Optimum fidelity for focuser?

[vlaiv]

How much would you say is optimum travel of focuser draw tube per knob rotation?

Recently, I played with non rotating helical focuser that I printed. I used 1mm pitch thread for focuser motion, and it has travel of 18mm - that translates into 18 full rotations to move it between two extreme positions.

On the other hand - I measured stock Skywatcher 2" refractor focuser (single speed) - and it moves about 30mm per revolution.

Interestingly, that skywatcher focuser is from F/10 refractor. Critical focus zone for such scope is 1/4 of mm (249um),

That would make something like 3 degrees of a knob turn. Way too small?

I'd say that critical focus zone should be maybe 1/16th of a turn for easy focusing.

That would make 1mm / turn for F/5 scope, ~1.45mm / turn for F/6 scope, ~2.56mm / turn for F/8 scope and ~4mm / turn for F/10 scope.

Would that be too fine for a single speed focuser?

 

[CedricTheBrave]

I know Bob Brown has dont some calculation stuff for his focuser project.

this video explains it 

 

[vlaiv]

2 minutes ago, CedricTheBrave said:

I know Bob Brown has dont some calculation stuff for his focuser project.

I'm using same equations for critical focus zone, but I'm not really sure how applicable that is for visual?

 

[markse68]

I think most Crayford focusers use a 4mm shaft or there abouts so about 12mm per revolution. Then either 8 or 10:1 reduction for the fine focus knob. I guess rack and pinions would use a larger diameter pinion so a bit coarser?

Mark

[Captain Scarlet]

I use three types of focuser: a Feathertouch Crayford on my 105mm refractor; a Feathertouch SCT microfocuser on my Skymax 180; I have three Baader Diamond Steeltraks: one on each of my newts and one on my Intes 6" Mak. I've measured them all to be able to ascertain how many turns I need to defocus by, say, 10 wavelengths. Their linear-travel-per-full-turn are as follows, with the numbers given being in fully-reduced mode. The coarse modes are x10 ish in each case, I think.

Feathertouch Crayford: 1.7mm per fine turn

Feathertouch SCT microfocuser: effectively 3.0mm per fine turn (i.e. 3mm movement in focal plane per fine turn, as it's the mirror that gets moved).

Badder DS: 2.1mm per fine turn

Hope that's helpful, Magnus

Edited December 5, 2022 by Captain Scarlet

[vlaiv]

It looks like I'll have to design reduction as standard.

I'm trying to assert if 3d printed rack and pinion system is feasible.

Problem is precision and I can't print gears of very small diameter. From what I've gathered online - lowest that I can go is module of ~1.

For pinion gear if I say use 9 teeth, that is going to be 9 * 1 * pi = ~28.3mm per turn (sort of inline with that SW focuser). However, I'd like to use that on slightly faster scope 70/500 comes to mind as potential next DIY scope, so I'll probably add 3:1 reduction by default so I get 10mm per turn - that is right between coarse and fine focus, and might work good?