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Astronomy TECHNOLOGY TODAY 39

Astro-imaging is definitely a hot topic inamateur astronomy these days – everyone en-joys the beautiful images and the camera’s abil-ity to reveal far more color and detail than thehuman eye can see allows us to enjoy and studystructure that we cannot observe otherwise.Astro-images are therefore interesting and scien-tifically useful.

The astro-images we produce ourselves area fun way to share with family and friends andalso great to use in outreach activities as everyoneis immediately drawn to astrophotos, especiallykids. Plus, you’ll have the coolest computer wall-paper and screensavers! Anybody can have a pic-ture of their kids, grandkids, pets, and such, buthow many can display an astro-masterpiece thatthey created themselves? Best of all, with rela-tively inexpensive equipment, you’ll produceimages that rival those taken by pros using thebest, most expensive equipment not so very longago – it’s a great ego boost.

One of the easiest ways to get started in as-trophotography is with a DSLR. These increas-ingly popular and affordable cameras offerseveral advantages, not the least of which is thatthey can also be used for conventional photog-raphy. This can help to justify the expense and,if you decide you don’t enjoy astrophotographyas much as you thought, you will still have agreat camera for conventional photos. Plus, if

you decide to sell it, the market is larger thanthat for used astro equipment.

Second, DSLRs can be used with a widevariety of lenses as well as telescopes and, sincethey use the same attachment methods as filmSLR cameras have used for years, any requiredadapters/attachments/accessories tend to befairly easy to find and somewhat reasonablypriced.

Third, although they may cost more thanlower-end dedicated astro-cameras, DSLRs

(generally) have much larger sensors resulting ina wider field of view and more forgiving imagescale. Additionally, depending on models,DSLRs are generally cheaper than dedicatedastro-cams with similar chip sizes. Another ad-vantage is that imaging can be performed withDSLRs using a minimum of equipment (nolaptop, filter wheels, external power source, etc.),which can be very convenient if you are travel-ing to dark skies to do your imaging.

Perhaps the biggest advantage though of

It’s Easier Than You Might Think!

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By John Moody

Getting Started In AstrophotographyWith DSLR Cameras

DSLRs is their flexibility – you can take a hand-held photo of a spectacular sunset, tripodmounted star trails, all-sky shots with a fisheyelens, piggy-back mounted Milky Way panora-mas, constellation photos and prime focus shotsof deep-sky objects. Finally, since many of ushave had some exposure to cameras and con-ventional photography, DSLRs are familiar toolsand may therefore seem less challenging thandedicated astro-cameras.

But DSLRs present disadvantages too.They are generally not to be as sensitive as ded-icated astro-cameras and tend to have higher“noise” levels, an aspect that is exaggerated bythe inability to control the temperature of DSLRsensors. Images produced with DSLRs musttherefore be calibrated with “dark frames” takenat a temperature as close as possible to that atwhich the image data was captured. Due tothese sensitivity and noise issues, processingDSLR images may be more challenging thanprocessing those taken with a dedicated astro-cam. Some DSLRs may also fail to functionproperly at extremely low temperatures. Also,when not using an AC adapter, it may be nec-essary to change batteries one or more times dur-ing an imaging session. However, in spite ofthese disadvantages, I feel that imaging with aDSLR is still a great way to get started in as-trophotography.

Starting with Fixed-Tripod Images

DSLR astrophotography can at first seemdaunting, especially once you start reading, re-searching and dealing with all of the jargon –guiding, polar alignment, image scale, calibra-tion, lights, darks, flats, bias, histograms, stretch-ing, layer masking, de-convolution, fieldrotation, and other terminology peculiar to theart can make astrophotography, DSLR or other-wise, seem confusing. In reality though, youneedn’t know and understand all of this minutiabefore you can begin taking pictures. Perhaps thesimplest way to get started in DSLR astropho-tography is by taking some very simple “pointand shoot” images of the moon, twilight scenes,planetary conjunctions, and constellations with asetup as simple as the camera mounted on aphoto tripod. In fact, this is a great way to learnabout the camera and its settings.

I usually shoot such “stationary” photos atan ISO setting of about 800 with a fairly shortlens (50 mm or less) at the lowest focal ratio withwhich the lens is capable of producing decent re-sults in “no flash,” “night,” or a similar mode.When you are shooting the moon, planets orbright stars (Sirius, Vega, etc) you can often evenget autofocus to work, although you may haveto get the planet/star/et cetera exactly in one ofthe Auto Focus (AF) points. If after reviewingsome shots on the camera’s LCD display I find

that I don’t like the results, I will switch the cam-era to manual mode and experiment with differ-ent exposure lengths until I get results that lookgood to me. The biggest limitation to this type ofphotography is that you’re relegated to relativelyshort exposures before star images start to trail.As long as you are not viewing the photo at fullresolution or printing it at an enlarged size, shorttrails may not become objectionable. Just howlong you can expose without noticeable trailingdepends primarily on the focal length you areusing.

To make wide-angle, stationary imagesmore interesting it helps to include familiar foreground references such as trees, an old barn or building, mountains, or even clouds. Also, shooting fairly low on the western horizonjust after sunset or the eastern just before sunrise can add some color and light to make the resulting image even more pleasing. See Image 1 for an example of a thin crescent moon with earthshine that I tookas a single image in “Flash Off” mode with aCanon 300D mounted on a simple photo tripod.

Capturing Star TrailsYou can also produce star-trail images with

the same simple camera and stationary tripod setup. The best of these tend to be centered near the celestial pole (Polaris in the NorthernHemisphere) and, just as with stationary photos, interesting foregrounds can really makethese images stand out. Star trails are as simple astaking many images over a relatively long timewith a very short interval in between, then stacking them. In this case you are actually taking advantage of the normal trailing you get when using a fixed mount. See Image 2(the horizontal red streak is an airplane) for an example created from 49 frames each 2-minutes long using the free software, Startrails,available at www.startrails.de.

Shooting multiple short exposures ratherthan a single long exposure offers the advantageof retaining the relative darkness of the back-ground sky. Startrails simplifies stacking and pro-cessing of a final image. It also facilitates creationof time-lapse movies as AVI files. In fact, this fea-ture can be used for creation of time-lapse moviesof any subject, not just of star motion.

40 Astronomy TECHNOLOGY TODAY

GETTING STARTED IN ASTROPHOTOGRAPHY WITH DSLR CAMERAS

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Piggybacking the DSLAnother fairly simple way to get some nice

astro-images and the logical next step after mas-tering fixed-tripod images and star trails is to ei-ther “piggyback” your camera on a drivenequatorially (EQ) mounted scope (see Image 3),or to simply place the camera on the mount in-stead of the scope. See Image 4 for an exampleof an image of Orion taken with a Canon DSLRand 50-mm lens piggybacked on an EQmounted Orion 8-inch Newtonian. If you usefairly short lenses (50 mm or so) even a relativelyinexpensive mount will provide decent results(long focal lengths require mounts of relativelyhigh accuracy for best results). Some extremelynice images of interesting areas of the Milky Wayand large Nebulae can be produced this way.

Shooting Longer Exposures – AnOverview

Once you start using a driven mount how-ever, you will almost certainly want to takelonger and longer exposures to capture ever dim-mer objects, and doing so will require takingmultiple sub-exposures and calibrating and

stacking them to achieve decent results. “Cali-brating” simply refers to taking some additionalexposures such as “dark,” “bias,” and “flat”frames and applying them to your sub-exposuresusing software such as Deep Sky Stacker, Images

Plus, IRIS, Nebulosity, or other similar programsto correct for issues such as vignetting, spots pro-duced by dust on the camera sensor, and to re-move sensor “noise,” all to make the final imagelook more pleasing. In my experience, the min-



Image 3


imum calibration necessary to obtaining decentresults is dark-frame subtraction. I won’t dealspecifically with the fine points of calibrationand stacking here other than to note that youmust shoot images in “RAW” or “sensor” modeto facilitate calibration and that you need to cal-ibrate individual sub-frames with dark frames,

and sometimes also flat frames, in order toachieve an optimum final image. We will coverthe finer details of stacking, calibration and post-acquisition processing in later installments ofthis series.

Once you have a stack of calibrated framesyou’ll need to perform some very basic post

image-acquisition processing and there are anumber of specialty software programs that as-sist with this “digital development” of the image.Many of these image processing programs canautomate both the calibration and digital devel-opment processes – clearly an advantage forthose just starting out in astro-imaging. I rec-ommend using software that can automate asmuch of the process as possible as it will make iteasier to quickly get some acceptable results andallows concentration on other aspects of imag-ing. The astro-imaging learning curve is steepenough as is – there are enough skills to masterthat there is simply no need to make things anyharder than necessary. Besides, it is easier to re-main motivated (not to mention more fun!) ifyou can get some decent pictures quickly. As youdevelop the desire for ever greater image perfec-tion and want to target ever more difficult tar-gets, you will encounter frustration aplenty, sodon’t needlessly add any when you are gettingstarted. Just have some fun!

When you master basic dark frame calibra-tion and stacking you can begin working onmore advanced techniques such as adding flats,

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darks and bias frames to your calibration stackand on more advanced processing techniquessuch as layer masking.

Prime-Focus AstrophotographyAs I’ve already said, once you have taken

some pictures using some of the simpler tech-niques and are comfortable with camera settingsand the basics of calibrating and stacking im-ages, you will undoubtedly want to graduate tolong-exposure prime-focus photographythrough a telescope to capture faint objects. Atthis point it is possible to spend nearly unlimitedsums of money and the equipment choices canbecome overwhelming, but by keeping in minda few basic but important facts it is possible tochoose a combination of equipment that willallow you to produce nice images at a reason-able cost. The things to remember when tryingto decide what equipment you will need forprime focus astrophotography are:1) The most important part of your astropho-tography gear is the mount.2) Increasing focal length generally equates toincreasing difficulty, frustration and need for amount of greater capacity.3) Even the most capable mounts cannot trackperfectly for unlimited periods of time.4) Fast focal ratios (lower “f” numbers) allowshorter exposure times.5) A good focuser is very important, as it has tosupport the weight of a camera, adapter, etc.,while allowing the astrophotographer to findand hold critical focus.

Given these considerations, many find thatthe ideal starter system consists of a relativelysmall, short focal length scope on the biggest,best EQ mount you can afford, preferably onethat can accept autoguider input. This wouldconsist of anything from a 60- to 80-mm Apoc-hromatic refractor between f/6 and f/7.5 to a 6-to 8-inch f/5 Newtonian on a mount such as theCelestron CG-5, Meade LXD75, OrionSkyView Pro, or the SkyWatcher EQ5 Pro,mounts that are capable of handling 20 poundsand more of telescope, DSLR and related imaging gear. Better yet would be mounts of the30 pound plus payload class of Orion’s Sirius/HEQ5 and Atlas/EQ6 mount, or Celestron’s new CGEM. If you need to cut cost,do it on the scope, not the mount. This may

seem counterintuitive, but even the best under-mounted optics in the world won’t produce aspleasing results as capably mounted modest op-tics. Plus, you can always upgrade to a betterscope later and then relegate your “starter” scope

to guide-scope status.When choosing a scope for astrophotogra-

phy you will need to pay special attention tothings you might not consider significant to vi-sual use. For example, a focuser that is capable of


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carrying the heaviest 1.25-inch eyepieces maynot fare as well under the much greater load ofa DSLR – the focuser of your imaging scopemust be capable of supporting the DSLR with-out slipping. You may also find that a motor-driven focuser, a feature many would consideroverkill for visual work, is very handy for as-trophotography. The scope should have at leasta two-inch focuser to prevent vignetting of thelight received by the relatively large sensors ofpopular DSLRs.

As mentioned above, you will want a scopewith a relatively fast focal ratio to allow shorterexposures, which becomes increasingly impor-tant with less capable mounts. Unfortunately,faster scopes are more likely to suffer from vari-ous optical aberrations and well-corrected “fast”scopes tend to be relatively expensive. Fortu-nately, you can keep the cost down and still geta high-quality scope by investing in an instru-ment of small to modest aperture.

If you choose a Newtonian instead of a re-fractor, you will want to be sure that the sec-ondary mirror is large enough to fully illuminatethe relatively large sensor of your camera, which

is one reason I recommend an aperture of at least6 inches – anything smaller is unlikely to have alarge enough secondary mirror to fully illumi-nate a DSLR sensor. At some point you will wanta coma corrector such as the Tele Vue Parracorrso stars nearest the edges of the image field, andespecially in the corners, look nice and roundrather than like little comets with short, broadtails. Until you invest in a coma corrector, youcan simply crop the edges of your images to re-move the most obvious evidence of coma, or dis-play the image as a smaller size.

If you decide you prefer a refractor, youshould consider investing in an Apo – anythingless will produce undesirable false color fringesaround brighter stars – and at some point youshould consider a field flattener/focal reducer toobtain even illumination across the entire fields ofyour images and to provide a slightly larger fieldof view. But again, as with the Newtonian op-tion, you can crop or otherwise display your im-ages in ways that minimize or mask suchimperfections. Fortunately, today’s market offersan embarrassment of riches in high-quality, com-petitively-priced small refractors that are won-

derful astro-imaging tools as well as great visualinstruments.

No matter what scope you use you will needan adapter system that permits mounting thecamera directly to the 2-inch (or larger) focuser.The most common method of attachment is a 2-inch to T-thread prime focus camera adapter (seeImage 5) mated to a T-ring that fits your camera.This is basically a two-inch eyepiece barrel withT-thread on one end. There are lots to choosefrom, but it is a good idea to choose one that hasthe lowest possible profile since a common issue(especially with Newtonians) is insufficient in-focus travel to allow a DSLR to reach focus.Scopestuff now offers adapters that combinethe T-ring and two-inch adapter. Indeed, it’snew “True 2-inch Barrel to Canon EOSAdapter” combines a very low profile with alarge unobstructed opening to minimize vi-gnetting of even the largest DSLR sensors, andOrion offers a “2-inch Zero-Profile Prime FocusCamera Adapter” that attaches to any T-ringequipped DSLR.

When you are ready to start imagingthrough a telescope you will need to master theskill of Polar Alignment of the mount. Whilethe topic is beyond the scope of this article,there are plenty of readily available Internet ref-erences with very good instructions on how toperform this procedure. I will simply refer youto the instructions that likely came with yourequatorial mount or Internet sites such ashttp://www.petesastrophotography.com. Sufficeto say that accurate polar alignment is necessaryto obtaining acceptable long-exposure images.There is a common misconception that guid-ing/autoguiding can substitute for accuratepolar alignment, but this is, sadly, not true.While it is true that you can obtain acceptableimages with a less-than-perfect polar alignment,the better the polar alignment the better the im-ages. Once you have learned to polar align yourmount you can begin taking photographs likeImage 6.

Regardless of the quality of the mount andfocal length of the scope, you will find that thereis a limit to the length of time you can exposean image before you get trailing or other unde-sirable mount related issues. You can often ex-tend these exposure limits by improving theperformance of your mount by performing


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such procedures as re-lubing it, polishing/lap-ping gears and other moving parts that are incontact with each other, adjusting clearances,and so on. Since the exact procedures aremount dependant, I cannot provide detailshere, but, again, there are lots of related re-sources on the Internet. A good place to startfor advice on “tuning” your mount for as-trophotography would be the “Mounts” forumon Cloudy Nights (http://www.clou-dynights.com).

In addition to “tuning” your mount, an-other way you can increase the length of timeyou can expose a single frame is to “guide” theexposure, either manually or automaticallyusing a guide camera and computer program ora self-contained autoguider. Guiding (whethermanual or automatic) is basically a matter ofcorrecting for a mount’s inevitable tracking er-

rors. Manual guiding is generally performed byusing either a second telescope mounted besideor on top of the primary imaging scope with acrosshair reticle eyepiece, or an off-axis guider,a device with a mirror or prism that redirectspart of the light 90 degrees through a crosshairreticle eyepiece. You simply keep the crosshaircentered on a guide star by making small cor-rections to the mount’s tracking using the but-tons of its hand controller.

Autoguiding is more common these daysand eliminates the tedium of spending hoursstaring at crosshairs (risking falling asleep andbumping the scope in the middle of a 4 hourimaging run at 3 a.m!). Autoguiding is per-formed by installing a second camera in place ofthe crosshair eyepiece and using a computerprogram such as PHD Guiding or any of the many commercial imaging programs that

can automatically control the mount and correct for tracking errors. There are also several self-contained autoguiding units thatpermit autoguiding without having to have aPC or laptop at the telescope, such as the LVISmartGuider announced in the November2008 issue of ATT, and the new OrionStarShoot Solitaire AutoGuider announced inthis issue.

Autoguiding equipment has become quitereasonably priced and, unless you are really ona tight budget or think you would enjoy man-ual guiding, it is pretty hard to justify not auto-guiding if your mount can support theadditional weight. Orion now offers a completeautoguiding package that includes an 80-mmscope, dovetail bar, Starshoot Autoguider cam-era, and PHD Guiding software, all for no morethan what a typical guide camera cost only afew years ago. Most autoguiding solutions dorequire a computer, but it does not need to beparticularly powerful one – even an inexpen-sive, used laptop will suffice. Indeed, I am cur-rently guiding with a computer that I purchasedused for only $200. But, if you are not using acomputer for other purposes, a self-containedguider is a great option.

Once you have tuned your mount, sortedout your autoguiding rig, mastered polar align-ment and the arcane arts of image processing,and are able to gather hours of flawless datanight after night, there is one other thing youmay want to consider and that is whether or notto modify your DSLR (or to purchase a profes-sionally modified camera) for enhanced red re-sponse to pick up more red nebulosity,specifically the Hydrogen Alpha wavelength.The reason I left this topic until last is that there

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are plenty of objects to photograph that do notbenefit from a modified camera and consider-ing the steep astrophotography learning curveand all the other issues you will deal with in theprocess of learning to take beautiful images, thisjust isn’t that big a deal unless you are sure youenjoy astrophotography and want to shootemission nebula. Galaxies, star clusters, and re-flection nebula don’t benefit at all from a mod-ified DSLR. But, if you eventually find yourselfwanting to get the most out of photographs of

emission nebula, as do most that get hooked on astrophotography, you’ll eventually also find yourself seriously considering this option.For more information on “spectrum-enhanced”DSLR options, please see the accompanying insert.

I know I said DSLR astrophotographydoesn’t have to be as complicated as many fearand, despite this, you’ve endured a long-windedarticle full of statements about things that are“beyond the scope of this article.” Nevertheless,

DSLR astrophotography is something you caneasily master while having a lot of fun in theprocess. Just follow a step-by-step, building-block approach – start simple and build fromthere. This approach is also, happily, much lessexpensive! There is nothing wrong with doinga bit of research on more advanced topics as youwork your way along the learning curve, but tryto avoid getting ahead of yourself and don’t letthe lure of long-exposure prime-focus photog-raphy sucker you in before you’re ready.



As Digital SLR cameras become increas-ingly sophisticated and ever capable with eachpassing year, the popularity of spectrum-en-hanced DSLRs is also increasing. StandardDSLRs are equipped with infrared (IR)-cut-off filters that enhance standard terrestrial im-ages, but that also block some to the redHydrogen Alpha (Ha) spectrum that is criti-cal to optimum reproduction of astronomicaltargets such as emission nebula. Modifying aDSLR by removing its IR-cutoff filter, or byreplacing it with one that passes more of theHa spectrum, results in a “spectrum-en-hanced” DSLR that more fully takes advan-tage of the DSLR sensor’s capacity to detectthe bandwidth criticalto imaging some of the night sky’s most cel-ebrated targets. Thoseinterested in spectrum-enhanced DSLRs havetwo basic choices: (1)Modify their DSLRsthemselves, or (2) investin a DSLR that hasbeen professionallymodified.

As to ATM modi-fication of DSLRs, wecaution that, while itcertainly can be done,there are some thingsbetter left to the pros. Topresent an extreme anal-ogy, we know of more than one example ofDIY dental root canal procedures. But, al-

though those masochists mayhave saved themselves a few hun-dred dollars, we’re left to wonder,“Why?” (Actually, we’re left towonder far more than that!) Anal-ogy aside, the fact is that we knowof a number of technically profi-cient astrophotographers whohave successfully modified theirDSLRs and, if you are absolutelyconfident in your technical profi-ciency and don’t mind voiding the factorywarranty on your camera, conversion filtersare available from such sources as BaaderPlanetarium (www.alpineastro.com).

For those of us who areless adventurous, profes-sionally modified DSLRsare readily available. AstroHutech offers spectrum-en-hanced versions of theDSLR models that are mostpopular with astrophotog-raphers, including those byCanon, Fuji, Nikon, Pana-sonic, and Olympus. AstroHutech installs custom de-signed filters that exactlymatch the light path of thefactory-installed filters thatthey replace, thus insuringthat the camera’s autofocuscontinues to work properly.Actually, the typical Astro

Hutech spectrum-enhanced DSLR uses atwo-filter system: a rear filter that replaces the

stock IR-cutoff filter and a front filter.Astro Hutech offers two options for the

rear filter: a clear, fully multi-coated filter thatpasses all bands, including the IR, and an “as-tronomical” filter that blocks UV and IR,while allowing recording of the astronomi-cally important Ha spectrum. The front filtersare mounted in a user accessible holder placedbetween the camera’s lens bayonet and the in-ternal viewing mirror where they can be eas-ily inserted or removed as the user requires toreturn the camera to standard daylight pho-tography functionality. Or, the user canachieve the same functionality by installingthe appropriate conventional in-front-of-the-lens filter instead of a “front” filter. Eitherarrangement permits the user to enjoy the as-trophotography benefits of a spectrum-mod-ified DSLR that still operates as afully-functional daylight camera as well.

Best yet, Astro Hutech’s spectrum-modified DSLRs carry a full one-year warranty from that well-established company. For more information, visitwww.astrohutech.com.

Spectrum-Enhanced DSLRs

Spectrum-Enhanced Canon 20D with internal thermometer installed. Shown with and without external readout unit attached.

Cutaway view of the front filter andrear filter locations in typical AstroHutech Spectrum-Enhanced CanonDSLR.

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dedicated astrocameras

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