Motorised Focus Step Size

[Atmos]

Attached my new RoboFocuser to my telescope this evening to calculate how large each step size is. Just as a rough figure I have come up with ~5.2 microns per step.
This is connected to the 1:1 end of the focuser, doesn't fit onto the 10:1 end without getting a new adapter with one end having to be 1/4 of the size of the other.

The Critical Focus Zone on my telescope with the reducer should be ~85 microns which gives ~13-14 steps within focus. In my mind having end 5 steps within focus should be fine but what seems logical in my mind doesn't always relate very well to the real world haha

So putting it out to all you experienced automated astro-imagers, is having ~14 steps within focus fine or should I bite the bullet and push that down to 130-140? To myself it seems a bit like overkill but it isn't something that I have ever played around with :sadeyes:

[AlexN]

13-14 steps should be plenty in my opinion...

[multiweb]

I'd recommend a micro focuser, not coupling it on a 1:1 shaft.

[Atmos]

I had you in the back of my mind when you mentioned that to someone else a while back. Right now it is just a matte of whether it'll work fine without having to buy another part :)

[multiweb]

I had some torque issues on a 1:1 rack and pinion. Another consideration is how much backlash you have and if 14 steps is enough to deal with it.

[Shiraz]

if you have enough torque without the 10:1 (as Marc notes), 14 steps should be plenty - the actual cfz is determined by the seeing more than the scope in many circumstances, so it is probably wider than the calculated value anyway. I also have seen advice suggesting that hundreds of steps in the cfz is good, but can't for the life of me understand why.

[multiweb]

Maybe better sampling? Also more leg room to compensate for mechanical issues, slippage, backlash, etc... and the piece of mind that if your focuser barrel seizes in the cold the 10:1 shaft will slip in the three ball bearing and you won't strip the smallest cog of your motor gearbox.