Electronic focuser in use. problem?

[Jackstar]

Hi,


I have a general question regarding using an electronic focuser.

I have one that does an annoying thing.

With this model, to calibrate, you set a "0" point where the focus tube is racked in.
You then send it to the outermost point and set that,

Lets say this point is "150000"


Now, what happens is:
i send it back to "0", it goes 2mm short of "0"

i then tell it to go to "120000" it goes 2mm beyond .
i then tell it to go back to "0" it goes 4mm short
etc etc.


To get around this when it goes back to "0" i then adjust the focus knob to take up the 2mm, then i have to re calibrate the "0" with a click on the software.


This is the annoying bit.
Is this normal with any of the electronic focusers?

I have tried leaving it out at the point of focus but then when i connect on NINA it seems to think it is set at "0" when it isn't.


Thanks


Jack

[bojan]

I do not use commercial electronics focuser, but from what I know about encoders, the behavior you are describing is certainly not normal.
It looks like your encoder is missing pulses..

[The_bluester (Paul)]

Is the focuser a crayford style? If so is it slipping as it is racked inwards?

[Imme (Jon)]

Slip or backlash in the focused mechanism I’d suggest

[The_bluester (Paul)]

I was thinking slip, I would expect if there was backlash it would return to a different position the first time, but then it should continue returning to the same (Wrong) position after that rather than creeping further and further away from the original zero position.

[Jackstar]

Thanks for the replies so far,

Is the focuser a crayford style? If so is it slipping as it is racked inwards?

It is the focuser on the Orion eon 115mm


"Rock-solid, rotatable 3" dual-speed (11:1) Crayford focuser provides precise focus control and stable support of imaging accessories"



With regards the slippage, there is a tensioner on the telescope to put pressure on the mechanism, would you say this needs slackening?



I will look into the slippage/backlash, they sound like things i can fix or deal with.




Thanks Bojan for the suggestion on the encoders, hopefully it's not those but i now have them in mind.


Jack

[Imme (Jon)]

If 15000 is fully racked out what happens when you go 0-7500?

Still 2mm?

[Jackstar]

what happens when you go 0-7500?

Imme,



I will test this tomorrow and get back on here, the max is closer to 190000 but i don't go out that far as focus is around the 120000


Jack

[Merlin66 (Ken)]

Jack,
For those of us not familiar with all the different electronic focusers, can you say which one you’re using...

[iborg (Philip)]

Hi

The encoders I am familiar with have holes for optical sensors to count. If yours has the same type of setup, has one (or more) of the holes become blocked with muck?

Philip

[The_bluester (Paul)]

To fill out on what Jon has written, if the maximum number of steps you have set would rack the focuser past the fully racked out position, being a crayford style it will slip once the end stop is reached, until the focus motor stops moving. That would effectively move the "0" point.

You probably do not need the focus motor to run all the way to the out travel limit, if for instance your gear is in focus racked 20mm out from fully "In" then you can probably set the step limit to stop it perhaps 5mm further out than that. But you should certainly have it stop a few mm before the "Out" limit of the drawtube. If it was a rack and pinion I would say keep it from quite reaching either physical stop to be sure it can not try to drive it into a hard stop.

[Jackstar]

Thanks all so far,


The electronic focuser is the sesto senso 2.


i avoided saying it initially as i wanted to know whether this was something normalish or whether i needed to look specifically at my equipment.


It is seeming like it is something to do with how i have it set up or running.



It is pretty new and well covered when not in use. I would discount dirt.


I will work on those issues discussed so far, this will probably take me a few days between work etc.


just to clarify the term "slippage" is this associated with the way it is installed, motor running but not fully engaging? therefore i should uninstall it from the focuser and start from there?


thanks again


jack

[The_bluester (Paul)]

Actually I realise I have got one thing backwards a bit, if you racked the focuser "Out" more steps than it actually takes to get all the way out, it will then be further "in" than expected.

Crayford focusers slip by design, if you rack the focuser all the way in or out, you can keep turning the shaft and it slips on the drawtube as they are friction driven. The trick is to ensure that you have enough tension on the shaft that it does not slip when racking the focuser in (As that is normally against gravity) or slowly slip downward with gravity when the shaft has not been moved, bit not so much as to make it hard to operate.

If there is a focus lock/brake screw, this needs to be loosened right off or it will drag and make the focuser slip. There will prbably be other tension screws that set the tension of the shaft against the drawtube but I would have to look up the Eon, I am not familiar with it.

The other end of the scale is a rack and pinion focuser, they do not slip, but if you drive them past either travel limit you can damage the focuser or the focus motor!

[The_bluester (Paul)]

Page 3 of the manual here https://www.telescope.com/assets/pro...9513_06-15.pdf

Figure B shows the drawtube lock knob (The bigger one) and the tension screw (The machine screw half hidden by the lock knob) The lock knob needs to be fully released and the tension screw just tight enough to ensure that the drawtube does not slip when focusing inward and the scope pointed straight up.

[Jackstar]

Thanks Paul,



That is excellent information, i will look at this first.
I have been reading up on the slippage and see that it can come from various points.
When i installed it, i used the bush/coupler that fitted the best, but i may go back and recheck this area.


thanks again


Jack

[Jackstar]

Update on my slippage and focuser.


I have spent a couple of hours today going through all of this in the daylight.
I am happy that the motor unit is installed correctly and that the slippage left is not too much of an issue.
using the adjustment screw identified underneath the focuser i have got it to almost be 0 slippage.

I now understand the design of the crayford focuser a bit more too having done a bit of reading.



Thanks all for your help with this..Jack

[The_bluester (Paul)]

Good to hear, at least with a crayford type you can re calibrate it pretty easily if is slips. Set the focus motor to zero and then if the drawtube is not quite racked all the way in a gentle push to the stop and it is lined up where the focus motor thinks it is again.

[SB (Chris)]

Hi Jack
I had the same issue initially with my ZWO EAF but solved it quickly by calculating backlash and entering this into the focus settings.

Chris