Heads up on using ZWO EAF with RASA/C8 SCT

[mldee]

Just got an ASI EAF with the ZWO-designed mounting bracket for the C8. Got it all together and then realised that the ZWO firmware only allows 60 thousand "steps", which translates to about 10 turns of the C8 focus shaft. Maybe 65 thou if you change the default setting. Found some entries on Cloudy Nights saying this was real, and a headache, as end to end focus on the C8 is about 47 turns or such. This means the EAF will only adjust over about 1/4 of the range needed

Go search CN for more detailed descriptions of this and other users' complaints and suggested solutions, etc.

Apparently ZWO says to preset the SCT manually to approx best focus before tightening down the coupler and then use the EAF as a "focus fine tuner". Depending on whether you're Hyperstar, .63 reducer, or full bore, the basic focus position will first need to be set. Bit of a PITA, but hopefully will still allow full autofocus function in NINA etc for each mode of operation. You can also reset the software counter to zero and then go through another iteration of 60K steps, then rinse ands repeat, plus there's a "reverse" option. This is what I did this morning. Slow but works.

I did get mine into good focus this morning on some local cross-valley scenery and will try autofocus with NINA next week when I have some time.

I'm a bit disappointed in the lack of mention of this limitation by vendors and in the couple of Youtube videos that describe EAF + SCT install, so just be prepared for a bit of head scratching while setting up the focus software. Google is your friend.

[Dave882]

Thanks for posting this. I’ve got the same setup and just waiting for a bracket to install it. Very interested if anyone’s got a workaround for the issue

[The_bluester]

While a bit of a pain initially to set up, 10 turns of the shaft is heaps of travel for an auto focus routine, you don't need anywhere near the full travel in use. Focusing my SCT with Voyager and a Celestron focus motor it was only going a couple of hundred steps either side of focus (The Celestron motor only had 1000 steps per rotation instead of nearly 6000 like the EAF) 5 turns away from best focus and I doubt it would even detect any stars.

I would get everything ready for the focus motor, get it in focus manually then move the motor to about 30K steps via the driver before fitting it so that best focus should be around half travel for the motor. The only time it would be a pain after that would be if you change backspacing for some reason, but you could do the same thing, set the focus motor to mid travel then remove it and focus manually before you re fit the motor. You would only need to do it once each time you change the setup. And after that you can drive the focus motor to 30,000 at the start of a session and know it is going to be close to focus.

On the C series you need to make sure to use backlash compensation, and in the case of the EAF I would use a couple of thousand steps (Half a turn of the focus shaft or so) and set the backlash direction so the final move is always to move the mirror up the baffle tube towards the corrector plate against gravity. That way the focuser assembly is in tension after each move otherwise the mirror will probably move down the tube a little over time which will impact focus and probably elongate stars too due to how the Celestron focuser works.

[evltoy]

I installed my ZWO on my C8 last week. I did also notice that the max steps you can only do on the EAF is 65000 steps or 11-12 turns. Each full turn is around 5775 steps.

the 8" SCT does 42-43 turns.

With my Planet setup (ASI462, ADC) I need to be 19 clockwise turns in to be in focus.

What I did here was go 18 turns in, installed the EAF and set the counter to 0 as I dont require to go less than 18 turns for focus.

Now I have a 2x Barlow coming and will add this to the image train. I expect I will need 22-23 turns in, which is within the 65000 steps the EAF can do.

One thing I have yet not done is setup my Nikon 600 DSLR on my SCT. I expect that I will still fall within the 65000 steps or at worst, I may need to hard set my SCT to 16-17 turns then reset the EAF counter to 0…. Very simple process.

The initial setup is very simple, but to fine tune the EAF to all your combination does take a little time. The good thing is once the hard work is done you note down the step number for each combo and you can always be very close to focus on each of your setups all the time.

Cheers
Wayne

[AnakChan]

This issue was highlighted to me this past weekend too by Joshua Bunn who's doing some custom work on autofocusing my Pentax 125SDP. This is a major disappointment by ZWO.

Aside from the ZWO-proposed workaround of manual rough focus and EAF for fine AF, has anyone had a conversation with ZWO on firmware upgrades to improve this limitation?

[evltoy]

I personally don’t think it’s a firmware limitation. it might be the amount of allocated physical memory they have put in the boards. Due to its high resolution the memory on the board can only have 10-12 turns on an sct.

There is maybe two ways zwo could resolve this;
Decrease the current resolution to allow more turns but not as fine on focus (firmware update)
Or increase the memory to allow more turns with the current resolution.

Saying all that…. I could be Absolutely wrong on my thoughts.