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Why are there Few Light Weight Refractors

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#1 Westseen

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Posted 23 October 2021 - 06:51 PM

Why are the refractor telescopes offered in the market all so heavy?

 

Here is a great example:

https://www.apm-tele...ptik-in-fassung

140mm F/7 Doublet Lens Cell, weight = 2kg

 

https://www.apm-tele...40-f7-fpl53-ota

140mm F/7 Doublet Telescope + Tube rings = 9.8kg

 

I don't understand why no one would make and market the telescope near 6-7kg.  

Lens:                                           2.0kg

Tube: 2mm x 165mm x 900mm = 933 cm3

          =>1.5 g/cc Carbon Fiber = 1.4kg

          =>2.7 g/cc Aluminum =       2.5kg

Baffles:                                        0.1 - 0.2 kg

Focuser & Adapter:                      0.6kg  Starlight 2.5"

Dew Cap 1/5 Tube:                      0.2-0.5 kg

Mount Support:                            0.5-1 kg

________________________________________________________________________       

Total weight =                               4.8 - 5.9kg  + 1kg (if you want extra supports)

 

I know some older Borg scopes were very light.  The current Borg 107 (with 4 elements no less) weighs 3.5kg.  So what gives?  Why does no one want to manufacture really light weight refractors?  I am practically ready to just design my own refractors because I don't see what is so difficult.  But, maybe there are some finer points I don't understand. Please enlighten me or I am going to run off and make lightened refractors.


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#2 bobzeq25

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Posted 23 October 2021 - 07:13 PM

That scope has a 3.7 inch focuser.

 

But mostly, the issue is mechanical rigidity.  More people will buy that scope for imaging, than visual.  Which requires that everything be more rigid.  The 3.7 focuser is part of that, and also allows for large sensor cameras. It doesn't make economic sense for them to make a lightweight scope for visual.


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#3 PPPPPP42

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Posted 23 October 2021 - 07:17 PM

You can get a carbon fiber tube, but most of the weight is in the glass anyways.

The high end glass they use in the good refractors gets exponentially more expensive the larger or thicker the blank, so if there was a way to do it with thinner glass the experts at the high end companies would be doing it to save money and market a lighter scope especially when you get into the triplets.

I guess if you REALLY wanted to you could make carbon fiber rings and remove the carry handle and the like. As was said stripping from other areas would cause stability problems.


Edited by PPPPPP42, 23 October 2021 - 07:19 PM.

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#4 GOLGO13

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Posted 23 October 2021 - 07:18 PM

Great question actually. I had a 130mm Vixen F7.4 which was very light weight. My guess is with the original focuser it was under 13 pounds.

 

So many of these scopes are pretty heavy. The new TSA-120 I got was reasonable and a triplet. Fully outfitted with 2 inch accessories it's under 19 pounds.

 

But yeah, many are too heavy for my taste.


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#5 Star Shooter

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Posted 23 October 2021 - 07:31 PM

Do not forget the front tube ring that holds the lens cell. And the rear tube plate, often called the focuser adapter. That should be another 1 to 2 kilograms. Then there are the rings and dovetail plate. 


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#6 teashea

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Posted 23 October 2021 - 07:46 PM

The lens cell needs to be rigid.  

 

There are some efforts in this regard.  Take a look at the Takahashi FC76 DCU and the Takahashi FC100 DC.  Generally the focusers on this lighter weight telescopes are lighter than on telescopes designed more for AP.  


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#7 ngc7319_20

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Posted 23 October 2021 - 08:38 PM

Why are the refractor telescopes offered in the market all so heavy?

 

Here is a great example:

https://www.apm-tele...ptik-in-fassung

140mm F/7 Doublet Lens Cell, weight = 2kg

 

https://www.apm-tele...40-f7-fpl53-ota

140mm F/7 Doublet Telescope + Tube rings = 9.8kg

 

I don't understand why no one would make and market the telescope near 6-7kg.  

Lens:                                           2.0kg

Tube: 2mm x 165mm x 900mm = 933 cm3

          =>1.5 g/cc Carbon Fiber = 1.4kg

          =>2.7 g/cc Aluminum =       2.5kg

Baffles:                                        0.1 - 0.2 kg

Focuser & Adapter:                      0.6kg  Starlight 2.5"

Dew Cap 1/5 Tube:                      0.2-0.5 kg

Mount Support:                            0.5-1 kg

________________________________________________________________________       

Total weight =                               4.8 - 5.9kg  + 1kg (if you want extra supports)

 

I know some older Borg scopes were very light.  The current Borg 107 (with 4 elements no less) weighs 3.5kg.  So what gives?  Why does no one want to manufacture really light weight refractors?  I am practically ready to just design my own refractors because I don't see what is so difficult.  But, maybe there are some finer points I don't understand. Please enlighten me or I am going to run off and make lightened refractors.

Thats the skeletonized focuser meant for Dobs,  The long-throw refractor focuser is 1.5 kg.

 

And sometimes there is also a binoviewer-friendly 5" metal drawtube, besides the Feather Touch focuser.

 

Finder scope?

 

Have you every tried to keep one of those light weight Borgs collimated?  I had to re-align the objective lens every time I was out.  The metal in the front cell was very thin and flexy.  Wasn't really up to the task.  They are probably great for wide-field imaging.  But if you want nice star tests, etc., its not easy.

 

The other thing to think about was (e,g,) the Celestron CR-150 scope phenomenon.   The whole thing was very light except for the objective lens.  The tube mount ring / balance point was near the objective, and the eyepiece swung wildly around depending on the target elevation.  Many folks (including me) ended up putting a 5 pound weight on the focuser to tame the situation.


Edited by ngc7319_20, 23 October 2021 - 08:41 PM.

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#8 fftulip

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Posted 23 October 2021 - 09:35 PM

I too wonder why refractors are made relatively heavy.

 

Here is an example that I have:  TMB 8 inch (203mm) f/6 achromat, I think it uses a phenolic tube.

Total with rings and dew shield, cap: about 33 pounds or 15kg, not too bad for an 8 inch refractor but it was still too heavy for me to easily handle and put on a mount.

Removed the dew shield and cap, made of really heavy materials: -5 pounds.  (I use a lightweight flexible dewcap instead)

Replaced the massive metal backplate with a phenolic backplate, still stiff but lighter and changed the lightweight 2 inch focuser to a more massive 3 inch focuser: about -1-2 pounds.

Added two strap handles for easier handling: about +1 pound

Total weight of OTA with rings: about 27-28 pounds, or 12.5kg

Not bad, about the same weight as my 6 inch triplet!  I believe I could make it even lighter using a carbon fiber tube but in that size it would be an expensive custom order.

 

Why not use some of the ideas incorporated in reflectors, such as truss tubes covered by fabric for blockage of light?

Ok maybe it's simpler to incorporate baffles in a tube but why not just use light absorbent fabric?



#9 SeattleScott

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Posted 23 October 2021 - 10:08 PM

Istar briefly had their truss tube light weight refractors. I don’t get the impression they sold well.

Vixen and Tak make nice reasonably light doublets.

Scott
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#10 Lagrange

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Posted 24 October 2021 - 03:44 PM

I too wonder why refractors are made relatively heavy.

 

Here is an example that I have:  TMB 8 inch (203mm) f/6 achromat, I think it uses a phenolic tube.

Total with rings and dew shield, cap: about 33 pounds or 15kg, not too bad for an 8 inch refractor but it was still too heavy for me to easily handle and put on a mount.

Removed the dew shield and cap, made of really heavy materials: -5 pounds.  (I use a lightweight flexible dewcap instead)

Replaced the massive metal backplate with a phenolic backplate, still stiff but lighter and changed the lightweight 2 inch focuser to a more massive 3 inch focuser: about -1-2 pounds.

Added two strap handles for easier handling: about +1 pound

Total weight of OTA with rings: about 27-28 pounds, or 12.5kg

Not bad, about the same weight as my 6 inch triplet!  I believe I could make it even lighter using a carbon fiber tube but in that size it would be an expensive custom order.

 

Why not use some of the ideas incorporated in reflectors, such as truss tubes covered by fabric for blockage of light?

Ok maybe it's simpler to incorporate baffles in a tube but why not just use light absorbent fabric?

I suspect there's an element of customer perception involved. Big professional reflecting telescopes have used truss tubes for a long time so having your Dob look like the 200" at Palomar is no bad thing. Refractors on the other hand are nearly always traditional solid tubes so people want to buy a scope that looks the way it should.

 

There's the other factor that a solid tube with good baffling is plays an important role in enabling a good refractor to deliver high contrast images.


Edited by Lagrange, 24 October 2021 - 03:44 PM.


#11 Jon Isaacs

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Posted 24 October 2021 - 04:33 PM

Why not use some of the ideas incorporated in reflectors, such as truss tubes covered by fabric for blockage of light?

 

Ok, so I own 4 truss tube Dobs.. I love them.

 

Not a good design for a refractor.. No easy way to mount the scope. Trusses must be angled.. 6 trusses are a minimum.. the truss mounting points must be rigid and reinforced.

 

Newtonians have lax collimation tolerances, refractors are trickier.. the general perception is that Newtonians are finicky about collimation. The reality is they're easy to collimate and quite forgiving, that's why they get away with such flimsy tubes..

 

Tubes have thermal advantages.. baffling advantages.. trusses work best for something that's not long and thin, tubes work better for long and thin.

 

I could go on.. 

 

Jon


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#12 teashea

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Posted 24 October 2021 - 08:25 PM

The aluminum tube of a good refractor does not weigh that much as to justify substituting a cloth covered truss system.  The mounting system would be a mess.  It would be ugly.  The lens cell would not work well with a truss frame.  It would be, in summary, pathetic.


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#13 TONGKW

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Posted 24 October 2021 - 09:19 PM

-------------------- 

I have a Vixen Shuttle Scope 100s achromatic refractor OTA at 100 mm aperture and focal ratio f/6.8. The weight of the OTA is only 2 kg due of the use of very thin aluminum tube, a simple 1.25” helical focuser and no dew shield provided.
This OTA on a small Kenko KDS altAz mount and a Velbon photo tripod constitutes a very handy grab-and-go set up.

---------------------  

Vixen Shuttle Scope 100s_Kenko KDS.jpg

---------------------


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#14 Westseen

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Posted 24 October 2021 - 11:06 PM

That Shuttle Scope is really cool.  I don't know why a 2mm thick carbon fiber tube like the above would not be a standard offering.  Heck, a tube made out of magnesium is probably justifiable on high end scopes.  And as a bonus the thermal expansion coefficients for aluminum and magnesium are similar.

 

If the Borg scopes are a bit too flimsy, why isn't there a competitive product near 107 at 4.0kg instead of 3.5kg?  Have you ever felt carbon fiber trekking poles, you can put a hundred pounds on a 400g pole.  Adding 0.5kg of carbon fiber is an enormous amount of extra stiffness.  And if you need a metal use aluminum and machine out the excess material.  Or use magnesium.  Many high end refractors >$3k...  An extra $500 on the tube would create a lot of added value.

 

Maybe a lot of optics manufacturer's are experts in optics and there is a hole in expertise in housing/tube manufacturing.  

 

Again here is 130mm triplet:

https://www.apm-tele...at-lens-in-cell

3.05 kg

https://www.apm-tele...r-1301200-37zta

11.4 kg (25.1lbs with rings)

Lens = 26.7% of total weight

 

Or a 140mm triplet:

https://istar-optica...ochromatic.html

2.8 kg

https://istar-optica...refractors.html

OTA = 10.1 kg

Lens = 27.7% of total weight

 

So, sure the lens is not a negligible weight but clearly the housing and brackets are the main problem.


Edited by Westseen, 24 October 2021 - 11:07 PM.


#15 gwlee

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Posted 25 October 2021 - 12:03 AM

Small refractors tend to be light weight, and so are their mounts.


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#16 Jae

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Posted 25 October 2021 - 04:55 AM

“I am going to run off and make lightened refractors“

 

I’ll buy it!!

 

I totally agree, as I’m a visual refractor fan.   I loved Vixen FL’s as they were light, but most were a little long except for the older 102ED f 6.5 which I loved but the focuser could be better.
 

I bought a TMB 92L as the 92SS was too heavy.  But even the 92L weighed 8.5 lbs with the rings and 7 lbs ota alone.  Vixen rings were always lightweight.

 

I found some Williams Optics rings, that were much lighter  and Starlight came up with their lightweight feather touch focuser so bought that.  I got the ota down to 5.6lbs and with the lighter rings 6.3lb total, 2.2 lb reduction is not bad.   And it would be solid for imaging as well.

The dew shield weighs 1.4lbs, so if I could replace that with something lighter, I could shed another pound.

It made the 92L a real L - lightweight scope, lighter than many well built 80mm scopes, like the LOMO 80 I have.  So why would I use a 80mm when I can use a lighter 92?   I think the Astrotech 92 could be modified like my TMB 92L but why don’t they just make it lighter to start ?  I’d make my own refractors too !


Edited by Jae, 25 October 2021 - 04:56 AM.

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#17 havasman

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Posted 25 October 2021 - 05:32 AM

Because no matter how fervently a small group of users may want an item to be just so, if the much larger group of users is satisfied with makers' offerings that do not conform to those needs then there is no market for the item that conforms to the needs of the few. Or if there is a market it is a niche to be satisfied by a few outlier products like the Borgs.



#18 Jon Isaacs

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Posted 25 October 2021 - 09:14 AM

Because no matter how fervently a small group of users may want an item to be just so, if the much larger group of users is satisfied with makers' offerings that do not conform to those needs then there is no market for the item that conforms to the needs of the few. Or if there is a market it is a niche to be satisfied by a few outlier products like the Borgs.

 

I want a robust refractor.  Refractors aren't bicycles where grams count. Lightweight means compromise, tubes that dent, fracture and buckle.

 

I ran a materials science laboratory.. I had a carbon fiber bike frame hanging on the wall, it has been leaning against a wall and fallen over. It was badly fractured, irreparable. I kept it there as a lesson to the grad students.

 

Jon


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#19 StarDust1

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Posted 25 October 2021 - 09:29 AM

I do like robust refractors as well. As Jon stated, lightweight means compromise. at least with Aluminum...

If the refractors can be made with exotic material to make it light weight, without compromise, then I'm all for it...


Edited by StarDust1, 25 October 2021 - 09:48 AM.


#20 MrJones

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Posted 25 October 2021 - 09:37 AM

I don't agree that lightweight = compromise. Weight is just not a priority for most makers vs. optics and cost. No reason we can't have say titanium alloy tubes and fittings or better composites but they'd cost more and it's not obvious customers are willing to pay for it.


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#21 teashea

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Posted 25 October 2021 - 08:32 PM

I too wonder why refractors are made relatively heavy.

 

Here is an example that I have:  TMB 8 inch (203mm) f/6 achromat, I think it uses a phenolic tube.

Total with rings and dew shield, cap: about 33 pounds or 15kg, not too bad for an 8 inch refractor but it was still too heavy for me to easily handle and put on a mount.

Removed the dew shield and cap, made of really heavy materials: -5 pounds.  (I use a lightweight flexible dewcap instead)

Replaced the massive metal backplate with a phenolic backplate, still stiff but lighter and changed the lightweight 2 inch focuser to a more massive 3 inch focuser: about -1-2 pounds.

Added two strap handles for easier handling: about +1 pound

Total weight of OTA with rings: about 27-28 pounds, or 12.5kg

Not bad, about the same weight as my 6 inch triplet!  I believe I could make it even lighter using a carbon fiber tube but in that size it would be an expensive custom order.

 

Why not use some of the ideas incorporated in reflectors, such as truss tubes covered by fabric for blockage of light?

Ok maybe it's simpler to incorporate baffles in a tube but why not just use light absorbent fabric?

I had a TMB 105 CNC version.  It was a pig..... a pig with good optics but a pig.  It was as if it had been manufactured in a Russian tractor factory.  Terrible finish quality and so absurdly large and heavy - I am so glad I got rid of that.  



#22 teashea

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Posted 25 October 2021 - 08:34 PM

Small refractors tend to be light weight, and so are their mounts.

and that makes them attractive to me.  



#23 GOLGO13

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Posted 25 October 2021 - 08:51 PM

Depends how light we are talking with compromises...and depends on design.

 

I think there are probably plenty of light telescopes which are just fine. Sometimes scopes are over-engineered. One I can think of was the TeleVue Pronto I had for a short time. Wonderful scope optics wise. But man was it heavy...and because of that was a bit hard to balance and use. That seemed un-necessarily heavy. In fact I think the Pronto (70mm) weighed more than the Vixen 81s OTA wise.

 

I rather have a scope that is engineered to be exactly the weight it should be. I think sometimes they over-engineer scopes for a purpose (maybe imaging?). But for the most part I rather it be as light as is reasonable. 

 

I can think of a few examples of extremes. The TOA 130mm is 27 pounds, my TSA 120mm is 13 pounds. That says to me that the TOA is a bit over engineered maybe for a purpose.

 

Another example are some of the large dobs. A 16 inch Orion dob weight substantially more than say a 16 inch of a premium brand. Even between premium dob brands some are pretty darn heavy and bulky in comparison. In this case, it's about engineering and decisions on design. Those scopes must be crazy to setup.


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#24 Jon Isaacs

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Posted 25 October 2021 - 09:25 PM

I don't agree that lightweight = compromise. Weight is just not a priority for most makers vs. optics and cost. No reason we can't have say titanium alloy tubes and fittings or better composites but they'd cost more and it's not obvious customers are willing to pay for it.

 

Well...

 

I spent most of my adult life as research engineer running a materials science research laboratory and a major university.

 

- Specific Young's modulus:  Young's modulus is the measure of the stiffness (rigidity) of a a material, the "specific Young's modulus" is the stiffness per unit weight.  Steel, titanium and aluminum all have identical specific Young's modului. 

 

What this means is that for a given weight and diameter, tubes made from steel, titanium and aluminum will all be equally rigid.  Titanium might have a greater specific strength but strength in refractor OTA is never an issue. The greater wall thickness of the aluminum tube is actually an advantage and it means it is more difficult to dent.

 

- Coefficient of thermal expansion:  The reason A-P, TEC use aluminum tubes rather than carbon fiber is that the thermal contraction of the aluminum better matches the shift in focus as the objective cools. 

 

Carbon fiber is a difficult material to work with as it requires great care.. It fractures.. 

 

Jon


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#25 Jon Isaacs

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Posted 25 October 2021 - 09:29 PM

Another example are some of the large dobs. A 16 inch Orion dob weight substantially more than say a 16 inch of a premium brand. Even between premium dob brands some are pretty darn heavy and bulky in comparison. In this case, it's about engineering and decisions on design. Those scopes must be crazy to setup.

 

The 16 inch Orion is just plain heavy..  Material choices., steel, particle board, design choices... It weighs nearly as much as my 22 inch which has a mirror that weighs 60 lbs versus maybe 20 or 25 for the 16 inch.  

 

Jon




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