Steel tube Vs Aluminium Tube

[Northernlight]

Hi All,

I'm looking at 2 scopes a SW Quattro 10" and a OrionOptics VX10 as a new imaging scope. I'm leaning towards the VX10, but have concerns about the aluminium tube flexing.

Are aluminium tubes just as strong as Steel tubes ?   which is stronger Aluminium or Steel ?

 

Cheers,

Rich.

[johngm]

Hi Rich,

Regarding This question, i work in the automotive industry, and specialise in Aluminium Vehicles, and they have to go through vigorous VOSA Crash testing, so from this, they pass the regulation tests, at tons of force, so i think, IMHO, it would not mater, especially at the short focal lengths of these imaging Newtonian's. Also bear in mind, a tubular construction is very rigid. Also you have the added advantage of aluminium of being lighter. This is my input, but would be interested in others views on this subject.

John

[johninderby]

There is the OO carbon fibre tube option if you are concerned about stiffness.

[markse68]

13 minutes ago, johngm said:

Also you have the added advantage of aluminium of being lighter. This is my input, but would be interested in others views on this subject

And it won’t rust of course

[lenscap]

For a thin walled cylinder such as a telescope tube the bending deflection is inversely proportional to  (Young's Modulus x tube wall thickness).

But Young's Modulus for steel is typically about 3 times that of Aluminium.

So for tubes of the same diameter, an Ally tube needs to be about 3 times as thick as a steel one for the same rigidity, which manufacturers are well aware of.

 

 

 

Edited November 5, 2019 by lenscap

[alacant]

 

3 hours ago, Northernlight said:

aluminium tube flexing

Hi

I don't think it's the tube material which matters. We found that the tube flex came from too short a distance between the tube rings. Replacing the existing dovetail mount side with a longer, wider plate than the flimsy affair which is usually supplied [1] and bolting an aluminium box section to rigidly tie the rings along the top of the tube tamed even our f3.9; spread the load along as much of the tube as possible. To eliminate what little flex and mirror movement which remains, you can use an OAG.

HTH

[1] e.g. the rings on our steel tube sw-250 are 50cm apart.

Edited November 6, 2019 by alacant

[Ben the Ignorant]

I am no metallurgist but I have observed this comparing steel and aluminum telescope tubes: the region around the small newtonians' (130 and 150) focuser bends, probably because the kind of steel that can be rolled into a tube shape has to be springy.

I have never, however, noticed any bending with aluminum tubes, probably because they are made of an extruded or centrifuge-formed rigid alloy that doesn't have to be flexible in order to be shaped into a cylnder. Not even the cheap toyish 60mm/800mm achromat I restored had any visible flex, despite being quite long relative to its diameter.

Then again, my GSO 300mm dob with a steel tube doesn't flex around the focuser, and doesn't sag over its 144cm length, not enough to change optical alignment, anyway, the essential is remaining within tolerances. The GSO steel seems to be a thicker and more rigid kind of steel, and looks like it calls for tougher machines to be rolled into a tube.

Overall I prefer aluminum because the tube is seamless and kinda forged by the pressure of extrusion or the centrifuge process, I guess. All the high-end apos have aluminum tubes or a composite called Krupax in the case of APM. It seems to have the qualities of aluminum but doesn't get frosted or dewy when the air is very cold.

Edit: I own only one carbon fiber scope, and it's a small 80/560 so not much experience but when taking it apart to blacken it better, and add baffles, the material felt very rigid. None of steel's springiness, but the same mild flexibility as alu when conducting the very scientific testing of pressing the dewshields inside my hands to feel and observe the deformation. Carbon wins in lightness, yes, BUT it makes the scope unbalanced toward the focuser so I had to make a quite heavy counterweight for the scope's front end. Other properties come into play.

Edited November 6, 2019 by Ben the Ignorant

[Northernlight]

14 hours ago, alacant said:

 

Hi

I don't think it's the tube material which matters. We found that the tube flex came from too short a distance between the tube rings. Replacing the existing dovetail mount side with a longer, wider plate than the flimsy affair which is usually supplied [1] and bolting an aluminium box section to rigidly tie the rings along the top of the tube tamed even our f3.9; spread the load along as much of the tube as possible. To eliminate what little flex and mirror movement which remains, you can use an OAG.

HTH

[1] e.g. the rings on our steel tube sw-250 are 50cm apart.

 I do have an QSI 683 with built in oag, so have no concerns with differential flex of camera's. It's literally just the tube flexing around the focuses under load that i'm worried about.

[Northernlight]

16 hours ago, lenscap said:

For a thin walled cylinder such as a telescope tube the bending deflection is inversely proportional to  (Young's Modulus x tube wall thickness).

But Young's Modulus for steel is typically about 3 times that of Aluminium.

So for tubes of the same diameter, an Ally tube needs to be about 3 times as thick as a steel one for the same rigidity, which manufacturers are well aware of.

 

 

 

Lens cap, is this based on standard aluminium or a Aluminium Alloys ? as i imagine telescope tubes will use an Aluminium alloy of some sort.

[Davey-T]

3 minutes ago, Northernlight said:

Lens cap, is this based on standard aluminium or a Aluminium Alloys ? as i imagine telescope tubes will use an Aluminium alloy of some sort.

They probably use Dural aluminium alloy, same sort of thing used on armoured vehicles.

Dave

 

[lenscap]

23 minutes ago, Northernlight said:

Lens cap, is this based on standard aluminium or a Aluminium Alloys

There is not much variation in stiffness (Youngs Modulus) of common Aluminium alloys, typically close to 10 Mpsi;

https://www.amesweb.info/Materials/Youngs-Modulus-of-Aluminum.aspx

Duralumin is  about 10.6.    (Mild steel is about 30).

Of course you can improve stiffness by adding ribs or rings, not just thickness.

Edited November 6, 2019 by lenscap