beachbum Hey sorry, this forum doesn’t send notifications unless to log into it to see if there new posts. But basically Chen nailed it, and I would defer to his answer as he is the resident expert volunteer here. Any replies you get from ZWO will be canned responses.

For what it’s worth I have a small refractor, a 72mm. The whole focuser has a travel of 9cm (aka 900mm) which take 21,000 EAF steps to rack from one end to the other, so basically 23steps/mm. I set my AF step size to 50 steps and exposure to 3 seconds. So at the start of the AF routine my confuser moves out about 1cm out and starts moving back taking exposures building the focus curve up to 1cm past the starting point. This amount of movement, in my case, ensures that fine focus will be found somewhere within the range covered by the EAF.

i suggest you change the step to something ridiculous like 200 and then run the AF routine manually and watch your focuser just to verify that it runs. Once you verify it does run, run a sequence during the day just to verify APT actually triggers the AF when it’s supposed to. Once you have verified both of these are happening it’s only a matter of finding the right step size and expose time (don’t leave it at 200). But as Chen mentioned none of it will work if your scope wasn’t pretty close to fine focus to begin with.

beachbum one guy said 20 did not work for his focuser. He changed it to 40 and it worked. I was going to do that next go to a 40...

Mabuhay Rodney,

On my two Takahashi scopes' rack and pinion focusers, the EAF moved the drawtubes by about 4 µm per EAF step (so, about 250 steps per mm of drawtube movement).

On a belt driven Askar ACL200 lens with the same EAF motor, the lens only moves by 0.44 µm per EAF step. So over 2000 steps just to move the lens just by 1 mm.

If you'd attached the EAF to the slow motion shaft of the focuser, you are likely to see something closer to the Askar lens than to the R&P focuser on the Takahashi.

The number of steps wil also depend on your focal length. In general, the depth of focus is shorter with shorter focal lengths. So there is really no one-size-fits-all solution. You just have to figure it out for your own case.

The HFD I mentioned is the "Half Flux Diameter" of a star; a measurement of how large the star is in pixel units. There should be something that your program should be able to measure for you. It pretty much corresponds to the vertical axis of that hyperbola curve in the screen capture that you posted.

When you focus on a star, geometric optics predicts that the star size will change linearly with focuser travel. So, HFD will change linearly with EAF steps. However, as you approach focus, the star size becomes diffraction limited and focus no longer improves as you get to perfect focus. The diffraction limited region is the Critical Focus Zone of the optics. A good match to the HFD data points is a hyperbola; so that is what is used to estimate the actual focus without having to land on the exact focus while you are building that v-curve.

The one thing that you need to watch out for is the backlash from these stepper motors if you manually focus.

Because of backlash, most of the autofocus programs I know first moves the drawtube in one direction away from the focus point and then step the focuser only in one direction as it measures the star sizes.

Once it is done, and it can now fit a hyperbola curve on the star sizes to determine the EAF position needed for perfect focus, it then again moves the drawtube to that same initial point that it had started before sweeping for the EAF steps. From there, it moves to the final focus setting using the same direction that it makes for the star size measurements. This way, the backlash is cancelled out, and you don't have to worry even about measuring the backlash when using these autofocus routines.

You do need to measure the backlash if you are manually controlling the electronic focuser and rocking it back and forth to achieve focus, otherwise it is an exercise in frustration when looking for focus.

As you said, you will figure it out after you are familiar with how these gizmos work.

Chen

beachbum

I had never been to the Philippines, but had lived for some years in Thailand and Malaya/Malaysia. There are many common words in Malay and Tagalog. Certainly, Selamat is one of them :-). (Useless trivia department: "soap" is "Sabun" in both Malay and Tagalog, and "Sabu" in Thai.)

Clear skies,
Chen