I can't trust my EAF... Help...

AlexN · #1 09-07-2024, 02:01 PM

So You'd think throwing an EAF on a scope would just resolve all your focus issues, right... You'd just tell your imaging software to refocus every couple of hours, or on > 2°C temp shift, or on 10% HFR increase and all your subs should then be in focus... That's the plan...

So, throwing the 3rd axis balance issues that plagued me for ages (2.4kg telescope with 400g of off-axis weight attached to the rear left of the scope) aside, here's the situation....

I run an AF routine with focus absolutely spot on (bahtinov mask confirmed), the v-curve looks reasonable, perhaps 10 steps off on the backlash give or take, with a slightly flattened edge to the right hand side of the v curve. I usually get 2 points that are roughly around the critical focus zone... Once it gets to its final position, the HFR will be worse than original focus, sometimes as far as 100 focuser steps off.

I test this via first looking at the starting and finishing HFR's being significantly different, then throwing the bahtinov mask on the scope, checking the focus and manually adjusting 8~15 'fine focus movements' on the focuser controller in NINA until the bahtinov mask then confirms accurate focus.

This has been going on since I got the EAF, and it's particularly annoying.
I don't mind manually focusing, if I haven't spent $400 on a system to do it for me.. but having spent that money, having to manually fix focus when I check it after an AF run and the stars look bloated is not what I had in mind when buying an AF system for my scope.

This is the last thing I'm manually having to adjust during a night of imaging, and I'd really love to be able to fire off a target sequence, and then leave it to do its thing...

Anyone got suggestions?

For reference,
The gear is:
Askar 65PHQ (with and without the 0.75x Reducer)
ZWO EAF (5v)

Focuser load is:
Player One Artemis C Pro,
Player One Filter Drawer,
Player One OAG

Maybe 1.5kg total including the reducer.

In NINA, my settings are currently
Backlash set to 60 on the out direction, backlash compensation mode is set to overshoot, step size is 50, initial step count is 3.

Nikolas (Nik) · #2 09-07-2024, 03:23 PM

sounds like the backlash is off there is a very good techniques by these youtube clips you could try out and see.
https://www.youtube.com/results?sear...acklash+on+eaf

ChrisV (Chris) · #3 10-07-2024, 12:10 AM

You have a zwo eaf?
Overshoot is probably the right mode. And the direction of it matters.
I'm sure I've seen mention of using a much larger backlash setting for this eaf. Just look at you scope when you reverse direction, see how many steps it take for the focuser to start moving - then double that for the setting.
You want more than 2 steps wirhin the flattish part of the curve.

AlexN · #4 10-07-2024, 08:41 AM

It's usually 45-50 steps when I measure it manually, so I went with that. I guess I could add more initial steps, and a smaller step size so that it has more points in the critical focus zone...

Hopefully I'll get another clear night this week to make some more adjustments and test things out...

Bart · #5 12-07-2024, 03:04 PM

My two cents worth.

Is it Crayford style or rack and pinion? I found that the most reliable was rack and pinion with a sizeable backlash programmed in.

I have a 3 inch Feathertouch R&P with a SharpSky focuser and controller.

Only time it plays up is when the seeing is too bad to be imaging anyway and it wont get good v curves.

Oh, and set your backlash to be double of what you have and set it to the "In" direction.

The_bluester (Paul) · #6 13-07-2024, 08:13 AM

I would use a couple of hundred steps of backlash, set so that the backlash correction occurs when the focuser is being racked out, not in, and then if you can, make sure that the focuser travels inward when making the final move (With backlash accounted for it does not really matter, but it can make it faster by avoiding extra backlash moves)

Is your average focus position for any given filter/configuration stable step count wise, or is it changing? If it is changing, something might be slipping in the focuser train and all bets are off about focus quality in that case?

Second question, without wanting to start an argument (So I am not going to mention the package) a friend grappled with focus issues for a long time which turned out to be the focus routine in the software he was using, which was not well written. It took quite a while and you could even see the software determine the "Best" position, which it then ignored and the last part of the routine was to move out form that point and then step inwards towards focus taking focus shots as it went. The first shot where the HFR went up, not down it would go back to the position before that shot and stop there regardless of where the best position previously found was. A momentary change in seeing as it worked would often have it land hundreds of steps out of focus and it did it over and over again. Very frustrating.

AlexN · #7 14-07-2024, 10:33 AM

Quote:

Originally Posted by Bart

My two cents worth.

Is it Crayford style or rack and pinion? I found that the most reliable was rack and pinion with a sizeable backlash programmed in.

...

Its a 2.5" R&P... Currently its on the out direction. I have distinct amounts of trouble even manually focusing it, because when I make outward moves, it feels like the backlash compensation brings it back almost to where it was to begin with...

Quote:

Originally Posted by The_bluester

I would use a couple of hundred steps of backlash, set so that the backlash correction occurs when the focuser is being racked out, not in, and then if you can, make sure that the focuser travels inward when making the final move (With backlash accounted for it does not really matter, but it can make it faster by avoiding extra backlash moves)

Is your average focus position for any given filter/configuration stable step count wise, or is it changing? If it is changing, something might be slipping in the focuser train and all bets are off about focus quality in that case?

Second question, without wanting to start an argument (So I am not going to mention the package) a friend grappled with focus issues for a long time which turned out to be the focus routine in the software he was using, which was not well written. It took quite a while and you could even see the software determine the "Best" position, which it then ignored and the last part of the routine was to move out form that point and then step inwards towards focus taking focus shots as it went. The first shot where the HFR went up, not down it would go back to the position before that shot and stop there regardless of where the best position previously found was. A momentary change in seeing as it worked would often have it land hundreds of steps out of focus and it did it over and over again. Very frustrating.

At this point I'm willing to try anything...

It feels repeatable (ie - the position it determines is almost always ~4 'mini steps' too far out. So I have to manually move the focus position inward in order to get best focus.

I can run 5 AF routines in a row, then throw the bahtinov mask on and guarantee I'm too far out by a handful of steps.