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25 Jul 2018
Glad to announce of release of INDI Library v1.7.4 on 2018-07-25. A few drivers were added in this release as we continue to improve & stabilize existing drivers.
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TOPIC: Precise autofocus

Precise autofocus 2 months 6 days ago #28408

My FSQ-85 is very sensitive in terms of focusing. Even slight defocusing ends up in oval stars in the corners. Fortunately, the direction of the ovals indicates in which direction I need to shift: tangential ovals are shown in the extra-focal position, radial ovals indicate the intra-focal position of the focuser.

Currently, I use the autofocus routine only for coarse focusing. Afterwards, I need to correct the focus manually until I see round stars in the corners. With the autofocus routine I am not able to reach a focus point that is precise enough.

Any hints, which parameter setup I need to choose for the EKOS autofocus routine so that I can avoid a final manual fine tuning?

- Wolfgang

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Re:Precise autofocus 2 months 6 days ago #28409

I'd say you would need a flattener to avoid the non-planar issues. But I've never worked with such a high-end product :)

I tried the full-frame autofocus procedure for this. But it's difficult to converge with poor seeing, thus tolerance must be increased. If you increase exposure time you could probably get better HFR stability (stacking did not make a difference for me). Also you risk focusing on the target and joining close stars as a single blob when exposure time increases, because indeed the system is out of focus. Full-frame also has the annoying issue that it will not return a mean HFR (or perhaps it is something else that I did not understand).

I don't remember if autofocus keeps a star target, that you can pre-select, over full procedures. Even so for one procedure you may choose a star half-way through the field and have the procedure keep it over iterations. That will round the planar issues between center and extremes (if what you have is actually a planar issue, again that's perhaps not the case).

I think we should have two additional features in the focuser module: a HFR map providing feedback on the flatness of the field, and a ring-limited source selection, that chooses a star or stars in a parameterized annulus on the field of view.

The following user(s) said Thank You: sterne-jaeger

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Re:Precise autofocus 2 months 6 days ago #28410

The FSQ-85 has an integrated flattener. The problem is, that the flattener requires a perfect focus. A minor de-focus leads to oval stars. But with exact focus, the result is impressive.

OK, I will give it a try with longer exposure times. The problem is - exactly as you describe - finding an acceptable low tolerance with the given seeing. Currently 5% works fine on my TSA-120, but this scope is much more tolerant.

- Wolfgang

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