Catching Light

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Dan Lessmann

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Astrophotography Pointers

 

My journey into astrophotography has been very rewarding but there have been a lot of rocks in the road along the way.  Years ago I attempted film astrophotography and quickly gave up.  The costs of film development and wasted frames and the generally poor results I obtained with the equipment I could afford convinced me quite quickly that the costs and demands film work places on the equipment were not going to allow me to enter into this aspect of photography.  The technology I needed wasn't available yet but all of this changed when digital technologies became available and affordable.  Today, amateurs can far exceed the capabilities available only to professional observatories of 15 to 20 years ago and that trend is accelerating as the technology continues to advance.

 

In this piece, I assume that you are interested in astrophotography and just starting out at the foot of the digital astrophotography mountain (yes it's most definitely a mountain).  I'm going to attempt to provide you with some pointers and tips based on what I've learned on my own continuing climb up this rocky slope.  Because the technology is changing rapidly, I won't deal with specific equipment here as much as more general topics that will always apply to the hobby.  However, if you're interested in the path I followed and some of the mistakes I made specific to equipment selection, click here.

 

I also assume you're interested in digital astrophotography although many of the topics I'll discuss apply to film work as well.  But the differences digital technology brings into this field are substantial enough that the two are almost unrelated.

 

And finally, I assume that you're currently a daylight photographer with some knowledge beyond typical point and shoot in "auto mode" practices.  Feel free to keep reading if not but you'll need to supplement this information with some basic photography knowledge.

 

All of that said, I don't represent myself as being an expert astrophotographer or even a good astrophotographer.  Rather I'm simply a fellow traveler on the path on which you are about to embark and want to share what I've found on my own journey in the hopes of making your beginning steps a bit easier.  So let's begin.

 

Establish Your Goals

 

What do you want to shoot?  Wait.  Let me answer that question for you.  I bet you want to shoot everything in the sky!  Most of us do of course but the requirements of equipment and techniques are radically different based on the targets you're interested in capturing.  This page assumes that your long term goal is to image a variety of different targets on tracking equipment.

 

We can essentially classify astronomical targets from a photography point of view by combinations of the following two characteristics:

 

Luminosity - Or the brightness of the object you want to shoot.  Really bright objects, such as the moon, can be shot with very short sub-second exposures quite easily.  Dim, deep sky objects are much more challenging requiring long exposures of minutes or even hours and much higher quality and more expensive equipment.

 

Size - I mean by this its apparent size from here on the earth.  The size of your target  determines the field of view required for the shot.  This is measured in degrees and fractions of degrees of arc.  Astrophotography targets range in size from only arc seconds across to many degrees across.  So there is a huge range of size and a correspondingly huge range of required fields of view when shooting astronomical objects.  Just as in daylight photography, since the size of the sensor in your camera is fixed, the way you shoot different size fields is by varying the focal length of the optics used for the shot either by changing lenses or by using zoom lenses that have adjustable focal lengths.

 

The smaller an object, generally the more magnification is required to image it satisfactorily.  Large objects, the moon, constellations and such, can be shot with standard or long lenses.  Some constellations or groups of constellations may even require wide-angle lenses.  Small objects, planets and most deep sky objects, require higher magnifications and correspondingly higher quality and more expensive equipment, i.e. telescopes.  To the astrophotographer, telescopes are simply extremely long lenses and they are used in the same way as daylight lenses as far as the optics are concerned.  As with all types of photography, it's still all about focal lengths and focal ratios.

 

Just as there is no single perfect lens for daylight photography, there is no perfect telescope for astrophotography.  If you delve deeply into this hobby, you'll end up with more than one telescope and a variety of other optical elements just as you end up with more than one lens for daylight photography.  This is all to adjust the focal length and speed of the optics to control the field of view and exposure appropriate to the object you are shooting.

 

These two characteristics of luminosity and size can combine in a variety of ways.  The same is true of daylight photography but the range of luminosities shot in daylight work is much smaller than that of astrophotography.  I mean in daylight work and with only a few exceptions, when things are too dim the answer is to use a flash or just wait for the sun to come back up the next day.  You can't do that with astrophotography and instead must increase the exposure time and/or speed up the optics to compensate for the low light conditions.  The range of brightness in astrophotography is from very bright like the moon which approaches daylight photography levels to very dim approaching almost no light at all.  This range of luminosities and sizes is very challenging and requires different equipment and techniques to match the conditions.

 

Be Prepared for the Cost...

 

There's an old joke common in amateur astronomy circles regarding how the only good thing you can say about the cost of astronomical equipment is that it's cheaper than a boat.  That's generally true for strictly visual astronomy but not necessarily true for astrophotography. 

 

Astrophotography rigs can run from a few thousand dollars (actually even less with correspondingly lower capability) to amounts exceeding $100,000 quite easily.  $100,000 will buy one nice boat!  For a "quality" astrophotography rig capable of deep sky work, it's generally accepted that $10,000 or so is about the minimum.  A $10,000 boat is pretty nice too. 

 

Let's assume for the rest of this discussion that the camera you'll be using is at a minimum a digital single lens reflex camera or DSLR.  That happens to be the path I have chosen although I've drifted off into other paths as well and use a combination of equipment.  But, if your primary shooter is a DSLR, you'll probably want to add, assuming you want a truly comprehensive selection of focal lengths for your various targets, a few prime lenses to be piggybacked with your camera on a scope or tracking mount.  The best quality lenses often exceed the cost of the camera body and you need high quality, fast primes to get the best results.  Zoom lenses are also an option and provide the advantage of being able to vary the focal length through the zoom range provided by the lens.  But these lenses tend to be slower and usually are more prone to aberrations than primes.  This is a workable compromise but quality will be reduced accordingly.

 

Suffice it to say, that the more you spend, the more capability you are buying and the more you can do in your astrophotography pursuits.

 

...And Go Slow

 

The good news is that you can enter the hobby without spending that kind of green and then add on or upgrade slowly as your skills improve, budgets allow and interests grow.  Since I'm assuming that your long term goal is to shoot those small and dim deep sky objects, I'm recommending equipment and suggesting techniques that meet those most demanding goals but that's not how you have to start out and you probably don't want to try taking that big a bite anyway.  If you currently own a digital camera, chances are you have all you need to enter the astrophotography arena.  I don't really mean your typical point-and-shoot compact here, although even these will open some doors, but equipment that's a bit more sophisticated and that will lead you into more demanding applications. 

 

Suffice it to say that you may in time end up with one or more specialized astrophotography cameras and that's fine.  What I'm talking about here are minimal capabilities for your long term goals.  Accordingly, the camera you shoot with must have the capability of long exposures (a bulb setting) and a degree of control over several other parameters.  It needs to be optically versatile meaning you can change lenses for different focal lengths and ratios.  It needs to have a cable release and it optimally needs the ability to download its images to a computer for evaluation in the field as they are shot.

 

Here then are some genres of astrophotography that are listed more or less in order of increasing cost and equipment complexity.  These equipment requirements are by necessity incomplete as the exact requirements will be determined by the equipment you select:

 

Short Exposure Star Field Snap Shots - Short exposure is relative.  Here I'm talking about exposures no longer than 15 to 30 seconds or so and probably no shorter than 5 seconds or so.  These are done from a fixed tripod for quick snapshots of constellations and other interesting star patterns.  You won't get much detail with these short shots but an exposure longer than about 15 seconds will result in star trailing from the relative motion of the earth depending on the focal length at which you are shooting.  Wider angle, shorter focal length shots can be longer of course and you can stack multiple frames after the fact to increase the detail to a limited degree.

 

Star Trails - Here you're after that star trailing and use it as the subject for your shot.  Star trails are images of the night sky taken with a camera on a fixed tripod.  Here's an example.  All you need for this type of shot is a camera capable of longer exposures, a cable release, a tripod and a clear night sky.  This particular image was done with a series of 60, 30 second exposures for a total exposure time of 30 minutes.

 

As time progresses and the earth turns under the sky, the stars carve out arcs of light in the image that can be quite beautiful and rewarding.  The more frames you take, or the longer the exposure,  the longer the arcs of the trails.  For digital cameras and multiple exposures, you'll need some sort of post processing software to stack up multiple images and build your trails.  Chances are your camera came with a software that will work but Photoshop is probably the preferred one and it's very useful for any type of digital photography.  There is also software specifically designed for stacking star trails but, assuming you choose Photoshop, you will in time become an expert in its use and in the use of a variety of other software applications for that matter.

 

Lunar Imaging - The moon is an inviting target for the beginner for several reasons.  It's big, it's bright and it's beautiful with lots of fascinating detail.  There are various types of lunar imaging ranging from wide field night shots that happen to include the moon in the view to very detailed close ups of specific lunar features.  If you can shoot star trails you can shoot those wide field shots as well.  But anything more gets you into the realm of requiring a telescope or a long lens.  Which one and what type?  That depends on what you want to shoot on the moon and where you plan to journey in your astrophotography travels after that. 

 

In general though, smaller features on the moon require lots of magnification.  Look into afocal or eyepiece projection photography for some options to take advantage of the magnifying power of telescope eyepieces.  Tracking, while convenient, is not that important for lunar photography because the moon is so bright.  Lunar features generally require short exposures of less than a second.  This means you can shoot images of this type with a typical Dobsonian mounted, manual tracking telescope and simply nudge the telescope between frames to re-center the subject.

 

Planetary Imaging - Planets are bright but they are small.  This shot of Saturn was shot at f/20 and a focal length of 5,000mm through a 10" Schmidt-Cassegrain telescope.  For these long focal lengths you would probably prefer tracking and the larger the aperture of the scope the better to keep the optics as fast as possible and exposures as short as possible.

 

Big telescopes and tracking mounts mean the costs are now going up.  However, the good news is that planetary images, because planets are bright, are shot with short exposures as well so these shots too can be done with nothing more than a Dobsonian.  And, believe it or not, the best planetary cameras are inexpensive webcams recording video frames that are then graded and culled for moments of quality seeing.  These quality frames are then stacked and processed after the fact.  Of course webcams require a computer to operate them, generally a laptop for outdoor field work, so there's a catch.  But you're reading this online so you may very well have that requirement handled already.

 

Wide Field Star Shots - You've probably seen this type of image, say an image of the summer Milky Way spanning several degrees of sky.  These shots are done with a long exposure capable camera, a cable release and standard lenses with the addition of a tracking equatorial mount.  Usually fast prime lenses work better than zoom lenses for this type of shot.  While you must have a tracking mount, at these short focal lengths, the mount does not have to be exceptionally accurate or even terribly well aligned.  So the investment and knowledge required is not necessarily that great for this type of shot.  To shoot these truly well is another matter.  Wide field night sky shots are an art form in and of themselves and many photographers devote their efforts to refining this skill into a very fine art indeed foregoing all other forms of astrophotography.

 

Should you choose to try this type of photography you will find that you need an external power supply for your camera and your mount.  Long exposures will very quickly drain the camera's internal battery especially when it's cold and a battery introduces a heat source in the camera that should be avoided to reduce noise prevalent in longer exposures.  Further switching batteries while the mount is tracking pretty much guarantees that you'll mess up the frame alignment for subsequent shots.  So begins the seemingly never ending purchases of miscellaneous accessories that end up being a major cost item in this hobby.

 

Deep Sky, Large Objects - A deep sky object is any object that is beyond the boundaries of our solar system.  A large deep sky object, as I'm defining it here anyway, is any object that can be shot through a relatively short telescope, say optics of 1,000 mm focal length or less.  These are generally smaller refractors or reflectors but could also be long lenses.  These optics range widely in price, quality and capabilities and you can spend a fortune on such scopes quite easily.  But you don't have to just yet.  Just keep in mind that quality and cost go together and the quality of the images tags along for the ride.  Let your budget guide you and be willing to accept that the equipment you end up with will, for now, introduce its own set of limits.

 

On the other hand, the telescope mount requirements are becoming much more demanding.  Some objects in this category are still comparatively bright but none are nearly as bright as the brighter planets or the moon so the exposures are getting longer and are requiring better mount tracking.  Brighter objects in this genre are open star clusters requiring shorter exposures than most other objects.  Using shorter exposures (still much longer than lunar or planetary work) means the mount's tracking does not have to be as good as is the case with truly long exposures.  This shot of the Pleiades is an example of open star clusters that fall within this category of relatively bright objects.

 

Other large objects include a number of nebulae and a few galaxies.  These objects are much more difficult to capture than open clusters because the light is spread out over a large area rather than clustered in point sources of stars.  The Great Orion Nebula and the North American Nebula are examples of this type of object.  Both of these images were shot at relatively short focal lengths through refractors in the range of 600 to 700 mm.  With this type of object, your mount's tracking capabilities become yet more important and you normally need some sort of manual or active guiding to shoot these successfully.  Oops! That's another camera (or a dual chip astro camera) and maybe a second telescope or other equipment along with software and a computer in the field.  More cost.

 

Deep Sky, Small Objects - This includes all planetary nebulae, most galaxies and a lot of the other nebulae, i.e., probably the majority of what you want to shoot.  This type of photography has to be done at long focal lengths and at longer exposures.  This shot of M63, the Sunflower Galaxy is an example.  This was shot through a C14 Schmidt Cassegrain  telescope at a bit over 3,000mm focal length. This places the maximum demand on your mount and its guiding capabilities and is by far the most demanding and most expensive type of astrophotography.

 

So the Four Basic Types of Astrophotography Are...

 

Let's sum all this up.  Here are the four basic types of astrophotography as I classify them anyway.  The equipment demands and costs go up and the knowledge required increases as we progress down the list:

 

Fixed Photography - Primarily star trails and snap shot exposure constellation shots.  Shot from a tripod mounted camera with a cable release through standard lenses and with no tracking.  This is essentially daylight, tripod mounted photography at night with some special additions in the processing steps and longer exposures than normal.  It's a relatively easy step to transition from daylight work to this type and that makes it a good first step for the beginning astrophotographer.

 

Short Exposure Photography - Bright objects, usually the moon and planets, that may be shot through a telescope and often afocally but that don't necessarily require tracking because the exposures are typically less than a second.  This takes what you learned shooting star trails and adds a bit of equipment, a bit more knowledge and some new post processing skills.

 

Tracked Wide Field Photography - Long exposure, short focal length shots of degrees wide star fields or constellations.  The short focal lengths minimize the need for truly accurate tracking even though the exposures are getting longer.  This adds long exposures of several minutes to the previous types and again brings new post processing skills and acquisition techniques into play.

 

Tracked and Guided Narrow Field Photography - Long exposures shot at long focal lengths almost always through a telescope and with accurate tracking and guiding.  This step up is more about equipment and it's use than additional photography skills.  It's mostly an investment in top of the line gear of a quality needed for accurate guiding and tracking.  It may also include dedicated astronomical cameras requiring new post processing skills beyond the previous steps.

 

I should note that these classifications are more guidelines than anything else.  The lines between them are very fuzzy and I encourage you to experiment and push your current equipment to the limit before adding the complication and cost of the next step up.

 

Your First New Acquisition Should Be Knowledge

 

A bit of forethought and planning goes a long way in this hobby as there are many, many costly wrong turns you can take.  I've taken a few of them myself.  Fortunately there are also many resources available to you to help keep you on the straight and narrow:

 

Join an Astronomy Club - If you're not already a member of your local club, join today!  Chances are your local club has a number of astrophotographers among its members and there's nothing like a little first hand mentoring to help you on your way.  Many clubs have astrophotography special interest groups formed specifically for this purpose.

 

Read, Read and then Read Some More - There are a variety of fine texts on the subject of astrophotography addressing equipment selection, image acquisition and post processing techniques.

 

Join Web Based Discussion Groups - There are many discussion groups around that are loaded with helpful people of all levels of skill.  The astrophotography community, actually the entire astronomical community, is made up of a great bunch of friendly folks who are tightly knit and more than willing to help each other out whenever they can.

 

Your goal in attaining knowledge is three-fold: 

 

First, learn everything you can about how to push your current equipment to its absolute limit. 

 

Second, learn everything you can to try to make sure that any new equipment you acquire will still be useful with future upgrades.  For instance, you might buy today a relatively inexpensive astronomical camera as your prime shooter but with an eye toward making it your guide camera when you acquire a better camera down the road. 

 

Third, learn everything you can to hone your skills.  The one piece of equipment in your astrophotography rig that can't be replaced is you.  But your capabilities as an astrophotographer can be enhanced with knowledge and practice and this is the least expensive way to improve your astrophotography results.

 

Start From the Ground and Work Up

 

Let's assume for the moment that you've studied and you've established your goals and know that, in time, you're going to be going after those small, way out there dim fuzzies.  Let's also assume that you've established a budget and, over the long run, it's somewhere close to that minimum $10,000 amount for a quality rig but your initial expenditures have to be substantially less than that.  It's tempting to then pull out your Visa card and go buy that way cool telescope and camera while skimping on the mount.  Don't do it!

 

The place to start is at ground level.  Plan on spending a minimum of half of your long term budget on the mount and let that be your first acquisition even if you have to skimp for now on the scope and camera equipment.  Having a mount that will do a good job of tracking an object in the night sky eliminates a huge amount of frustration when you're starting out and you can always upgrade to better cameras and scopes later on.  The mount you select, perhaps more than anything else determines what you can shoot and how well you can shoot it.

 

Catching Light Is Important but this is Not Where the Magic Happens

 

I can pretty much guarantee that when starting out you'll be disappointed in most of your images as they come off the camera.  This is completely normal so don't worry.  The real magic of astrophotography does not occur at the scope.  Instead it occurs the next day at the computer during the post processing of those images.  This is true of any type of astrophotography.  A good image post processor can take even a mediocre acquisition and turn it into a masterpiece you'd be proud to hang on your wall.  Conversely, the best acquisition will be suitable only for the recycle bin if the post processing is done poorly.

 

Unfortunately, there is no set recipe for good image processing.  There are techniques that can generally be applied to all images but every image is different and requires different techniques just as every gourmet dish is different and requires different spices.  The key is knowing what spice and how much of it will improve that particular dish. 

 

This is by far the steepest learning curve there is in the hobby.  Most of the texts I have read address post processing techniques and the math behind them rather than acquisition skills and I'm still learning something new every time I process an image.  A gourmet chef that is good at creating new and tempting dishes can do so only by having a solid grounding in the culinary arts.  The same is true of image processing arts (this is as much an art as a science). 

 

To be able to know whether or not an image needs a pinch of Gaussian blur or a teaspoon of unsharp masking requires that you have a solid grounding in just what is happening to the image when you add these ingredients.  With knowledge and practice, you'll begin to recognize just what ingredients you need to add to your dish to bring out its full flavor without over seasoning in the process.

 

On the Other Hand, I Did Say Catching Light Is Important

 

Having stressed the importance of image post processing, I don't want to minimize the importance of acquisition skills.  A gourmet chef worth his salt will go to the market and select his own ingredients as needed to meet his own high standards of quality.  He knows that high quality ingredients will improve the dish beyond his own culinary skills.

 

The same is true of image acquisition.  The better the quality of the images going in to the post processing, the better the final image will be coming out the other end.  You must learn how to evaluate the quality of your raw images just as a gourmet chef must learn to evaluate the quality of his ingredients.  There are a few skills you need to be able to tell whether or not your raw images are of the quality needed to be included in your recipe.

 

You Must Understand the Types of Exposures to Be Captured and Their Purpose - If you don't know what I mean by light frames, flat frames, dark frames and bias frames and what each is used for you need to go back and hit the books until you do.  This is the foundation on which all other acquisition skills are built and you will need to shoot and use all of these types of frames to produce the best results you can get with your equipment.

 

Focusing Skills - Any raw image that is out of focus will drastically decrease the quality of the final image.  There are some techniques you can use to work around this to varying degrees but it's much better to have every image in critical focus in the first place.  Focusing is not so much a skill as an exercise in patience.  Consider that the range of critical focus for a typical telescope and camera is only a few thousandths of an inch in depth.  Your goal then is to determine whether or not the raw image you just shot is within that very narrow and critical range.  This takes time and it takes patience.  Tweak the focus a bit and shoot another.  Better or worse?  Tweak the focus and shoot another.  Better or worse?... ad infinitum!  Or at least it seems like it goes on forever.  You must also reevaluate focus from time to time during each session.  Components contract as they cool in the night air and focus changes over time as the object climbs higher in the sky or sets in the west.  This can change your focus shifting it out of that critical few thousandth's range. 

 

There are, for additional costs of course, both software and equipment that can aid you in this important skill and I encourage you to look into this.  But first learn to focus manually.  It's hard, at first to know whether or not an image is focused and it's difficult to achieve that focus in the first place but this skill becomes easier in time and this ability is important for evaluating the performance of any automated equipment you end up using.  Regardless, be patient and stubborn about focus!  Demand critical focus of every image and don't compromise.

 

Exposure Evaluation Skills - When it comes down to it, deep sky astrophotography is the application of the art of photography in the absence of light.  What I mean is we're taking a camera sensor that is sensitive to light and we're asking it to work in the dark.  Anyone with a modicum of common sense would say we're just asking for trouble and they'd be right.  I know of no more demanding application of photography. 

 

What we're after is the occasional photon of "signal" that finds its way to the camera sensor amongst all of the photons of "noise".  This is commonly referred to as the signal to noise ratio or S/N and a high S/N is the holy grail of quality exposures.  Noise comes from a variety of sources including your equipment while signal comes only from the object you're attempting to image.  The key to a quality exposure then is to maximize signal while minimizing noise. 

 

But how do you know whether or not you've achieved this with a particular exposure time, exposure count, filter arrangement or any other number of variables that you can control among all of the many variables that you cannot?  You must learn to evaluate a raw exposure and determine whether or not a longer or shorter exposure or different filter arrangement is more appropriate. 

 

You must be able to evaluate the quality of the sky during your session and, from time to time, you must be willing to toss it in for the night because the conditions are so bad.  Better to do that than to add bad data to an acceptable image turning it into an unusable mess.  These skills come from knowledge and from experience so again, be patient with yourself and persistent in your demand for quality.

 

Equipment Use Skills - Your particular equipment selection will determine how you use it but your skill for being able to do this in the dark and often in the cold is directly related to the quality of the images you produce. 

 

What you'll find as you become more experienced is that quality is achieved by passing your exposures through a series of bottlenecks or gates until eventually one is too narrow for the exposure to squeeze through.  That gate determines the quality of your exposure.  So your goal is to make those gates as wide as possible by configuring your equipment optimally for the conditions at hand.  This allows your exposures to pass through each gate without getting snagged up and each gate passed increases the quality of your exposures. 

 

Poor polar alignment is such a gate.  So make it nice and wide by learning how to drift align your mount and take the time to do it. 

 

Poor scope collimation is a gate.  So learn how to collimate your equipment and check and tweak it as necessary every time you set up. 

 

Bad focus is a gate.  Learn how to obtain critical focus and do it every time. 

 

Bad tracking and guiding are both gates so learn how to use periodic error correction on your mount and optimize the mechanics and setup to minimize these problems in the first place. 

 

Less than optimal exposure settings are a gate...  There are many others and each must be addressed to allow your exposures to slip through.

 

After you've optimized everything under your control, you'll eventually hit some other gate that is not under your control; seeing conditions or light pollution or something else.  There's always a limiting gate somewhere.  But if you effectively manage those things that you can control, you'll be imaging at skill levels of those astrophotographers whose work you've come to admire.  They may have an edge on equipment, that's yet another gate, but you can do good work at any budget within the limitations those budgets define and ultimately that's what you're after... 

 

The best results you can get... from the equipment you can afford... under the conditions in which you shoot.

 

Sounds Hard Doesn't It?

 

Well it is hard!  There is much to learn, and a huge number of trials with the results often being more failures than successes most especially when we're first starting out.  But sticking with it and surviving those trials and the inevitable failures to produce our own masterpiece of some part of the heavens is the source of the satisfaction of the hobby!  As they say, if it was easy everyone would do it!  And I believe I'm safe in saying that if it was easy you probably wouldn't be interested.  The true satisfaction of this hobby is overcoming the challenges such a difficult pursuit sets before us through the skills and knowledge we attain as we progress in the hobby.

 

I hope you found this information helpful and feel free to drop me a line and let me know how you're doing.  I'll be happy to answer any questions that I can and those that I can't answer we'll learn about together.  I wish you clear skies and beautiful images in your future!

 

--Dan Lessmann

 

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Last Updated: 11/12/2014  -  Copyright 2004-2013 by Dan Lessmann.  All rights reserved.  Please click here for my usage policy.