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).
Sky-Watcher Black Diamond ED80
Meade 8" LX-90
Sky-Watcher EvoGuide 50 (guide-scope)
ZWO ASI290MC (non-cooled primary camera)
ZWO ASI120MM mini (guiding)
Canon EOS 7D
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.
TSA-120 + FSQ-85 + GSO 150/750 | Avalon Linear + M-zero | Moravian G2-8300 + ASI 1600mm pro | KStars/INDI on Raspberry Pi 4 with Raspbian 10
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.