Cosmic Bird Watching: Visual Observation of the Deep Sky

Who am I?

My name is Jay Freeman. I live in Palo Alto, California, and write computer programs for a living. I have a Ph.D. in physics, via doctoral thesis data from an astronomy experiment that flew on a spacecraft, so you could call me an astrophysicist. But all my professional work was done using light the human eye cannot see -- I have retained amateur standing in the visual wavelengths.

I have been doing visual amateur astronomy for over forty years. I started when I was eight, with a 50 mm spotting telescope so junky it made modern beginner refractors look good, and that's hard to do. Last time I added up my logbook, I found I had made about twelve thousand observations of almost five thousand different objects. Most were deep-sky objects. I have used about twenty different telescopes and binoculars enough to know them well; these have had apertures from 50 mm through fourteen inches.

What is Deep-Sky Observing?

"Deep-sky" means "beyond the solar system". It includes galaxies, star clusters, and nebulae. Some people add double stars or variable stars too, and I won't argue, but for the sake of brevity I will not discuss those objects here.

What about the "observing"? I should make clear that I only mean visual observing. I have little knowledge of photography or electronic imaging, and that's not what I like to do.

People who do the kind of observing I am talking about are not really doing science. Many deep-sky observers would recognize something of scientific interest if they saw it, and would be able to contact scientists who might want to study it further, but that's not why most of us are there. If galaxies were birds, then what we do would be called bird watching, not ornithology. "Cosmic bird watching" might be a good catch-phrase for what deep-sky observing is all about.

Why Observe the Deep Sky?

The only real answer to that question is "Because!". This is a hobby -- who needs explanations? Some people do cosmic bird-watching because the birds are pretty, some do it to see lots of different species, some like to study a few favorites in detail, and some like to handicap themselves deliberately in some way, such as by restricting telescope size or observing time, to make things more fun and more challenging.

Deep-sky observing has different aesthetics and challenges than lunar or planetary work, though it is hard to describe just what the differences are. They mostly stem from deep-sky objects being generally much fainter than the Moon or most of the planets. Lunar and planetary work usually involves looking for details that are hard to see, on objects that are easy to see. Deep-sky work often involves trying to detect objects that are hard to see in the best of circumstances.

Some objects are almost impossible to see. We use terms like "faint fuzzy nothing" and "lumpy darkness" to describe them. We make obscure references to the "elusive Elvis nebula". (Some people think that the "King rate" selection on a sidereal drive is for tracking the Elvis nebula.) The general consensus is that a good deep-sky observer would make a fantastic spiritualist medium -- once you've found some of the stuff we look at, seeing ghosts is easy.

There are even some pragmatic reasons why an amateur astronomer might pick deep-sky work over other sides of the hobby. For example, deep-sky observing does not require perfect seeing -- the air does not have to be perfectly steady to do it. It does not require as large and as perfect a telescope to look for deep-sky objects as to chase down fine and subtle lunar or planetary detail. There are lots more deep-sky objects than there are large moons and planets in the solar system, too, and they are available year 'round. For example, there are often periods of several months when no bright planets are well placed in the evening sky. Things like that don't happen with deep-sky objects.

How Hard is Deep-Sky Observing?

The perception of how difficult it is to detect various deep-sky objects has changed dramatically over the years, mostly in the direction of "it's not as hard as they say." Some of this change is because equipment has gotten better and cheaper over the last several decades. High-quality telescopes and binoculars, and in particular, large telescopes, are much more common now than they were a generation ago, and cost much less in comparison with other consumer goods.

Yet not all of that change has to do with hardware. For example, a generation ago, the Pelican Nebula, located east of Deneb and just south of the brighter and better-known North American Nebula, was considered by many to be unobservable visually, with any equipment. These days, it's not uncommon for observers at dark sites to report it with the naked eye. Today's eyes aren't any better than eyes a generation ago. Today's skies aren't any darker, rather the reverse! Thus at least some of the increased ease of deep-sky observation stems from increases and improvements in observing skills, and from those skills becoming more widely known.

Let me illustrate the benefits of observing skill and experience. At a star party I once attended, a relatively inexperienced observer had bought a 15- or 18-inch Dobson, and was looking for the Horsehead Nebula. It's pretty faint, and he was having trouble locating it. I know the field for the Horsehead, so I offered to help. Yet when I stepped to the eyepiece, what do you know? He had it centered! The object he was looking for was right square in the middle of the field of view, but he couldn't see it!

Now, the reason I could and he couldn't was surely not that I have superior vision: I know that I do not, because I have compared notes with many other experienced observers, and we can all see about the same things in similar circumstances. But I do have a lot of experience, and I have deliberately worked hard to try to develop my observing skills. In my opinion, that made all the difference. I hope you will excuse my talking about it, because I am not really boasting -- there's nothing I have learned to do that you can't learn to do just as well, and perhaps knowing of my experiences will make you decide to try.

Let me give another example. At another star party, the owner of an eighteen-inch Dobson had chased down NGC 6822 -- Barnard's Galaxy -- which is fairly large in angular size, but has a relatively low surface brightness. It is fairly difficult to see, and the owner of the big Dobson was legitimately proud to have located it. I asked if he would like to see it in my telescope, and led him over to the 90 mm refractor that I had set up on the other side of the observing area. At first he couldn't see a thing, but after I reminded him about averted vision, shielding the eyes from stray light, and a few other tricks, he did indeed see the galaxy in the smaller telescope.

Think of that: Knowing how to see, allowed users of my tiny telescope to detect something that other people were finding difficult with five times as much aperture. That's a big difference.

Actually, Barnard's galaxy was discovered -- by Barnard -- using five-inch refractor visually, and of course, Barnard didn't already know it was there. Yet he did know how to see, and was therefore able to discover it with an aperture that many people consider too small to see it with, even when you know where to look.

Knowing just where to look for an object does make a lot of difference: With that advantage, I have seen Barnard's galaxy with a 7x50 binocular. I didn't tell that to my friend with the 18-inch Dobson, though: He was bigger than I was.

Anyhow, I hope it won't surprise you, that in my opinion, the most important part of my presentation will be about observing skills. Yet there are a few other things I want to talk about first.

How Fancy is the Required Equipment?

Although perhaps forty Messier objects are visible to the naked eye, and several score other deep-sky objects as well, nevertheless, I do recommend that you use a telescope or a binocular for deep-sky observing. To help decide what equipment to use, I am going to suggest three rules. I will give the first two immediately, and save the third for later. The first two rules may seem contradictory, so I will talk about them at some length. But first, let me state them:

RULE ONE: APERTURE WINS.

RULE TWO: You don't need large aperture to do deep-sky observing.

Let me restate rule one: APERTURE WINS, and it wins big. There is no substitute for large aperture, for visual observation of deep-sky objects. With rare exceptions, mostly for objects that are large in angular size, a telescope which collects a large beam of light will show a better and more easily seen view of deep-sky objects, than will a smaller instrument. There are a lot of manufacturers of small telescopes -- sometimes very expensive small telescopes -- who would like you not to believe it, but in my opinion they are all wrong. An excellent small telescope is -- at best -- a small telescope still. I repeat: APERTURE WINS.

On the other hand, just because aperture wins doesn't necessarily mean that the biggest telescope is the best one to get. There are many reasons why. Lots of aperture costs lots of money, perhaps more than you can afford, particularly if you are just starting out in astronomy and aren't sure how much you are going to like it. Possibly you do not have enough space to store a big telescope, or a car large enough to haul it to an observing site, or time, strength, and energy to set it up and take it apart when you are done. Maybe you like using a little telescope because it is more convenient. Or maybe you prefer a small telescope just because the small size makes it more fun and challenging: After all, some folks who go fishing take great delight in landing humungeous fish with light tackle -- why shouldn't observing be like that?

These remarks lead to rule two: You don't need large aperture to do deep-sky observing. Again, I hope you won't mind if I draw examples from my own experience.

One thing I like to do to familiarize myself with a new telescope or binocular, is to do a Messier survey with it -- to go through all of the deep-sky objects discovered by the great French comet-hunter, Charles Messier, and his close colleagues. I have done that twenty times so far, with twenty different instruments. Three had apertures of 50 mm -- 7x50 and 10x50 binoculars, and a Meade 50 mm refractor. The first Messier survey I ever performed, when I was a quite inexperienced observer, was with the 7x50 binocular, and I did indeed find all 109 Messier objects. That proves it doesn't require either great aperture or great experience to do it. On the other hand, it did take persistence, dark sky, and a whole lot of tricks.

One instrument I have used a lot lately is an exquisite small refractor, made by Vixen, whose objective is only 55 mm in diameter. I did a Messier survey with it, too, and I have also finished another survey of fainter objects -- the Astronomical League's Herschel-400 list. That is a selection of 400 of the brighter of the 2500 or so objects discovered by the great observer, William Herschel. Some were very difficult, but I managed to find them all.

I have an internet friend in her late teens. She was given a 50 mm Jason refractor a few years ago -- one of the so-called "drug-store junk refractors", that are quite properly shunned by serious amateur astronomers. She was a complete beginner, and the 50 mm Jason was her first telescope. Nonetheless, she complete a Messier survey with it, as her first observing project.

Yet there are many experienced amateurs who think it takes a six- or eight-inch telescope to see all of the Messier objects, and one half again or twice as large to tackle the Herschel 400. I hope I have convinced you that isn't so. Now, people who observe in light polluted conditions may need a larger telescope to do deep-sky work -- though my Messier survey with my 55 mm refractor was done entirely from my yard in suburban Palo Alto. Yet for the most part, folks who complain that they can't see deep-sky objects with a telescope six inches or more in aperture, are not complaining about the telescope, they are complaining about their own lack of ability, or rather, their own lack of will to sit down and learn how to see all the things their telescope can show them.

And that brings me to one area in which small telescopes do not fail in comparison to larger ones. That is the third rule:

RULE THREE: You can develop observing skills just as well with a small telescope as with a large one.

Rule three does not provide an escape from rule one -- APERTURE WINS -- but it does provide you with something interesting and challenging to do with a small telescope. As I have already discussed, there are plenty of things to look at with such an instrument, and rule three means that while you are looking, you can learn and practice skills that will transfer to larger equipment when you get to use it.

The point of rule three is that it takes just as much effort, and just as many tricks, to see a 12th magnitude galaxy with a two-inch telescope, as it does to see a 15th magnitude one with an eight inch. All that changes is the scale of things -- the eight inch might just as well be looking at the same object as the two inch, only four times as far off. If you develop the skills to see 12th or 13th magnitude objects with a two-inch telescope, then when you get hold of an eight inch, you will be able to apply those skills at once, to work to 15th or 16th magnitude on the same kind of objects.

Specifically, What Equipment Does It Take?

In case I already haven't said so, APERTURE WINS, so if you are going out to buy a telescope for deep-sky work, get the largest one you can afford, that you will actually use. By that last, I mean, don't get one too cumbersome to store, haul around, and set up.

For a couple of decades, the cheapest large-aperture telescopes have been Newtonian reflectors mounted in some variant of the kind of altazimuth mounting pioneered by John Dobson. There are at least four commercial manufacturers of Dobson-mounted Newtonians with apertures as small as six inches, and even more who make larger units. People who don't already have a telescope often find a six- or eight-inch Dobson a good first choice. One of those is small enough to fit in most automobiles, and light and compact enough for almost anyone to set up.

The four commercial manufacturers I had in mind are Celestron, Meade, Murnaghan (who bought the old "Coulter" brand name), and Orion. As far as I can tell, none of their Dobson telescopes are truly first rate, but the prices are low enough that they are all good values. I don't think it makes much difference which brand you buy.

There are several sources of used telescopes that you may want to consider. Many clubs run ads for used instruments available locally, in their newsletters and bulletins. If you have Internet access, there is a web site devoted to classified advertising for used equipment; namely, http://www.astromart.com. There are some others, too, and incidentally, most of the major telescope manufacturers have web sites.

Consider making a telescope. Newtonian mirrors are not that hard to make, or you can buy finished optics from several sources. The mechanical parts of Dobson telescopes are very simple. Commercial units are inexpensive enough that making one may not save a lot of money, but it is fun, and you may well end up with a better telescope than if you bought one.

Yet I hope I have said enough so that if you already have some other kind or size of telescope, or want some other kind, you will know that you can do deep-sky work with it just fine. Even a small binocular will show hundreds of deep-sky objects, and will let you develop your observing skills. Many people already have a binocular lying around the house, or can mooch one from family or friends for a while.

Binoculars are actually rather complicated instruments for the astronomical performance they deliver. Large ones -- say 100 mm aperture and up -- tend to be pretty expensive. I think that large binoculars are poor values as astronomical instruments, though I hasten to say that I'm not telling you not to get one if you happen to like the views. Yet smaller binoculars are mass-produced in enormous quantity, so the prices drop a great deal. You can buy a reasonable 7x50 or 10x50 binocular for a good deal less money than the cost of a decent beginner telescope. Furthermore, small binoculars are useful for other things than astronomy -- like sports, bird-watching, and spying on the neighbors. So small binoculars are reasonable choices for beginning astronomers on a low budget.

No matter how you obtain a telescope, you will need an assortment of accessories to use with it. Possibly the most important ones are a decent set of warm clothing -- particularly a good hat -- and a nice thermos bottle for your favorite hot beverage. But there are some more conventional ones that require thought, notably eyepieces, finders, and star atlases.

Many people assert that the eyepiece is just as important as the objective when it comes to telescope performance, yet I think that statement is seriously misleading. A bad eyepiece will certainly ruin the view, but even so, no matter how good the eyepiece, it cannot increase the amount of light gathered or the amount of detail resolved by the objective. Also, decent eyepieces are not hard to manufacture. Thus I do not recommend making eyepieces a high-budget item, at least, not unless you have lots of extra money, or you already have as large a telescope as you can handle. Otherwise, if you have extra money, get a bigger telescope, because -- RULE ONE: APERTURE WINS.

Eventually, you will almost certainly want enough eyepieces to provide a nice series of magnifications, from very low to very high. Some magnifications are more useful than others, however. You should probably get those first.

I think everybody's eyepiece box should include a low-magnification, wide-field eyepiece, if only because you will need one to find objects in the first place. Such an eyepiece might give a magnification of 0.15 to 0.25 times the telescope clear aperture in millimeters. For an f/5 Dobson, that will mean a focal length of 20 to 30 millimeters. Some of the wide-field designs in this focal range have lenses big enough to warrant a two-inch barrel, too -- but don't get one unless you have a telescope with a two-inch focuser, otherwise the small focuser will block the light, and you will be wasting those big, expensive eyepiece lenses.

When I recommend eyepieces for general use, I generally next suggest one with a magnification about equal to the telescope aperture in millimeters -- that might be a 4 to 6 mm eyepiece for a typical Dobson. But that magnification is a bit much for most deep-sky work, though it is entirely appropriate for looking at globular clusters, or at many solar-system objects. The next eyepiece beyond the wide-field one, that I generally use for deep-sky observation, has a magnification of about 0.7 times the aperture in millimeters; in my experience, that is about right to show the cores of galaxies against the sky background.

Choosing additional eyepieces for deep-sky work also depends on what you want to look at. To resolve globular clusters, or to show small planetaries as non-stellar, you might want more than the telescope aperture in millimeters. It also depends on sky brightness -- with light pollution, the sky background at low magnification will be bright enough to make contrast poor and wash out images. In such circumstances, you may not use your low-magnification eyepiece for anything but finding objects -- but you will need it badly then, for the same sky brightness will make your finder less useful than at a darker site.

That brings me to the subject of finders. It is important for deep-sky observers to be able to point their telescopes in the right direction, and finders are one of the most useful tools for doing that. I like the kind that magnifies, in contrast to unit-magnification, "reflex-sight", types, like the TelRad. I also like to look straight along the tube, with no prism or diagonal to change the direction of the light beam. That last is because it you can use a straight-through finder with both eyes open, and let your brain project the crosshair image, as seen through the finder, onto the sky. Thus you can use any magnifying finder as a reflex sight, too.

Many people prefer unit-magnification finders, though -- my dislike of them puts me in the minority. So if you have any doubt what is the right kind for you, try both before you make up your mind. But do learn how to use a straight-through magnifying finder as a reflex sight. That is a very useful trick.

Many people also like to use analog or digital setting circles, or a "go to" control interface, to find objects. I have generally found that I can find things about as fast using charts, finders, and the main telescope optics, as with either kind of setting circle, and often a lot more accurately. "Go to" control interfaces are quicker, but they are also rather expensive. I'd say that if you want to use any of these tools, go ahead, but I personally would not call them really necessary. The money you spend for them might better be spent on a larger telescope, because -- remember rule one -- APERTURE WINS.

Charts, and knowing how to use them, make a lot of difference. I use several kinds. First, when I am planning a night's observations, I often use a simple planisphere just to remind myself what constellations will be visible, and at what times.

Second, I make occasional use of an atlas whose individual charts show large areas of sky -- many constellations at once -- as an aid to orientation, for those embarrassing times when I am looking at a more detailed chart of a small part of the heavens, that contains no bright stars I recognize. The one I happen to pull out when I wondering "where is delta Sextans, anyway?", is usually an old Norton's Star Atlas, but many others will do.

For final homing in on an object, I like to use charts that show stars approximately as faint as I can see in my telescopes' finders -- it is much easier to interpret between sky and chart when exactly the same stars are visible on both. At the moment, my favorite atlases for this purpose are the Uranometria 2000.0, with a stellar magnitude limit of about 9.5, and the new Millennium Star Atlas, which has a fainter limit, but with inadequate coverage of objects from the NGC and IC catalogs. Those catalogs have inconsistencies and irregularities in coverage which are reasonable cause for celestial cartographers to shy away from them, but the NGC and IC are likely to be with us for a long time to come, so I think the creators of Millennium Star Atlas should have taken more care to deal with them.

A lot of people prefer atlases that don't have quite as faint a stellar magnitude limit as Uranometria. The best-known of these is the Tirion Sky Atlas 2000. I don't like these atlases -- there aren't enough stars for me to make good use of my finder. They make good place mats, though, for those times when you want to use the flat surfaces of your Dobson as a picnic table.

Computer planetarium programs can do all the things for you that charts can, but it is sometimes awkward to work with computers in the field. To be fair, I should mention that I am rather more down on computer assistance than are many of my fellow observers, simply because I write computer programs for a living, and far too many of them. I tend to look for hobbies that don't have anything to do with electrons.

I should perhaps mention one other accessory: People sometimes ask about light-pollution filters. There are several kinds, and they do work, though they work better for nebulae than for star clusters and galaxies. I have only encountered two or three objects which I could see with such a filter, that I could not see when I removed the filter, so I do not consider them really necessary, but if you observe in light-polluted conditions, you might try one or two, and decide whether they are worth it for you.

What are Good Observing Sites?

The kind of stuff you can see depends on where you observe. The basic requirement for an excellent deep-sky site is that the sky be dark and clear, with the emphasis on dark. For most of us, that means a drive of several hours to get to where we will set up. It takes local knowledge to find these sites, but it is worth looking for them.

Yet decent deep-sky work can be done when the sky is brighter, at least on objects with high surface brightnesses. Globular clusters and open clusters can often be observed well from within cities, even when the Moon is up. A little more darkness permits looking at galaxies with bright central regions. I don't give up too much of my telescopes' capabilities when I observe these objects at magnifications of 0.7 times the telescope aperture in millimeters, from my yard in Palo Alto. The main problem in my yard is finding a spot sheltered from direct illumination by street lights.

What are Some Good Programs and Observing Lists?

You don't have to have a program or an observing list to use a telescope, but some people feel silly setting up and then spending most of their time wondering what to look at next. For these folks, a little planning may help.

For beginners, a relatively easy, short observing program is to observe all the Messier objects. By the time you have done that, you will likely know what kinds of things you particularly like to look at, and can seek out specialized sources which list them. The Royal Astronomical Society of Canada's Observer's Handbook has a number of lists of deep-sky objects in its back pages, grouped both by type of object and by degree of difficulty. Sky Catalog 2000.0 has more comprehensive lists, by type. And there are enormous numbers of specialized catalogs and observing lists on the world-wide web.

One list to be aware of is the one of all 2500-odd deep-sky objects discovered by William Herschel. It is available on the web, and perhaps in other places. The interest here, besides its size, is that it was made by visual observation, by one observer, who for the most part used a telescope not much more powerful than the ones we use today -- an 18-inch reflector with speculum-metal mirrors. All the objects on this list are within range of amateur telescopes, and you can get most of them with a six inch.

Burnham's Celestial Handbook is a three-volume observer's guide to the deep sky. Some of the scientific information in it is getting out of date, but it still has excellent lists of objects to observe, grouped by constellation, and it includes the southern hemisphere as well as the northern. The faintest stuff in Burnham is generally not as faint as on the big Herschel-2500 list. Burnham includes only about half of what Herschel found.

What skills are useful, and how do you develop them?

Even some experienced amateur astronomers think seeing things comes free and easy, with no more effort than opening your eyes: But as current popular slang so evocatively articulates,