Thinking of going big? Of course you are, and right you should be. Nothing makes up for aperture under dark skies if it’s deep sky objects you’re after. Some may make an argument that refractors show slightly sharper planetary images, but simple physics says the more light you gather (aka, the bigger the mirror), the brighter the image will be. So, how bright and how big is big enough? Let’s take a look at some practical considerations.
Questions to consider before making a final decision on scope size include: What do I most enjoy viewing? Do I observe more at, or away from, my home? How much weight can I comfortably lift? What eyepieces do I currently use? Can I locate deep sky objects by reading a map, or do I depend on computers to point me where I need to go? And finally, do I mind having lines of people waiting to look through my scope, or would I rather observe alone?
The Basics
If you’re a deep sky aficionado, then a big scope will reveal more detail on the faint fuzzies, period. A scope’s light gathering capability is determined by the size of its primary mirror in the case of reflecting telescopes, or its primary lens in the case of refracting telescopes. And you don’t have to double in size to double in light gathering capability. Remember, the area of a circle is π (pi) multiplied by the square of its radius (πr2). With that in mind, here’s a quick reference table of increased light gathering with a number of mirror sizes, each compared to a 4″ telescope:
*Limiting Magnitude – This estimate is based on good seeing, magnitude 6 skies, a 6mm dilated pupil, and 40x per inch of aperture. 40x per inch of aperture requires a well-figured primary mirror. For more information on limiting magnitude, see www.cruxis.com/scope/limitingmagnitude.htm
So, What Does That Mean At The Eyepiece?
The first step is to understand the above table, yet that alone doesn’t tell the whole story. Low contrast objects require not only dark skies and decent transparency, but also aperture. Think about M51, the Whirlpool Galaxy. Under dark skies and pristine conditions, an 8″ telescope will reveal a hint of the spiral arms with averted vision and high scrutiny. Through a 12″ under the same conditions, the arms are easy with direct vision. Through a 16″, knots in the arms can be made out. Through a 20″, the knots are much brighter and M51 begins to look like a black and white photo. Through a 24″, it’s possible to begin to make out faint coloring in the spiral arms, and the core of the galaxy is so bright, one wonders if it’s going to ruin their night vision!
Nebulae, globular clusters and any of the 109 Messier objects are perfect targets for large telescopes. I have found that a 12″ delivers color on the brightest of nebulae, and the color gets easier to see and more vibrant as the telescope size goes up. On globular clusters, an 8″ will resolve M13 and M3, while a 12″ will resolve most of the rest of the Messiers. With a 16″, all of the Messier globs are easily resolved, as well as many of the NGC’s. With a 20+”, you start loosing count of resolved globs!
Planets
Who can resist a peak at Saturn or Jupiter? Well, once again, aperture rules.
As a rule, as the primary mirror increases in size, the ability to discern detail increases. To fully recognize the potential of the large scope, a finely figured primary mirror is necessary. A great amount of discussion has occurred regarding smaller refractors and their reputation to outperform larger Newtonians. This mustn’t always be the case, however, and it would be a serious error to believe the superior view through a refractor is constant, impervious to variables in design, optics and weather. Those in the pro-refractor camp often claim their allegiance is due to the inherent design inferiority of a Newtonian. Nothing could be further from the truth.
A large mirror, such as is found in some Newtonians, must not only be properly supported from underneath, but also on its edge as it is being tilted within the telescope. Many a Newtonian builder neglects to provide the appropriate support. A consequence of an improperly designed mirror cell or edge support system will be any of several detail and contrast robbing aberrations, most notably different orders of spherical aberration and astigmatism.
An important aspect of large aperture Dobsonians (Dobs) is that the larger primary mirror requires far more time to cool down than a smaller refractor. Most of the older large Newtonians out there compound this because it was once thought that the mirror had to be relatively thick, otherwise aberrations would be introduced by the mirror cell (we now have finite computer analysis programs that will plot a perfect mirror cell of any size – most specifically David Lewis’ PLOP program). Thanks to the research of Bryan Greer (research article published in the May and June issue of Sky and Telescope) and others, we now have a better understanding of the ways larger optics shed heat. One of the more straightforward discoveries of this research was that the reason larger mirrors take so long to cool is mostly due to their thickness and not overall diameter. So if we choose the thinner mirror for faster cool down, we again shift our attention to the mirror cell. A thin mirror that is not supported properly from underneath will cause a slight deformation in the surface figure, which in turn causes light rays reflecting off the mirror’s surface to not come into focus at a single point. A star test would then readily reveal different orders of spherical aberration, degrading the view at the eyepiece. Now consider the mirror’s edge support. A sling is historically used to support the edge of large primary mirrors, often made out of Kevlar or metal banding. Through the work of Nils Olif Carlin (www.cruxis.com), we now understand that as much care should be given to choosing the proper edge support as goes into the design of the rest of the mirror cell. If this part of the mirror cell is neglected, you once again will experience different optical aberrations at the eyepiece as the telescope is moved from horizon to zenith.
Another point to consider is that bad atmospheric seeing can cause one to believe that a large telescope is performing poorly on the planets. It is true that a larger mirror will seemingly amplify poor seeing conditions, but patience at the eyepiece (waiting for the seeing to settle momentarily and for the planetary image to “pop”) will once again prove the larger mirror to outperform the smaller one.
So, let us review: A big telescope with a thin mirror, excellent mirror cell and edge support, built with an active cooling system (fans to provide air motion within the mirror box) and a night of good seeing – Viola! It’s a recipe for a night of planetary viewing that will leave you and other observers arguing about the spokes in Saturn’s rings!
Portability
I often hear of an amateur astronomer selling his scope because it’s just too much of a hassle to get out and observe with. The size and weight limit varies from astronomer to astronomer, so observers must carefully consider for themselves what may be too heavy or too much hassle to result in pleasurable observing.
An 8″ is usually considered the “biggest of the small”, while a 12″ is often referred to as the “smallest of the big.” I agree with this sentiment. An 8″ – 12″ tube-style Dobsonian is a one-person job and both easily fit in a mid-sized sedan, but the 12″ may push the weight limits of some. The 8″ scopes on the market today are around 65 lbs fully assembled, while the 12″ telescopes weigh in around 100 lbs. If you plan to use an equatorial mount, make sure to factor in an additional 30 lbs or so above the overall weight (and prepare to spend an extra 20 minutes or so setting up). Forget about 14″ – 20″ tube-style telescopes – portability is key and unless you have a small observatory, an equatorial mount is probably not feasible due to the sheer size and weight it encompasses.
Truss-style telescope weights vary significantly from vendor to vendor. One telescope I can be sure of knowing the weights of is one that I build, a New Moon Telescope. A fully assembled 16″ NMT is just under 100 lbs, the heaviest component you would lift weighing in at 60 lbs, and the collapsed scope readily fits in the same mid-sized sedan that would cart a smaller tube-style scope around. A 20″ is 134 lbs, the heaviest component weighing slightly over 80 lbs, and this is the size at which to start relying on detachable wheelbarrow handles to maneuver it. A 24″ would weigh roughly 165 lbs and a 27″ almost 200 lbs. When going this big, remember to reflect on what type of SUV, truck, or trailer you might like to own, because car-hauling is doubtful. Any of the scopes through a 20″ can be stored in a bedroom or living area (and the 8″ and 12″ even in a closet), fully assembled, should you choose to showcase them as pieces of furniture. From the 20″ and up, consider utilizing a storage shed, garage, or an observatory (should you be so fortunate!). Keep in mind, telescopes 20″ and larger necessitate a large car in which to travel, or ideally a truck, trailer or SUV, so if you’re an apartment dweller with no access to a storage unit, you’ll want to stick on the small side. Likewise, if you have your own observatory in your backyard, the sky is the limit on the size of scope you could choose, as portability will not be a factor.
Eyepiece Preference
One factor that may be overlooked when considering the purchase of a new telescope is the choice of eyepieces. The longer the overall focal length of the scope, the smaller the field of view (and so the higher the magnification), so the limited field of view of Plossl eyepieces quickly become frustrating when you start using telescopes in the 20″ range. Another factor about your eyepiece collection is the capability of the eyepiece for correcting coma. Coma is an aberration you get with any Newtonian, in which the stars in the eyepiece start looking like tadpoles as they near the edge of the field of view. Everyone seems to have a different tolerance level of coma, but there are ways to correct for it. The easiest – buy all high-end eyepieces. TeleVue, Pentax, Explore Scientific, and a few others are building eyepieces that contain coma-correcting elements (and of course FAR wider fields of view than the typical Plossl) and these usually perform well down to a focal ratio of F4.5. Faster than F4.5, you may need to invest in a specific coma-correcting eyepiece such as TeleVue’s Paracorr (I cannot recommend these enough). All of this being said, you could observe happily for the rest of your life with three high end eyepieces and a barlow lens with as large a telescope as you wish to endeavor (my opinion only of course!).
Further Considerations
Familiarity with the skies will also likely determine the size of the scope to purchase (or build, of course). The obvious determining factor here is cost. If you are brand new to astronomy and can’t tell the difference between Cygnus and Sagittarius, you should probably wait to invest in a $15,000.00 set-up, even if you can afford it now. A modest familiarity of the sky is needed when using any telescope, and wisdom has shown that beginners typically have an easier time with a simple pair of binoculars or a small telescope. In fact, many of the 8″ tube-style telescopes on the market right now are under $400, and perfect for a beginner. Purchasing a telescope like this will give you the time under the stars you need to learn the constellations and familiarize yourself with pointing, moving and using a telescope. If you are a more advanced amateur, however, bigger scopes and better optics start to make more sense. You have probably amassed a few decent eyepieces and know your way around the sky well enough to invest in a larger scope that will open the skies to you exponentially. Even if you aren’t a star hopping pro yet, there are digital encoders available and GOTO capabilities that can be added to even the largest of telescopes. Think of it this way: with a good NGC and IC map (or encoders), you could go to a dark site every night for the next 20 years and upon each visit discover a new deep sky object! And with some of the more obscure objects, you may be one of only a handful of people that have EVER seen said object through a telescope! And remember – the bigger the mirror, the brighter and more picturesque the object is going to look. And of course there’s always that chance of discovering a comet…
Finally, big scopes draw crowds, and crowds are the future of amateur astronomy. If you can point at a few nebulae, open clusters, or galaxies and give a 60 second presentation on what you are looking at, you will quite possibly change the lives and perspectives of countless people. So that’s my final “big scope” pitch: Big scopes change lives!
