Best focuser options for the UC-108 telescope - Forums

Topic starter

Jun 4, 2026

Alternative Focuser for the William Optics Ultra-Cat-108?

Hi all;

What is the best focuser you have found for the U-C 108?

I have used both the PrimaLuceLab SESTO SENSO 2 which I found slipped and the the ZWO EAF, (older model). I am not happy with the amount of backfocus I have to work into the Hocus Focus formula as well as the lack of repeatability. If 3822 gets me 1.67 HFR, I would expect, a second run within a few minutes would be in the ball part but that is not the case with these focusers.

I am looking at the ZWO EAF Pro, (just came out), attached to the U-C using the belt based system provided by the William Optics EAF Mounting Bracket.

This gets rid of that adapter that connects the two shafts, (EAF to WO).

Can anyone provide some feedback on their best focuser experience and what they are using and their experience with the belt based mounting bracket if your using it?

Thanks in advance.

Bob

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Brian Puhl

IOTD Staff

Jun 4, 2026

ZWO EAF shouldn’t have any issues. Ditch Hocus Focus and go with regular NINA autofocus routine. It’s miles easier to program and gives repeatable, reliable results every time.

Also, what do you mean backfocus you have to work into the hocus focus formula? Back focus is static. Do you mean back lash? Back lash is a simple mechanic, you can put any number into the equation, and as long as it’s bigger than the actual physical backlash, it will work. For example, you only needed to put in 100 probably, but you put in 200 anyways, just to be safe. If you’re not getting repeatable results and the focus point is shifting consisistenly in one direction, you probably haven’t tightened down the collet properly and the shaft is slipping.

Unless theres some unusual mechanic about the Ultra Cat, I would stick with the EAF. It’s a very good, simple focuser. You gain nothing by upgrading to a newer one.

Feel free to switch over to the default NINA autofocus routine, give it a run. If you can’t get it dialed in, post a screenshot in here and I can give you settings to get you close. I suggest bin2 which will make things run faster, should allow 1 second exposures, and give you wider sample of the focus range.

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Bob Anderson

Topic starter

Jun 7, 2026

Hi Brian

Thanks for the response, I’m traveling right now and will be back home tomorrow. Will increase back lash from current 60 to twice that and see if it helps improve the shape of the curve and get a better focus. Autofocus Step size has been tested in the 30 to 50 range so that I have high end of the curve around 5 times the target. So if the skies can support 1.65 HFR, then getting to 5 times that has been my objective to get a good curve.

Too often, I look at the final position and know I need to move the focuser manually one way or another to get best focus.

May I ask what Curve fitting strategy do you recommend. Parabolic, Hyperbolic or one of the Trends?

Thanks in advance.

Bob

Bob Anderson

Topic starter

Jun 9, 2026

Hi Brian

I had a an ideal night for testing, (high clouds, barely see the stars but cameras can see through).

Made another run through all the fittings to make sure no mechanical issues. Rock solid.

First, made the mod you recommended on the Backlash. set at 120, (Out), delivered excellent lines for the V-curve. Verus before, I was not running out the backlash.

Second, kept kept the curve fitting to Parabolic. I tested Hyperbolic and found no benefits.

Redid a number of focus runs, complete repeatability each time, with in 2-3 micro steps. (eg: 3345-3348).

Did a couple of Aberration Inspector runs, worked out correctly as fed the data into the Wanderer ETA which provides for electronic Tilt and Back focus adjustments. Worked perfectly. Love it when I am in the 0.01 to 0.10 tilt range on ASTAP looking at a just captured image.

Brian, thanks for the advice, most appreciated.

Bob

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