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How much does your dob weigh per inch of aperture?

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#1 Jeff Porter

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Posted 31 March 2013 - 11:31 PM

Just curious to find out what some of the different dobs weigh per inch of aperture. Here is what I have...

17.5" StarStructure TL = 96 lbs. so
96 lbs/17.5" aperture = 5.5 lbs/inch of aperture

Orion XX12...
86.5 lbs./12" aperture = 7.2 lbs./inch of aperture.

Orion 6"
35 lbs./6 aperture = 5.8 lbs./inch of aperture.

I would love to hear from everyone from the Coulter owners to the ultralights.

Jeff P

#2 David Knisely

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Posted 01 April 2013 - 12:31 AM

My Orion XX14i weighs in at around 120 lbs, so that would be about 8.6 lbs per inch of aperture (and it is a heavy beast, although not quite as heavy as the XX14g). Clear skies to you.

#3 FirstSight

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Posted 01 April 2013 - 12:52 AM

80 lbs (base + OTA) / 12 inches aperture = 6.67 lbs/in.

Presumably, we don't count accessories such as finderscopes or Telrads or the removable wiring and batteries needed for e.g. dew heaters and fans; just the telescope structure itself.

#4 Ed D

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Posted 01 April 2013 - 04:51 AM

My 6" tube Dob weighs 30# fully loaded - 5# per inch

The 6" strut Dob should weigh 22# or less when finished - 3.7# per inch

Ed D

#5 Jon Isaacs

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Posted 01 April 2013 - 06:26 AM

Just curious to find out what some of the different dobs weigh per inch of aperture. Here is what I have...

17.5" StarStructure TL = 96 lbs. so
96 lbs/17.5" aperture = 5.5 lbs/inch of aperture

Orion XX12...
86.5 lbs./12" aperture = 7.2 lbs./inch of aperture.

Orion 6"
35 lbs./6 aperture = 5.8 lbs./inch of aperture.

I would love to hear from everyone from the Coulter owners to the ultralights.

Jeff P


In a scaled design, weight scales with the cube of the aperture. If everything is scaled up then a 12 inch Dob should weigh 8 times as much as a 6 inch. A small scope has a real advantage in a linear measure. One measure would be the ratio of the cube weight divided by the cube of the aperture. Some parts are not scaled, focusers... another measure might be pounds/square inch, the weight for a given amount of light gathering.

Note, in the interest of simplicity, the numbers below are in terms of "lb/inches squared", i.e. the aperture squared, this proportional to the area of the mirror.

Two examples:

- Jason Comet Hunter 480. This should pass muster and a Dob, it's on an alt-az mount. It's got a steel tube and metal mount. It's a 3 inch scope that weighs 4.75 lbs, it's weight to aperture ratio is 1.59 lbs/in, weight per square inch of mirror area is 0.53 lb/inch squared. Even with the 1.59 lb/in number, it could easily be much lighter. I have a 3 inch F/3.7 Newtonian OTA that weighs about 1.5 lbs, with a simple mount, it could be under 1 lb/inch.

This scope comes in at about one third the weight/aperture of the Star Structure but the Star Structure is clearly the lightweight design, the Comet 480 is not.

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- 16 inch Dobstuff. This weighs about 90 lbs depending on the configuration, it's weight to aperture ratio is about 5.6 lbs/in. It's weight per square inch of mirror area is about 0.35 lbs per inch squared.

The mini-dob was not designed to be particularly light, steel tube, steel mounting, but at under 5 pounds, it really didn't need to be. On the other hand, the Dobstuff, needed to be light. The optics began their life as Meade Starfinder Dob, I believe that scope must have weighed about 150lbs, 9.4 lbs/in or 0.59 lbs/inch squared. It was basically unmanageable and something had to be done.

Looking at the scopes listed:

- 17.5 inch Star Structure: 0.313 lb/inch squared, 5.5 lb/in

- 12 inch Orion Dob: 0.601 lb/inch squared, 7.2 lb/in

- 6 inch Orion: 0.972 lb/inch squared, 5.8 lb/in

- 14 inch Orion Truss: 0.612 lb/inch squared, 8.6 lb/in

- 6 inch custom: 0.833 lb/inch squared, 5 lb/inc.

- 3 inch Jason: 0.59 lb/inch squared, 1.6 lb/in

- 16 inch Dobstuff: 0.35 lb/inch squared, 5.6 lb/in

If one is looking for a measure of the effectiveness of a lightweight design, I don't think the linear lb/in is reasonable. The 17.5 inch Star Structure at 90lbs is well executed design that goes a long way to minimizing the weight and yet the linear metric does not show that. To my mind, if the smaller scopes were built with such attention to detail, they could be considerably lighter, at 0.313 lb/inch squared, a 12 inch Dob would weight 45 lbs, not at all unreasonable and an even light scope could be built.

Looking the various weights/inch square, I think it does a better job as a metric for sorting out what is a lightweight design. However, the weight/inch cubed might be more appropriate, if one were comparing different animals, this would probably be the metric one uses. The Star Structure comes in at 0.018 lb/inch^3, at weight over inches cubed ratio, a 12 inch Dob would weigh 31 lbs, possible but really pushing the limit. But then, the Star Structure is really pushing the limit for a 17.5 inch scope, it's just that our expectations are skewed because of the simple practical fact that a large Dob has to be manageable. I don't know how much a 25 inch Obsession weighs, probably between 225 lbs and 250lbs. If a 25 inch Dob were built using the same technology used for a steel tube Dob, it would probably weigh at least 500 lbs.

Just some stuff to think about...

Jon

#6 JimMo

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Posted 01 April 2013 - 07:06 AM

My 14.5" is a behemoth and I have no idea what it all weighs. I'm sure it's well over 100 lbs with a deep mirror box so lots of 3/4" plywood which is doubled for the altitude bearings. I've contemplated rebuilding it but it is easy to transport with the wheel barrel handles and holds collimation extremely well with no flex of the structure at all. In other words it's a tank. :p Let's say 120 lbs. which is 8.28 lbs. per inch.

#7 tezster

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Posted 01 April 2013 - 07:14 AM

My 10" DobStuff weighs around 40lbs., so 4lb/inch. I'd love to see some 'suitcase' scopes, Sumerian optics designs, and other home-made light-weight designs. :)

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#8 nicknacknock

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Posted 01 April 2013 - 07:15 AM

3.7 per inch :) lightweight structure does have a few advantages...

#9 isawit

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Posted 01 April 2013 - 07:16 AM

Apertura AD12 12"/86.1 lbs. = 7.175 lb. per inch of aperture

#10 Mirzam

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Posted 01 April 2013 - 07:22 AM

68 lbs, 14" aperture homebuilt travel scope = 4.85 lb/in. It helps that the Mark Cowan quartz mirror only weighs 11 lbs.

JimC

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#11 ctcables

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Posted 01 April 2013 - 07:57 AM

32/14.5 is 2.2lbs can pick it up with one hand and go with no problem. Working on a 20" I hope to get in around 40lbs + 10lbs for the bace with drive. We will see how it turns out.

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#12 nirvanix

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Posted 01 April 2013 - 09:14 AM

13.1" original Coulter weighs 150 lbs, so about 11.5 lbs per inch.

#13 Jeff Porter

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Posted 01 April 2013 - 09:35 AM

Jon,

Your points are well made. I didn't think to use either of the methods you mentioned but they do give a better representation of the differences between the various models. I really like the idea of weight/(aperture squared), so I will compile a list using that ratio from lightest to heaviest based on that criteria.

For everyone else, keep em coming.

Jeff P

#14 FirstSight

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Posted 01 April 2013 - 10:36 AM

Weight and bulk per inch of aperture is not purely advantageous (if relatively light) nor disadvantageous (if relatively heavy). For transportability, lower weight and bulk relative to aperture is an advantage, higher weight-to-aperture tends to be a disadvantage. For stability in breezy conditions or resistance to flexure, higher weight-to-aperture tends to be an advantage. A lighter-weight design can compensate by using stiffer, higher-quality materials, though usually this compensation comes at higher cost in materials and required quality of design and workmanship.

Winter Star Party (the site for which is directly on a wind-exposed south-facing beach berm) is an excellent test of the relative values of weight/bulk/materials/design/aperture/transportability. You can easily see who's thriving and who's not with what scopes on a temptingly clear, transparent night with just enough breeze to challenge the ability of scopes to provide stable views above about 150x, as well as getting a good idea what kind of vehicle capacity and human effort was necessary to transport them the distance from home to the site and set them up.

#15 Project Galileo

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Posted 01 April 2013 - 11:12 AM

Loaded my modified LB16 weighs in at 120 lbs. That is 7.5 lbs per inch.

#16 BluewaterObserva

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Posted 01 April 2013 - 12:40 PM

Way to much.... Wood adds up quicker than I expoected.

Self-made 13.1" Truss dob.

#17 Tad S.

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Posted 01 April 2013 - 01:23 PM

My modified XT8i now has a 200mm mirror and a few accessories that bump the weight to 42 lbs. That comes out to 5.27#/in.

Stock the XT8i is 41.6 lbs for a 203mm mirror (5.2#/in).

--Tad

#18 careysub

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Posted 01 April 2013 - 06:47 PM

From a spreadsheet of compiled weights (mostly from a thread here about a year ago, I added the HO offerings and the Orion XX16g recently). Ordered from the lowest Lb/In^2 "figure of merit" to the highest"

---------------------Aperture-Wt. (lb)--Lb/In---Lb/In^2
Hubble Optics UL18------18------72------4.00-----0.22
Hubble Optics UL16------16------60------3.75-----0.23
Hubble Optics UL14------14------48------3.43-----0.24
Webster D18-------------18------92------5.11-----0.28
DobStuff 17.5 ----------17.5----89------5.09-----0.29
Obsession 20 UC---------20-----124------6.20-----0.31
Obsession 18 UC---------18-----105------5.83-----0.32
Obsession 15 UC---------15------79------5.27-----0.35
DobStuff 14-------------14------70------5.00-----0.36
Obsession 20 Classic----20-----147------7.35-----0.37
Obsession 18 Classic----18-----126------7.00-----0.39
Starmaster 20-----------20-----156------7.80-----0.39
StarStructure 15 TL-----15------88------5.87-----0.39
Obsession 15 Classic----15------89------5.93-----0.40
Obsession 12.5 Classic--12.5----65------5.20-----0.42
Starmaster 18-----------18-----138------7.67-----0.43
Starmaster 14.5---------14.5----96------6.62-----0.46
Starmaster 16-----------16-----121------7.56-----0.47
StarStructure 12.5 TL---12.5----78------6.24-----0.50
LightBridge 16----------16-----128------8.00-----0.50
Zhumell 12--------------12------75------6.25-----0.52
Zhumell 16--------------16-----134------8.38-----0.52
Orion XT10--------------10------53------5.30-----0.53
LightBridge 12----------12------80------6.67-----0.56
Zhumell 10--------------10------60------6.00-----0.60
Orion XX12--------------12------87------7.25-----0.60
Starmaster 11-----------11------74------6.73-----0.61
Orion XX14i-------------14-----120------8.57-----0.61
LightBridge 10----------10------65------6.50-----0.65
Orion XX16g-------------16-----195-----12.19-----0.76

#19 buddyjesus

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Posted 01 April 2013 - 06:55 PM

http://royalsociety....wton-telescope/

would love to find out how much this one weighs personally.

#20 kfrederick

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Posted 01 April 2013 - 08:42 PM

The HST is weightless so it wins

#21 Achernar

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Posted 01 April 2013 - 09:43 PM

Just by itself, my 15-inch is about 90lbs, or about 6 pounds per inch of aperture, throw in the batteries and other stuff in, or on the base, it's closer to 120lbs or 8 pounds per inch of aperture.

Taras

#22 Datapanic

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Posted 01 April 2013 - 09:58 PM

I don't own a Dobsonian, but thought I would blurt out the weight of the Horsetrail Cave Newtonian, just for comparison:

OTA: 130lbs
Dec Shaft Counterweights: 100lbs
Equitorial Mount/Stand: ~120lbs

It has wheels.

#23 omahaastro

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Posted 02 April 2013 - 12:05 AM

My 30" weighs about 300 lbs. It has a full thickness Sitall primary. It weighs about 70 lbs at the wheel barrel handles, which can make for an adventure going up/down the trailer ramp. I hope to implement some kind of winch solution somewhere between now and before my back goes out.

#24 Jeff Porter

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Posted 02 April 2013 - 03:16 AM

The HST is weightless so it wins


:funny: :lol:

#25 Jeff Porter

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Posted 02 April 2013 - 03:28 AM

I really appreciate all of the responses. Some of the ultralight designs are very impressive. My good friend Jay has a 14" Dobstuff that is smaller and lighter than my 12". The mirror box or my 17.5 is just barely too big to fit into my Honda Accord, so I haul that one around in my Odyssey.

Maybe I'll have an ultralight one day. This thread really shows the efficiencies that some are able to attain.

Jeff P.






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