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Gordon S.
member
Reged: 08/23/08
Posts: 11
Loc: Brevard County, Florida
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Whenever someone who is not into astronomy finds out I have a telescope, they invariably ask me, "How big is it?" I find this question tricky to answer. What do they mean by that? Are they asking so they can visualize in their minds the physical scale of the instrument or are they simply asking me how good the views are? For that matter, how good compared to what - to the 60mm Tasco they had as a kid or to the Hubble?
Now, we all know that the size of a telescope is generally rated according to a couple of different measuring scales. The first one is aperture, which correlates highly with the quality of views the telescope provides. The other scale is what I call the fuss factor, which I'll loosely define here as a group of parameters concerned with such things as bulk and weight, ease of transport, setup time and bother, and perhaps even focuser height at zenith. Along with aperture, each of these parameters contribute to an equation that determines whether a given instrument is, in broad categories, small, medium, or large. There are no absolute lines in the sand to be drawn here. It's more about current sensibilities and consensus.
The Kriege & Berry book on telescope building offers a couple of attempts to give its readers some sense of what it means to live with and use Dobsonians from various aperture categories, but those assessments are about ten years old now and warrant a fresh look. A lot has changed in a decade. Both the ATM community and the pro vendor community have matured in knowledge and sophistication. So I wonder how our perceptions of size and of what we expect from our equipment have morphed in the last ten or so years.
I'd like to know where you would draw the class division lines and why. What are the natural thresholds and plateaus in each of the two measurement scales and what causes them? In terms of both aperture and fuss factor, where do the various sweet spots reside and what defines them?
To get things started, here's my thumbnail estimation of the basic size categories for today's Dobsonian reflectors. It's based on what I've seen recently in the field. I'm keeping it simple and broad here, understanding that many other scope designs exist and that further refinements are possible. Perhaps later, someone else will address this for other mount types and reflector designs, too. Anyway, here goes...
Dob Size Categories Based On Aperture:
==============================
4.5" - 8": SMALL - Good for "basic" viewing of planets and brighter DSOs. Many common globular clusters and galaxies are often merely detected, not showing much detail. Challenge objects like the horse head nebula are very tough to see, even in dark sky and with proper band pass filtration.
10" - 16": MEDIUM - Excellent resolution on planets. Globular clusters begin to resolve to individual stars. Deep space probing becomes easier. Viewing the horse head becomes doable in reasonably dark sky. Also, mass produced imported mirrors currently top out at 16 inches, which by virtue of the large number sold may create a natural line of distinction.
17.5" - 25": LARGE - To varying degrees, this class of aperture provides all the high quality views most of us will ever want. Color in some nebulas becomes apparent to many.
28" & up: VERY LARGE - Reserved for the very serious observer with an insatiable appetite for best-in-show views, these apertures push the limit of what can be seen visually with even remotely portable gear. Color is easily seen in many objects.
Dob Size Categories Based On Fuss Factor:
================================
4.5" - 10": SMALL - Easily transported and set up quickly by one person. Fits on the back seat. Tubes are not too cumbersome to bear hug. Seated or flat footed viewing for most adults.
12" - 18": MEDIUM - Heavy, but still fits through any door. Better in a collapsible design. Still considered a one-person setup. Depending on focal ratio, a block or short step stool is sometimes needed for viewing at zenith.
20" - 30": LARGE - Requires extra planning for storage and transport. Very heavy. Setup still manageable by one physically fit adult, but help is useful. Ladders are an integral part of the viewing experience. Thermal equilibration is a constant challenge.
32" & up: VERY LARGE - Extremely heavy. Big vehicle or small trailer required. Dedicated moving apparatus required. Setup help required. Stable ladder required. Head examination optional. Full thermal equilibration unlikely.
Okay, it’s your turn. I invite you to disagree with me and set me straight.
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Gordon Schafer
Obsession #361 18" f/4.5 - Pegasus, AN, FT
Discovery PDHQ 12.5" f/5 - DD-II
Orion ST-80 f/5
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Tom Clark
super member
Reged: 11/14/07
Posts: 144
Loc: Chiefland, Florida
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Hi Gordon and welcome to CN!
Very interesting and thought provoking thread. For the most part you are very accurate in your size definitions. I would make some minor adjustments, but they are negligible.
8"-10": SMALL beginners telescopes
12" - 18": MEDIUM starting to give good views
20" - 25": LARGE great observing
28" & up: only getting better
Of course I am a bit jaded, having been a visual observer for over 25 years, and having owned many of these size scopes. Most of our experience has been from attending over 40 major star parties all over the country, and observing through 1000s of scopes over the years.
I still have 8 and 12" travel scopes, use a 24" for traveling to star parties (the WSP this year), and have owned four scopes 32" or larger. If you want to SEE, aperture rules. For imaging, nothing I own can keep up with my buddy Charlie Warren's 60mm refractor. I do think that visual observing will be changing drastically in the next decade as video cameras make more headway. The future will be changing…
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John Fitzgerald
In Focus
Reged: 01/04/04
Posts: 1257
Loc: AR
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Here's my take:
I copied and edited your entries, hope you don't mind.
Dob Size Categories Based On Aperture:
==============================
4.5" - 6": SMALL - Good for "basic" viewing of planets and brighter DSOs. Many common globular clusters and galaxies are often merely detected, not showing much detail. Challenge objects like the horse head nebula are impossible to see, even in dark sky and with proper band pass filtration.
8" - 12.5": MEDIUM - Excellent resolution on planets. Globular clusters begin to resolve to individual stars. Deep space probing becomes easier. Viewing the horse head becomes doable in very dark sky under near perfect conditions.
13.1" - 18": LARGE - To varying degrees, this class of aperture provides all the high quality views most of us will ever want. Color in some nebulas may become apparent to good observers.
20" - 25": VERY LARGE - Reserved for the very serious observer with an appetite for spectacular views, these apertures push the limit of what can be seen visually with reasonably portable gear. Color is easily seen in many objects.
Over 25" - Not really portable, don't get used much unless already set up and accessible like in a garage or observatory. Give views exceeded only by professional instruments that are not used much visually.
Dob Size Categories Based On Fuss Factor:
================================
4.5" - 10": SMALL - Easily transported and set up quickly by one person. Fits on the back seat. Tubes are not too cumbersome to bear hug. Seated or flat footed viewing for most adults. I concur with you on this one.
12" - 18": MEDIUM - Heavy, but still fits through any door. Better in a collapsible design. Still considered a one-person setup. Depending on focal ratio, a block or short step stool is sometimes needed for viewing at zenith. In the 15 to 18" sizes, a truss design keeps them manageable, otherwise (in solid tube designs) they belong to the LARGE category.
20" - 25": LARGE - Requires extra planning for storage and transport. Very heavy. Setup still manageable by one physically fit adult, but help is useful. Ladders are an integral part of the viewing experience. Thermal equilibration is a constant challenge.
26" & up: VERY LARGE - Extremely heavy. Big vehicle or small trailer required. Dedicated moving apparatus required. Setup help required. Stable ladder required. Head examination optional. Full thermal equilibration unlikely. BIG fuss factor, unless used in an observatory or wheeled assembled out of a garage.
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?
Observing since 1966
Messier Cert #898
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Gordon S.
member
Reged: 08/23/08
Posts: 11
Loc: Brevard County, Florida
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Tom -
Believe it or not, I sort of had you in mind when I started this thread. Based on your obvious Guru status and skills as a PTM (Professional Telescope Maker) along with your unrivaled knowledge of, and attendance at all of this country's major star parties -- AND the fact that you personally went all the way, constructing for yourself an amazing 42 inch dob, I can think of no one more qualified to weigh in on this topic.
As you confess, you probably are a little jaded, and rightfully so. You may not have much use for the smaller end of the spectrum, but I know there are many experienced observers out there who actually prefer a smaller telescope to a big one.
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Gordon Schafer
Obsession #361 18" f/4.5 - Pegasus, AN, FT
Discovery PDHQ 12.5" f/5 - DD-II
Orion ST-80 f/5
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Lard Greystoke
sage
Reged: 07/27/08
Posts: 202
Loc: Ohio
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I have just returned to the hobby after 20 years absence. I purchased one of the first commercial Dobs to be offered back when they were viewed as a step backwards (no clock drive, no German equatorial). When I was getting started:
Small: refractors less than 4", reflectors less than 6".
Medium: 6 to 8" reflectors.
Large: 10 to 12" reflector (back then, usually not a Dobsonian).
Super large: anything over 12" was either very expensive or home-built. They existed, but were nowhere near as common or available as they are now.
Other changes:
1) the internet
2) eyepiece technology
So count your lucky stars to be around today.
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Lard Greystoke
10" Odyssey Compact
"With Tantor, the elephant, he made friends. How? Ask me not."
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Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 10960
Loc: Los Angeles
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Having owned 18 scopes and looked through hundreds, while observing for 45 years under dark skies, I would add to the above a classification based on what the scope will show under real dark skies:
Basic small scope for casual lunar and planetary use, with observations mostly of the Messier objects and a few hundred to a couple thousand of the brightest objects beyond that: 4-5.9"
Medium sized scope for examination of the brightest several thousand DSOs, with a real "lifetime" (you'll never see everything the scope can see) light grasp: 6-10"
Large scope wherein the number of DSOs is more than can be seen in a lifetime, and which produces photo-quality images at times: 11-16"
Very large scope for serious observations of details in the brightest 30,000 DSOs and detection of up to 100,000 DSOs and more: 17.5-25"
Obscenely large scope where galaxy clusters and ultra-faint planetaries become not only objects to view but display details. Too many DSOs visible to see more than a fraction in a whole lifetime: 27+"
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Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie
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jdownie
professor emeritus
Reged: 02/24/06
Posts: 737
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What possible use is it to assign arbitrary categories to size, when the aperture perfectly and unambiguously does this for us? If the perceptions of the listener do not agree with the person telling of the "medium" category, then there is uneccesary confusion. I call for sticking with the numbers.
John
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ATM project - a terrible waste of good Pyrex.
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Tom Clark
super member
Reged: 11/14/07
Posts: 144
Loc: Chiefland, Florida
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Quote:
What possible use is it to assign arbitrary categories to size, when the aperture perfectly and unambiguously does this for us? If the perceptions of the listener do not agree with the person telling of the "medium" category, then there is uneccesary confusion.
John
The reason most of us bother to read the forums is to learn more about our hobby, especially the newer ones to take up the hobby. And for the ones who have been around awhile, it's because we enjoy sharing information about what we have learned over the years.
The interesting part of this thread is that if one person thinks that a 10" is a 'medium' size telescope, and learns that others may think that a 'medium' size is an 18", he may wonder why, and then try to see if he is missing something.
The only thing important to anyone is what suits him and his own eye. I have always promoted attending star parties, to go where you have the opportunity to look through hundreds of telescopes, and to see what truly satisfies you! Go look through some larger instruments if you dare, and just maybe you may become a bit more like Dobson, and enjoy sharing telescopic sights with others.
I've met thousands of people who think people interested in visual astronomy are nutcases, since all they think you can see are "dim fuzzy objects." In reality, there is the whole universe that can be seen by those who take the time and effort to really look.
I still use my 8" and 12" scopes, but whenever the opportunity exists, they sit idle if a larger instrument is available. I wish everyone had the opportunity to spend an evening in my observatory with me. Then perhaps they would understand why some of us have been compelled to do what we do…
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rmollise
Carpal Tunnel
Reged: 07/06/07
Posts: 1662
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Quote:
Dob Size Categories Based On Aperture:
==============================
4.5" - 8": SMALL - Good for "basic" viewing of planets and brighter DSOs. Many common globular clusters and galaxies are often merely detected, not showing much detail. Challenge objects like the horse head nebula are very tough to see, even in dark sky and with proper band pass filtration.
Challenge objects like the Horsehead are hard to see under dark skies with filtration and a 24-inch. 
I find your classes pretty sensible, if there is a need for such things, but I part ways on your inclusion of an 8-inch with "small." It may be small compared to giant dobs, but its capabilities do not match your description.
If skies are even halfway decent, you will do far more than "detect" globulars. In fact, under fairly good skies, a 5-inch will at least begin to resolve the showpieces.
Aperture is great, sure, but the amateurs of my dad's generation did one hell of a lot, saw one hell of a lot, and thought a 6-inch was a hell of a big scope.
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Uncle Rod
Watch for Rod's New Book:
Choosing and Using a New CAT--coming in December!
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pennyandchris
professor emeritus
Reged: 01/30/07
Posts: 505
Loc: Horsham, England
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Quote:
What possible use is it to assign arbitrary categories to size, when the aperture perfectly and unambiguously does this for us? If the perceptions of the listener do not agree with the person telling of the "medium" category, then there is uneccesary confusion. I call for sticking with the numbers.
John
I agree about categorisation. However, the marrying of performance to 'fuss factor' for a range of apertures will probably be very helpful to beginners.
I've already decided to get an 'extremely large' (compared to my other 'scopes) 12-inch Dob if/when funds allow, based on the portabilty / performance tradeoff.
I'll wait 'til I can afford it, because I know that a 'medium' 8" won't give the wow factor and any sort of 'hyper large' 16"+ dob will be too big for me to move and set up easily.
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Orion UK OMC140 Mak Cass
TeleVue Ranger
Coronado Ha and CaK PSTs
Meade LXD75 mount
Ambermile alt-az mount on wooden surveyors tripod
Manfrotto 074 photo tripod with 501 head
+ various binocs
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Dick Jacobson
member
Reged: 12/22/06
Posts: 87
Loc: Plymouth, Minnesota, USA
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I divide Dobs into two categories: standing-on-the-ground-scopes and ladder-scopes. The aperture where standing-on-the-ground tops out depends on the focal ratio and design of the scope and the observer's height, but is typically around 12 to 16 inches. Usually a standing-on-the-ground-scope can be loaded into the vehicle without mechanical assistance, while ladder-scopes usually require wheelbarrow handles or hand carts.
I think the largest standing-on-the-ground-scope is a real "sweet spot", combining good light gathering power with reasonable portability and ease of use. The Messier objects are bright enough to be interesting and many globulars can be easily resolved. The ladder-scopes are good for looking at galaxies, which generally don't look like much in smaller scopes.
For me, the pace of observing slows down when you go to a ladder-scope. With my 14-inch, I take quick looks at a lot of objects. My 20-inch slows me down because I have to move the ladder, and there is more light so I spend more time studying objects in depth.
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jayscheuerle
Carpal Tunnel
Reged: 01/16/06
Posts: 2990
Loc: S. Philadelphia, PA
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To the uninitiated, even a 6" dob will elicit a "THAT's a TELESCOPE?"
8" looks huge and 12" is the biggest telescope they've ever seen. They couldn't fathom someone using a ladder to look into a telescope.
So Orion, who markets to newbie as well as the seasoned, will tend to define scopes that way, while Obsession may take a different tack. - j
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12" Green Goblin (trusser w/Protstar secondary and OWL refigured primary)• 6" f/5 Eero2 ball-scope • 6" f/5 Frankenscope • Garrett Optical 10x50 binos • Edmund 8" yoke-mounted red-tube reflector • Edmund 6" GEQ red-tube reflector (on loan to Dad)
Gone, but with lessons learned:
Skyquest XT8 • NexSTar 8i • Eeroscope 6" f/5 ball(sacrifice was not in vain) • Vixen ED80sf • Edmund red-tube 4.25" f/10 • Edmund Astroscan
Facts are stubborn things.
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davidpitre
Pooh-Bah
Reged: 05/10/05
Posts: 1410
Loc: Central Texas
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Quote:
I divide Dobs into two categories: standing-on-the-ground-scopes and ladder-scopes.
That's how I first divide'm .
After that, I divide both categories into "big " and "small".
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David
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BluewaterObserva
Post Laureate
Reged: 05/18/04
Posts: 4763
Loc: Zuni Mtns, NM
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You would really have to come up with some sort of sliding scale, where aperture and F/ratio are both taken into account in my opinion.
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llanitedave
Humble Megalomaniac
Reged: 09/26/05
Posts: 10483
Loc: Amargosa Valley, NV, USA
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Great discussion! I'm glad to see categories using other than aperture alone. "Fuss factor" is a big deal.
A 16" solid tube scope is huge, whereas a 16" truss is "medium".
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"S.O.E." (Sauron's Other Eye) 16" Royce conical mirror: A permanent work in progress.
10" Homebuilt dob, old Coulter mirror
Next Project: The "Eye of Sauron" Observatory!
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BluewaterObserva
Post Laureate
Reged: 05/18/04
Posts: 4763
Loc: Zuni Mtns, NM
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Hmm but in many ways the 16" solid tube is more portable. That zero setup time is a biggy for me on that note.
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K. A. Scharf
member
Reged: 09/02/08
Posts: 26
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I built a 6" telescope back in the early 70's when I was in college. I first tried to grind my own mirror using an Edmond kit but gave up when I 'pitched' my mothers kitchen. So I bought the optics from A. Jaegers (VERY good mirror!). I got quite a few comments "is that a cannon?!"
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Tom Polakis
professor emeritus
Reged: 12/20/04
Posts: 551
Loc: Tempe, Arizona
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Quote:
Aperture is great, sure, but the amateurs of my dad's generation did one hell of a lot, saw one hell of a lot, and thought a 6-inch was a hell of a big scope.
Yep, I remember reading Walter Scott Houston's "Deep Sky Wonders" column in S&T when I was starting out in the 1970's. He would often refer to the 10" telescope he used in Kansas. My mind would always conjure up a (probably correct) image of a behemoth telescope on a mount that weighed a couple hundred pounds.
Today, 10 inches is the aperture of my small "grab-and-go" scope that is out of the truck and completely set up on a tracking platform in about three minutes!
Tom
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Tom Polakis
Tempe, AZ
Visual observing, DSLR photography, lunar & planetary imaging
http://www.pbase.com/polakis/
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EJN
super member
Reged: 11/01/05
Posts: 148
Loc: Pluto
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Quote:
"How big is it?" I find this question tricky to answer.
I use the following system:
"wee"
"not-so-wee"
"friggin' huge"
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Luigi
Carpal Tunnel
Reged: 07/03/07
Posts: 2061
Loc: Massachusetts
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Short
Tall
Grande
Venti
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17.5" f/5 Discovery Truss
IM715 7" f/15 MCT, Eon-120ED
Lunt 60mm single etalon HA
CG5A coffee grinder, Orion Skyview Alt-AZ
35,19,15 Pans.9 Nag. Meade 24.5 4kSWA, 4.7 5kUWA.
BO-TMB 7mm planetary.
Zeiss Diascope 85
Zeiss, Leica, Canon IS, Fujinon, Nikon binos
One each generic rescue Greyhound (pictured)
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