar features and work around any blurring induced by atmospheric turbulence.
While capturing images, use the smartphone’s display or the camera’s “live-screen view” to check your framing and focus. Some imagers like to use a camera’s optical zoom, if it’s got one, to capture the smallest surface details, though changing to a higher-power eyepiece can work just as well. But don’t use the digital zoom — smartphone users, take note! — because that doesn’t actually record finer details.
Finally, shooting lots of images is a good hedge against “things that go bump in the night.” Almost anything that can go wrong often will — ranging from knocking the telescope off target, losing focus, vignetting, weird internal reflections, power loss, and unexpected weather changes.
Kicking it up a Notch: The DSLR
A digital single lens reflex (DSLR) camera is an extremely powerful and versatile imaging device. Not only can you use it to take pictures of a wedding or stunning macro images of insects, but you can also take that same camera and produce planetary and deep-sky images that rival the best from dedicated planetary cams and CCDs. There are a numbers of reasons for the secret of the DSLR's success: a large, sensitive CMOS chip; interchangeable lenses (including zoom); saving images in a number of different formats including RAW (or unprocessed/uncompressed mode); and the capability to produce high-resolution AVI (video) files. Other smart phones and compact cameras also have large CMOS detectors, but they lack versatility of the DSLR. Nor are they compatible with the wide range of control and processing software tailored specifically for astronomical imaging.
As with many smaller cameras, the afocal method works well with DSLRs. It's usually better to use a shorter-focal-length lens — a 50-mm or 18-to-55-mm zoom — than a longer, heavier lens that is more prone to sag and harder to line up. Telephoto lenses often have a "sweet spot", where the sharpness is optimum, so experiment with different settings.
One huge advantage of a DSLR for lunar imaging (and for astrophotography in general) is that it can be easily adapted to insert directly into the focuser, using neither a lens or an eyepiece. Effectively the telescope itself becomes the camera's lens. This puts the camera at the telescope's prime focus,, and it's a far more rigid, precise setup than using the afocal method and trying to center the camera's lens over an eyepiece.
To use the prime-focus method, you'll need a T-ring adapter specific to your camera brand and a camera adapter that slips into the telescope's focuser (where the eyepiece would ordinarily go). Many suppliers offer these adapters for either 1¼- or 2-inch-diameter focusers; the larger size is preferable (if your telescope has a 2-inch focuser) to reduce the amount of vignetting. To increase magnification or focal length, you can insert a Barlow lens into the focuser first and then insert your camera-adapter combo into the Barlow. This setup is particularly useful for high resolution study of lunar structures or craters.
Of course, if you already have a telephoto lens for your DSLR camera, don't hesitate to use it. However, when shooting whole-disk images, conjunctions with planets, or lunar eclipses, you'll want to back off on the magnification and image the entire scene.
Image Acquisition and Processing
Most newer models of DSLRs now have a "live view" mode, which is critical for attaining a good, sharp focus. Use the 5× to 10× zoom function to really nail the focus. Then use a remote trigger, an intervalometer, or even the camera's self-timer to control the shutter without touching the camera. When combined with manual settings and bracketing, this is a complete package that you can set up quickly and use on the fly. More powerful still is to connect your camera to a laptop or other computer that is running camera-control software. Canon has by far widest range of control and acquisition astronomical software, though Nikon, Sony, and others are now coming out with useful products. Some of the most popular and useful applications include Nebulosity, Backyard EOS (for Canon and Nikon), ImagesPlus, and Maxim DL.
My personal favorite for lunar imaging is Backyard EOS, a very inexpensive program for controlling lunar, planetary, and deep-sky imaging. Once you've connected camera and computer, a whole range of operations can be accessed with only a click of a mouse. In the Frame and Focus mode, your screen displays a much larger version of the camera's "live screen" including the 5× and 10× zoom modes. This allows for very precise focusing, exceeding anything you could accomplish by using the camera alone. In the Planetary Image mode, the DSLR becomes a "dedicated planetary camera" capable of producing either single exposures or AVI files comprised of hundreds or even thousands of frames.
Once you've acquired and saved your images and/or AVI files, how do you go about processing them? Single-image files can be sharpened and enhanced by programs like Adobe Photoshop. It gets much more interesting when you want to stack (electronically combine) a series of images or to process AVI files. The stacking of images — whether just a handful or literally thousands of frames from an AVI file — is a technique that boosts the signal-to-noise ratio and dramatically increases the overall sharpness and contrast. The process involves having your computer select the sharpest and least distorted frames, aligning and combining them, and finally sharpening and boosting contrast. Although it can be quite complicated, free applications such as Registax and Autostakkert! can automate most of the procedure.
The final processed image can be astounding! You start with what seems to be a series of "noisy," blurry frames — and the end product is a detailed and wonderfully sharp image. But remember the old adage: "garbage in = garbage out." You'll get the best results by far when the sky's seeing and transparency are best. Stacking even thousands of frames under poor conditions will only result in a lackluster final image.
The digital-camera revolution is ongoing! So whether you have a smart phone, a "point and shoot" camera, or a full-frame DSLR, take some time to image our closest neighbor. You'll be quickly surprised by the results — and it might mark the beginning of a more serious and enjoyable study of the Moon.
Frequent S&T contributor Richard Jakiel observes everything from the nearby Moon to distant galaxies.