Threaded rod + nut focuser?

[vlaiv]

Here is an idea ...

It is mostly for 3d printed focusers but I can't see why it could not be used as fine focus in any DIY focuser.

All we need is a draw tube and focuser body, draw tube being suspended on bearings or Teflon pads so it can slide in and out.

Focusing motion is performed by threaded rod and nut - or two pieces of threaded rod.

Either nut or one piece of threaded rod is attached to draw tube and can't move / rotate. Other, longer piece of threaded rod is attached to focuser body via bearings and can rotate freely.

It is either threaded thru the nut or in close contact with other piece of threaded rod. Rotating it causes draw tube to rack in / out.

Maybe easiest way to explain it would be to imagine fastening the screw in something - as we turn the screw - it is driven inwards. Now just "fix" the screw and let the object move instead.

[vlaiv]

Forgot to add - it will move one thread pitch per revolution, so if one wants to have faster focuser - there is another option instead of standard metric threaded rods.

That is trapezoidal / lead screw + corresponding nut.

These usually have multiple starts and longer pitch. Standard 802 has pitch of 2mm and 4 starts - which means it moves 8mm per turn.

[Rusted]

HI,

The downside is the inability to change focus quickly over any distance.
Plus the need to handle the telescope continuously for some time.
Very fine focus is an impediment to finding best focus in my experience.
One ends up rocking back and forth. Rather than racking.
 

[Chriske]

Vlaiv,
Maybe I completely misunderstood, but I think you're referring to this...?

Two threaded rods, two (driving)nuts and two LMUU bearings
Three T2 pulleys, a closed timing belt and a few bevel gears.

[Captain Scarlet]

On 28/10/2022 at 21:02, vlaiv said:

Here is an idea ...

It is mostly for 3d printed focusers but I can't see why it could not be used as fine focus in any DIY focuser.

All we need is a draw tube and focuser body, draw tube being suspended on bearings or Teflon pads so it can slide in and out.

Focusing motion is performed by threaded rod and nut - or two pieces of threaded rod.

Either nut or one piece of threaded rod is attached to draw tube and can't move / rotate. Other, longer piece of threaded rod is attached to focuser body via bearings and can rotate freely.

It is either threaded thru the nut or in close contact with other piece of threaded rod. Rotating it causes draw tube to rack in / out.

Maybe easiest way to explain it would be to imagine fastening the screw in something - as we turn the screw - it is driven inwards. Now just "fix" the screw and let the object move instead.

I think you’ve basically described how the Skymax MCTs focus, with a threaded rod attached to a bulkhead on the sliding outer baffle tube on which sits the primary mirror, with the rod poking out through the back of the OTA and affixed with a knob.

[vlaiv]

1 hour ago, Chriske said:

Maybe I completely misunderstood, but I think you're referring to this...?

Yes - but my idea was simpler, not so sophisticated and nicely executed

I like the way you used 3d printed bevel gears and timing belt to make it ergonomically the same as regular focuser.

Mine idea was much more like focuser on SCT/MCT type scope or maybe spotter scope.

Only thing that I can't decipher from the image is what the belt is turning - how did you attach gt2 pulley to nut? Is pulley attached to threaded rod and whole rod is turning or is it just nut?

[Chriske]

It's simple, one alu pulley (on the right) is a regular one. the two other (printed) have threads inside.

[saac]

I think my little Meade ETX 80 uses the very system you are describing vlaiv. To achieve focus  the outer tube slides over the fixed inner tube being carried along by a threaded rod. This moves the objective lens relative to the eyepiece which is held in an assembly fixed to the bottom of the inner tube. It's fine when close to focus but a pain if having to move the tube through any larger distance. 

Jim