CNYO Feature: Going Big

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:

Mirror Size
Increase in light gathering over a 4″ mirror
Limiting Magnitude*
8″
4x
14.7
10″
6x
15.2
12″
9x
15.6
16″
16x
16.3
20″
25x
16.7
24″
36x
17.1

*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!

2013june25_ryangoodson_bioRyan Goodson is the owner of New Moon Telescopes (www.newmoontelescopes.com), manufacturer of custom Dobsonian telescopes. He is a member of several CNY astronomy clubs, hosts observing sessions from his dark skies in West Monroe, NY, and lectures regionally on telescope building. He can be reached at ryan@newmoontelescopes.com.

CNYO Observing Log: ShoppingTown Mall, 19 June 2013

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Greetings fellow astrophiles!

From the CNYO Facebook Group page on 17 June 2013:

Damian and I [Larry S.] have been talking about doing another impromptu observing session. We had some really good turn out for a Solar/Lunar observing session in the Shoppingtown upper parking lot. Wednesday’s forecast is looking promising. Anyone interested in doing another Solar/Lunar session at Shoppingtown at 6pm on Wednesday? I’m choosing Shoppingtown again, because I have a CNY Skeptics in the Pub meeting at Scotch and Sirloin at 7pm. Any one interested in joining us for a drink after is also welcome.


View Larger Map

The approximate location (at center) of the session.

CNYO hosted a half-dozen observers (and a half-dozen or so other stopper-by’s) at its second facebook-organized combined Solar/Lunar Observing Session in the parking lot of ShoppingTown Mall on Wednesday, 19 June 2013, just prior to the bimonthly CNY Skeptics In The Pub meeting at the Scotch & Sirloin.

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In attendance were Larry Slosberg with both his NMT 12” Dobsonian (and my custom Baader solar filter) and his Meade SCT 8”, myself with Baader-equipped Zhumell 25×100’s, and John Giroux with his Coronado Solarmax 60 II (which obviated the need for me to bring my Coronado PST, providing a low-magnification Baader view for onlookers instead through the binos). This event also featured the first official use of our CNYO Solar Observing brochure, which we will continue to update and have available at all of our Solar observing sessions (download the PDF for yourself at its CNYO post).

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The 11 day old waxing gibbous Moon hung quite pale blue but feature-rich through Larry’s 8” SCT. Outside of discussion with attendees, all attention was placed on the Sun, which was busy with several sunspots and prominences, include Sunspot 1772, which featured a surface prominence easily visible in John’s Solarmax. A gif of the 5 days prior and 5 following days is shown below from NASA SOHO images (the image for the 19th is in yellow).

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From NASA/SOHO images. Click for a full-sized version.

The Solar/Lunar Sessions are a perfect combination of interesting (and important!) objects and family-friendly observing times, making them one event we plan on committing to a more regular schedule this summer (with new potential locations under discussion). We will keep you posted on this website. Stay tuned!

CNYO Observing Log: Baltimore Woods, 14 June 2013

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Perhaps the last bug-free (of the buzzing kind, that is) observing session until later in the fall, Bob Piekiel hosted his monthly Baltimore Woods observing session on Friday, 14 June 2013 at the same time that the Syracuse Astronomical Society hosted a public viewing session at Darling Hill. An excellent Friday for taking in the CNY skies beyond the Syracuse skyline!

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Bob Piekiel performing collimation surgery.

Scope-wielding CNYO members in attendance included Bob Piekiel with 8″ and 11″ SCTs, Larry Slosberg with his trusty New Moon Telescopes 12″ Dobsonian, and myself with my 25×100 Zhumell binoculars (despite a last-minute re-collimation surgery, Bob couldn’t get his monster pair of Vixen 25×125 binos up to his satisfaction. I await the next session to take in that view!).

Beyond the several deeper sky objects we observed that evening, attendees were treated to (1) two ISS flybys (and the rather spectacular run of ISS flybys this month will be the part of an upcoming article), (2) four bright meteors (total count from among the observing group) that all appeared to radiate from the vicinity of Libra and Virgo (so the meteors all appeared to move from the SouthWest to the NorthEast), and (3) more than 20 satellites that crossed fields of view or were prominent enough during the ISS watch to jump out to most everyone. The sky was dominated by the 6 day old waxing crescent *always super* Moon, excellent at all magnifications and the best binocular object in our Night Sky.

As has been a recurring theme in some of these observing logs, the variety of available optics gives one the ability to experience the pros and cons of different equipment first-hand both in terms of setup and magnification.

The Zhumell big binos are easy to setup and provide excellent views of the Moon and some of the very open star clusters. We used the binos as a “sneak preview” of our observing of Saturn as we waited for enough bright stars to appear for Bob to get his GOTO scopes aligned. Even in the binos, Saturn is obviously “Saturn,” with the ring system, planet, and gap between the rings and planet visible. You will fight for the Cassini Division in less transparent skies, but a steady mount and patience will grant you a peak of this dark band between the two major rings.

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Larry S. and Michelle M. in a Dob setup action shot.

Larry’s NMT Dob is a 5-minute setup from car to observing, with only a modicum of labored transport from the parking lot to the ground. In moonlit skies, nebula, galaxies, and faint fuzzy objects require a good memory or a great trust in one’s Telrad. On the minus side, you spend extra time trying to find faint objects – this only being a real problem during Public Viewing sessions when you really don’t want to spend all your time finding “something” to see. On the plus side, you really learn the sky this way, you don’t have to worry about battery life and any of the problems that come from modern observing technology, and you can generally get a larger mirror for the same amount of carrying-intensive weight in a Dob over a Schmidt-Cassegrain Telescope (SCT), which means a generally better view.

Bob’s two SCT GOTO scopes are, after setup, perhaps the best way to facilitate a Public Viewing session, as you simply call for the object you want to see in the GOTO controller and, with some grinding of gears, you spare yourself from the hunt. In the case of his image intensifier, your GOTO scope might land on a galaxy too faint to even acknowledge seeing through a good eyepiece. Your GOTO purchase is validated when the intensifier then brings out subtle detail in an otherwise invisible object, a search that might easily aggravate a novice Dob user.

All of that said, we remind ourselves of the words of Stu Forster – “The best scope is the one you use.”

For my part, I spent time Naked Eye observing and adding to discussions going on around scopes. The confirmed sightings list contained Saturn, M3, M13, and M57 (the Ring Nebula, both with and without enhancement).

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15 seconds of fun with a red flashlight and green laser pointer.

Increasing dew around 10:30 p.m. left us to pack up gear and simply enjoy the naked eye Night Sky (and play with the long exposure setting on my Canon Digital Elph). The next even is scheduled for July 12/13 at Baltimore Woods, followed by Bob Piekiel’s “Star Search” event at Green Lakes on July 26th. We hope you can join us!