Why are focusers round?

[ollypenrice]

For something that is as old as the telescope itself, focusers seem to give a disproportionate amount of grief. Yesterday I was summoned to our robotic shed to fix a flexing and slipping Baader Steeltrack Crayford. On this focuser there are two ballbearings which run on steel strips placed 120 degrees apart on the round drawtube. 120 degrees from these ballbearings is the third point of contact, the drive roller itself.  The ballbearings have to be preloaded by the right amount so that they prevent the drawtube from flexing but I know from experience that if they are overloaded they crack.

However, if the outside of the drawtube were square it could run on roller bearings rather than ballbearings, giving a much larger surface area of contact. Would it look odd? Who cares if it worked better?

On this particular scope, an Orion Optics ODK, there is another bit of design nonsense. The focuser is low profile, a design which is prone to flex because of the short drawtube, yet it is mounted on a long extension tube on the back of the scope so there would have been plenty of room for a longer focuser. What am I missing?

Olly

[Sunshine]

I’m going out on a limb here and guessing it’s because light cones from optics come to a focus from optics are round. A square focused would have to be larger to clear a particular sized light conce, I’m guessing square focusers would be unwieldy large.

Edited May 3, 2019 by Sunshine

[gorann]

Great idea Olly, but you may not be able to get a patent. A few years ago Explore Scientific started to supply their top refractors with hexagonal focusers, but they are rack and pinion. Not sure how good they are.

https://www.explorescientific.co.uk/en/Telescopes-es/APO-Triplet-Series/ED-APO-FCD100-Series/EXPLORE-SCIENTIFIC-ED-APO-127mm-f-7-5-FCD-100-Alu-HEX.html

https://explorescientificusa.com/products/hex-focuser

Edited May 3, 2019 by gorann

[johninderby]

Bresser uses the hexafoc focuser which is the same as the Explore Scientific one. Had one and it was not bad ar all. Didn’t slip but could have had smoother movement. A good budget focuser though.

 

Edited May 3, 2019 by johninderby

[ollypenrice]

1 hour ago, Sunshine said:

I’m going out on a limb here and guessing it’s because light cones from optics come to a focus from optics are round. A square focused would have to be larger to clear a particular sized light conce, I’m guessing square focusers would be unwieldy large.

The inside can be round though.

John, Goran, thanks for the link. I also thought about something along these lines but had never seen this one before. Basically it just seems sensible to run the drawtube on rollers.

Olly

[JamesF]

From an engineering standpoint, it may perhaps be easier and more accurate to have something running on a single point of support, such as a ball bearing or wheel rather than on a roller.  Neither would prevent the use of a straight-sided drawtube though.

James

[johninderby]

Found a few pics of the hexafoc focuser on the scope I used to have.

[andrew s]

I think it is mainly historic. It is simpler to machine round components and lap them for precision. Accurate milling of flat surfaces is much more difficult.

It is easy to forget the original Crayford focuser was designed for the man with hand tools to self build, not as an ideal design!

I have never understood why the basics of kinematic design have not been widely applied in astronomy equipment for the amateur. It is not rocket science and would overcome many of the flex and shift issues we encounter.

Regards Andrew 

 

[Jessun]

Is it not simply easier to align something with three points of contact? Much like a three legged stool self aligns without wiggling? 

The roundness in general puzzled me somewhat though. Sure things screw on and off with ease but just as easily do they unscrew for various reasons... 

I'd be all for a non rotating assembly system that clicks in place. 

I'll buy your first focuser Olly 🙂

/Jessun

[andrew s]

1 hour ago, Jessun said:

Is it not simply easier to align something with three points of contact? Much like a three legged stool self aligns without wiggling.

/Jessun

Yes but it can slide in the plane and take off from the plane (try it upside down on the ceiling)

You need 6 suitably place point to fix an object in space. In a focuser you need two free for in out motion so that leaves four points. A Craford typically has 5 which is theoretically redundant but is good for loads that might bend the tube.

Regards Andrew