Piggyback Astroimaging Let a Telescope Take Your Camera for a Ride and Capture Amazing Sky Vistas

Astrophotography: part three Piggyback astroimaging Let a telescope take your camera for a ride and capture amazing sky vistas. by Michael A. Covington

he best pictures that I take with attach a manual-focus lens from an older doesn’t lose sensitivity during long expo- my telescope are not ones I take film SLR with an adapter to your cam- sures like film does. If you plan to use a T through the telescope. Instead, I let era. (Go to www.fotodiox.com for a film camera, you’ll need the fastest lens the camera ride “piggyback” on top of a wide range of adapters.) you can get your hands on. telescope that tracks the stars. The camera An off-the-shelf DSLR works well, but accumulates light during relatively long if you want to image emission nebulae, A mount with a polar view exposures, picking up deep-sky objects consider having it modified to extend its In piggyback photography, the type that cover moderately wide fields of sky. sensitivity to red light. Most DSLRs have of telescope you attach the camera to filters that block long-wavelength visible makes little difference, but the mount Cameras and lenses radiation as a way to make daytime color the telescope sits on is vital. With an Most astroimagers use digital single-lens photos look realistic. Unfortunately, this alt-azimuth mount, the telescope has reflex (DSLR) cameras for piggyback includes the prominent Hydrogen-alpha to twist with the stars even if its comput- photography. Film cameras also work emission line in bright nebulae. erized motors track the sky perfectly. well, although they prove to be more Astrophotographer Hap Griffin Because the field itself doesn’t twist, finicky than DSLRs. modifies cameras, Canon models in the stars appear to rotate around the You can use almost any lens. Most particular, to either remove or replace image’s center during long exposures. imagers use a medium telephoto, but you this filter (visit www.hapg.org). And To compensate, you’ll have to shoot many can get nice results with any focal length Hutech Corporation (www.hutech.com/ short exposures, typically 30 seconds at between 20mm and 500mm. Wide-angle AstroCamera.htm) sells similarly most, and then rotate and stack them lenses (those with shorter focal lengths) enhanced DSLRs. The modification has with computer software. The Pipe Nebula (Barnard 59/65–7/78) in southern Ophiuchus ranks among the finest dark nebulae in the sky. The author captured this view with a record more sky. For high-magnification little effect on stars and galaxies, but it If at all possible, choose an equatorial Canon 40D DSLR and a 105mm f/2.8 lens. He stacked five 5-minute exposures at ISO 800 and subtracted dark and flat frames. Michael A. Covington shots, a compact, high-quality telescope makes faint emission nebulae pop. mount. This type has its axis tilted to can take the place of a long telephoto lens. With a digital camera, an f/4 focal match that of Earth, so your target keeps The lens doesn’t have to be one made ratio is fast enough. The camera’s CCD the same orientation as Earth rotates. First, point the telescope in exactly ments with the mount only. Next, adjust If, after 5 minutes, the star drifts north specifically for your DSLR. You can sensor builds up light linearly and The mount’s polar axis must match the same direction as its polar axis. Don’t the mount so Polaris lies in the center or south no more than 20" (equivalent Earth’s axis to within half a degree, or blindly trust the 90° mark on the declina- of the finder scope’s field of view. Then to half the separation of Albireo’s [Beta somewhat better if you don’t make guid- tion circle; test it if you suspect a problem. move the polar axis 0.7° toward Kochab Cygni’s] components or half of Jupiter’s ing corrections. Fortunately, this isn’t Once you align the scope, leave it in that (Beta [β] Ursae Minoris), the brightest apparent diameter), you’re done. Other- hard to achieve. Here’s how I do it. position and make the rest of the adjust- star in the Little Dipper’s bowl. If your wise, if the star drifts north, move the I polar-align during twilight, and finder has the typical 5° field, you need to polar axis a fraction of a degree to the always with the drive and computer put Polaris one-quarter of the way right (or, if south, then left), re-center turned off. Although the telescope’s from the field’s center to its edge. the star, and try again. built-in computer will offer to run a Now check your alignment Last, perform the same test on a star polar-alignment routine, in my experi- with the drift method. The idea is that lies fairly low in the east or north- ence, it’s often confusing and not much that stars will appear to drift north or east. This time, if it drifts north, move help. Computers can overcome polar- south if tracked with a misaligned mount. the polar axis down, or vice versa. Once alignment errors when finding stars, So turn on just the right ascension motor, the drift rate falls below 20" per 5 min- but they can’t compensate smoothly or simply turn off both motors and track utes, you’re done. when tracking them. by hand-turning the right ascension knob. Now you can turn on the telescope’s If your mount includes a tiny finder computer, conduct a one-star initializa- scope built into the polar axis, use this Stop that drift tion (because the pole already provides finder to align on Polaris and then shift Unclamp the telescope from its 90° north a fixed point), and start taking pictures. slightly to the true pole. Otherwise, position and aim it at a star high in the proceed as follows. southern sky. Put an eyepiece with cross To guide or not to guide hairs in the focuser and rotate it so one In pre-digital days, you could tell who A camera that Longtime astrophotographer Michael A. piggybacks on an cross hair runs east-west and then the serious astrophotographers were: The Andromeda Galaxy (M31) is the biggest and brightest galaxy visible from the Northern Covington is a senior research scientist and equatorially mounted center the star. Try to keep the star the ones with stiff necks from spending Hemisphere. The author recorded M31 with a Canon 40D DSLR and a 300mm f/4 lens. He stacked associate director of the Institute for Artificial telescope is the best centered by turning just the right hours staring in an eyepiece, keeping a two 6-minute exposures at ISO 800 and subtracted dark and flat frames. Michael A. Covington Intelligence at the University of Georgia. way to photograph ascension knob. star centered on cross hairs by constantly fairly wide swaths of sky. Bill and Sally Fletcher 56 Astronomy© 2011 • March Kalmbach 2011 Publishing Co. This material may not be reproduced in any form www.Astronomy.com 57 without permission from the publisher. www.Astronomy.com in its gears, which produces a “periodic you may not need to make them even error” that recurs with every turn of the with longer focal lengths. Many astro- shaft. And third, in really long exposures, photographers simply refine the mount’s refraction becomes noticeable — Earth’s alignment and performance so they atmosphere causes objects near the hori- never have to make corrections while zon to appear a fraction of a degree taking an exposure. higher than they really are. Others, including me, still do so. With film, we had to guide perfectly (There’s no substitute for peace of mind.) for half an hour or longer. In the digital But we no longer gaze into an eyepiece age, we can take sets of 2-minute expo- and press buttons — we use autoguiders. sures and use software to stack them. So, An autoguider can be a self-contained Michael A. Covington The region surrounding Gamma Cygni (the errors that don’t show up in 2 minutes instrument or a software package (such bright star at center) teems with star clusters won’t show up at all, occasional jerks in as PHD Guiding, available for free from The Pleiades star cluster (M45) shows up nicely and nebulae. The author took this image with the guiding can be left out, and any grad- www.stark-labs.com) used with a web- even under suburban skies. The author took this a Canon 20Da DSLR and a 105mm f/4.5 lens. He ual drift disappears when you align the cam or planetary video camera. The image with a Canon Digital Rebel DSLR and a stacked three 3-minute exposures at ISO 800 pictures for stacking. As for periodic autoguider watches a star through the Nikon 300mm f/4 lens from a residential area. and subtracted dark and flat frames. error, you now make the corrections main telescope and issues corrections He stacked six 3-minute exposures at ISO 400 and subtracted dark frames. Michael A. Covington once, with a cross-hair eyepiece, and to keep the telescope pointed at it. The pressing buttons. It was the only way to the computer will remember and replay piggybacked camera reaps the benefits make the telescope track the stars per- them on subsequent cycles. And short of perfect tracking. Choose your targets by looking for fectly. Fortunately, those days are gone. exposures keep atmospheric refraction You have a wide range of celestial objects plotted as shapes and not mere We had to make those guiding correc- from becoming a problem. objects to target with your piggyback dots or symbols in star atlases. Although tions for three reasons. First, polar align- The bottom line? With a focal length setup. You’ll get nice results photograph- you need a dark sky to record galaxies ment is never perfect; stars always drift a of less than 200mm, you don’t need to ing nebulae, star clusters, and galaxies and nebulae, star clusters and the little north or south during a long expo- make guiding corrections. With a high- that span anywhere from roughly 1° to brighter parts of the Milky Way produce sure. Second, the motor has irregularities quality mount and good polar alignment, several degrees. A good rule of thumb: fine pictures even from suburban sites.

Exposure and processing Tracking the sky with an alt-azimuth mount Tracking the sky with an equatorial mount When you shoot the sky, use the same procedures you would for any deep-sky photography. With a DSLR, set the cam- era to ISO 800, tell it to record raw files (not JPEGs), and turn off “long exposure noise reduction” so the camera doesn’t take dark frames automatically. When you’re ready to take a picture, To celestial pole use live focusing (Canon calls it “Live Straight up View”) if your camera has that feature. Otherwise, take short test exposures to verify focus; don’t trust the infinity mark on the lens. Then, under full manual

control, take a series of 2- to 5-minute Michael A. Covington : Roen Kelly exposures followed by a matching set The Lagoon and Trifid nebulae (M8 [below] and M20, respectively) in Sagittarius make stunning Alt-azimuth mount Equatorial mount of dark frames with the lens cap on to targets for piggyback photographers. The author shot this scene with a Canon 20Da DSLR and a

Astronomy 300mm f/4 lens. He stacked eight 4-minute exposures at ISO 800 and subtracted dark and flat frames. Equatorial mount record CCD noise. Canon and other With an alt-azimuth mount, With an equatorial To celestial Declination companies make timers that can be set the telescope (and any mount, the telescope pole Altitude to take a series of sky exposures with one ily the same ISO speed. Let the camera (free from http://deepskystacker.free.fr). attached camera) keeps the Azimuth (and any attached same orientation relative to camera) rotates with press of the button. auto-expose (using “A” or “Av” so your It outputs a TIFF file that you can edit the horizon and must track the sky. The target Many astroimagers also take a set of selected f-stop remains in force) while with Adobe Photoshop or similar image- the sky by simultaneously thus keeps the same Right ascension “flat fields” to compensate for edge-of- you photograph something featureless processing tools. moving in two axes (left-right orientation relative field darkening and dust on the sensor. and gray. I usually take flat fields during The final step is to adjust brightness and up-down). The celestial to the camera’s field target thus rotates in the of view. You should take these through the same twilight with a white handkerchief in and contrast. “Auto Levels” in Photoshop camera’s field of view. lens at the same f-stop, but not necessar- front of the lens. You can also hold a often works miracles, but you’ll probably light box in front of it. want to make final adjustments manually. This is part three of Michael Covington’s Now you need to combine your expo- Then save the result and enjoy it. imaging series. He will look at prime- sures and subtract the dark frames. Many software packages do this well, but my To read the previous two parts of this Polar axis focus astrophotography in a future issue. series, visit www.Astronomy.com/toc. current favorite is Deep Sky Stacker

Polar axis angle = 58 Astronomy • March 2011 observer’s latitude www.Astronomy.com 59