Wobbly focus - star selection?

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ChrisR Oz
Posts: 158
Joined: Tue Sep 28, 2021 11:03 am
Location: Melbourne

Wobbly focus - star selection?

#1

Post by ChrisR Oz »

Hi Robin,

I also had problems with the Focus Assistant lately (v 8395). It gives wavey looking "curves" using the MultiStar FWHM measure. I played with various setting, check for saturation, changed exposure and gain, all to no significant avail.

Today I had Sky Simulator (w/ Telescope Simulator, Focus Hub + ASCOM Focuser) running for the Plate Solve issue and thought I'd run a focus series on this ideal simulator (set focuser backlash to only 1 count). And "low and behold", I get the exact same wobbly focus curves (should focus at 25000) ...

Sky Simulator focus curve
Sky Simulator focus curve
Focuser issue 1.JPG (48.3 KiB) Viewed 260 times

Previously, when playing with the Sky Simulator, the focus was more text book, even with a little backlash added to the simulation. This time it's pretty terrible. Seems like a (recent) bug to me ...

Watching the simulator frames pop up, I noticed that the choice of stars changes with each frame (about 12, but varies by a few). I have noticed that FWHM measures often show smaller values for weaker stars. Hence, if you add a few weaker stars to the "average" it will probably drop a bit. This may (in part) explain the "wobblies". Is there a way to lock the selection (aside from tracking a bit of movement) to avoid this random error source?

Cheers, Chris.
Celestron EdgeHD 8, reducer 0.7x, Star Sense, CGX-L mount, Focuser, CPWI; Starlight Xpress AO, OAG and Filter Wheel; ZWO 294MC/294MM Pro and 174MM mini; SharpCap Pro, PHD2, Powermate 2x, Baader Neodymium, Astronomik CLS-CCD, ZWO UV/IR, Duoband filters.
ChrisR Oz
Posts: 158
Joined: Tue Sep 28, 2021 11:03 am
Location: Melbourne

Re: Wobbly focus - star selection?

#2

Post by ChrisR Oz »

... ran another simulated focus scan and watched carefully which stars were selected, and used the red box to restrict it to 6 stars mostly. But for some frames, one particular weak star dropped out, thus using just 5 stars. These consistently showed higher FWHM by about +0.1 to 0.2.

Granted, this is quite a small focus range.
Cheers, Chris.
Celestron EdgeHD 8, reducer 0.7x, Star Sense, CGX-L mount, Focuser, CPWI; Starlight Xpress AO, OAG and Filter Wheel; ZWO 294MC/294MM Pro and 174MM mini; SharpCap Pro, PHD2, Powermate 2x, Baader Neodymium, Astronomik CLS-CCD, ZWO UV/IR, Duoband filters.
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Re: Wobbly focus - star selection?

#3

Post by admin »

Hi Chris,

there's no way to lock to the stars being used - SharpCap just selects the stars that are detected in each frame (aside from ignoring those above the 95% max brightness level). What you can try is turning up the sensitivity of the star detection, which should help keep all the stars being detected. As for the values being different for dim/bright stars, the biggest problem is when the peak brightness of the star gets into the range where the camer is no longer linear - that means brightness is lost from the peak, leading to the FWHM/HFD measuring larger. You could try turning down the max brightness to 90% or even 85% to help with that a bit I think.

cheers,

Robin
ChrisR Oz
Posts: 158
Joined: Tue Sep 28, 2021 11:03 am
Location: Melbourne

Re: Wobbly focus - star selection?

#4

Post by ChrisR Oz »

Hi Robin,

Linearity using the simulator is probably not an issue in this case. In my case, using the ZWO 294MC Pro, your sensor analysis shows good linearity, and I used the new photometry tool to show that the stars were about mid-way in ADU range.

Cheers, Cris.
Celestron EdgeHD 8, reducer 0.7x, Star Sense, CGX-L mount, Focuser, CPWI; Starlight Xpress AO, OAG and Filter Wheel; ZWO 294MC/294MM Pro and 174MM mini; SharpCap Pro, PHD2, Powermate 2x, Baader Neodymium, Astronomik CLS-CCD, ZWO UV/IR, Duoband filters.
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