A total lunar eclipse is a wonderful sight to behold—and if you have clear skies on Sunday night, you’ll be able to take in the event with your eyes alone or with binoculars. But some will want to capture a photographic souvenir.
There are many ways to record the eclipse, but if you want to get a close-up shot, the best approach is to attach your camera to a telescope. If you have a digital SLR (DSLR) or “mirrorless” camera and a scope with a motorized tracking mount, you can employ prime-focus photography to get a lunar-eclipse image like the one shown above. Here’s how to do it.
The ideal telescope is one with a focal length that allows the Moon’s disc to very nearly fill the short axis of the image sensor. For a “full frame” DSLR camera, something around 2,200 millimetres is nearly perfect. That means an 8-inch scope working at f/11, or a 10-inch f/9. If your DSLR has a “cropped” or “APS-C sized” sensor (as most do), then a telescope with a 1,500 mm focal length fits the bill. An 8-inch f/7 or 10-inch f/6 is a good choice.
For the photographs shown here, I used a home-built 8-inch f/4.5 Newtonian with a focal length of around 820 mm. The image scale was a bit small, so I attached a 1.5× teleconverter to my camera, which boosted the effective focal length up to 1,370 mm.
If your telescope’s focal length is less than ideal you can still get pleasing results, though you may have to crop your photos a bit to compensate for the smaller image scale.
For photography with your telescope, you need two extra pieces. First, a T-ring-adapter for your specific make of camera. This is a simple fitting with female T-threads in front and a bayonet flange on the rear to match your camera’s lens mount. T-rings are available for most popular camera brands and models. You’ll also need a prime-focus adapter tube. This piece screws into the T-ring and allows you to attach your camera to your telescope’s focuser.
Depending on the specifics of your camera and telescope combination, and the brightness of the eclipsed Moon, the optimum exposure time will vary considerably. Expect your exposure to be several seconds long, depending on the ISO setting you’ve chosen. That’s why you will most likely need a motorized mount capable of tracking the Moon to prevent the image from blurring during the exposure.
DSLRs have improved greatly in the past few years, so if your camera is of recent vintage, you can probably use ISO 1600 or greater without fear of introducing a lot of image noise. Older cameras will likely need to stay at ISO 800 or lower. As you increase the ISO, you can reduce the exposure time, and vice versa.
Set your camera for daylight white balance (usually a Sun symbol on your camera’s menu display). “Auto,” or any other setting, will give your images an unnatural colour cast.
Firing your camera’s shutter is likely to introduce image-blurring vibration. If your camera is equipped with a mirror lock-up feature or exposure delay mode, here’s where it comes in handy. In any case, you want to be sure to use a remote shutter release if you have one.
If your camera lacks these features, here’s a handy trick. Get a piece of black cardboard bigger than the front opening on your telescope. Set your camera’s shutter speed for one or two seconds longer than required for the correct exposure. Hold the card in front of the scope so that no light makes its way down the tube. Now, fire the shutter and wait a second or two for vibrations to die out, then move the card out of the way to begin your exposure.
Experimentation is Key
Fortunately, a lunar eclipse lasts for a long time—photographing one is nowhere near as frantic as with a solar eclipse. That means you have time to try different exposure and ISO combinations until you find the one that works best for your set up. How will you know if your exposure is correct? The most reliable way is to check your camera’s histogram display—especially the graph that corresponds to the camera sensor’s red channel. Watch for “clipping” — the histogram data crowding up against the right side of the graph.
Finally, to maximize your chances for success, it’s a good idea to setup your gear a night or two before the eclipse and perform a dry run session photographing the Moon. There’s nothing like actually going through the entire process to ensure that your gear is working properly and that you know how to use it.