Hi all,

Have a question about the Ekos focus module:

I've had some clear nights and have been shooting data like a demon. I'm not happy with how my stars appear. My focus routine is to use a Bhatinov mask to get close, then fine tune with the auto focus module in Ekos. 2 things bother me. I can't seem to get a v-curve (it is either a straight line or bounces around all over the place.), and my HFR value doesn't drop below 3-4. The other night it wouldn't go below 5. In Jasem's video, he was getting an HFR value below 1. As a consequence my stars appear bloated. Round, but bloated.

I'm using a SW Black Diamond ED-80, ZWO ASI290 (uncooled) & Sesto Senso focusser.

What is other peoples experience?
Is there something I can do to improve my HFR value? note: I've followed the settings from Jasem's video when doing an auto-focus routine.
What s the maximum magnitude I should use?
Does it matter where in the sky I focus? ie, zenith versus lower in the sky

Thanks in advance.

PS - I was shooting 180 second subs, with no filters (I know not he best but am just starting out).

Rene

My stars have been looking bloated, but I think that is something else. And the last time I had the focusing module created large donuts and called it a success (2x too). Really odd. Taking screen shots the next time I'm out. If it's not cloudy nights it's +95 degrees!

First of all you should be somewhere close to the focus. As soon as you get donuts, the focuser creates nonsense.

You have to experiment with several parameters:
- exposure time: if it's too short, you get problems with noise. I typically use 6sec with 1x1 binning
- step size: it should be big enough such that the HFR value changes, but not too far. This depends upon the focuser you are using.
- tolerance: depends upon the sky quality. Start with 10-15%, but less leads to more precise results. I use 5%.
- experiment whether full frame or sub frame gives better results.

The HFR number is in pixels not arc seconds so it will depend on your sampling. For me Its 1.28"/pixel and some nights I get a HFR of 0.9 and some nights 1.5.. Usually its around 1.1 or something like that. It depends on the seeing conditions.
I usually get a sort of V curve. If you dont, then I'd suspect some mechanical problems like slippage or backlash.
Derek

I'd like to add there is no real V-Curve in Ekos, you'll just get half the V-Curve and the algorithm will then oscillate around the critical focus zone.

Many important things mentioned already, though not backlash. Does your focusser have some? Are you compensating for it? Without the AF indeed seems to jump around wildly without success.

Also, if you're careful with the Bahtinov it should already give you a focus as good as the AF can do...