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INDI Library v1.8.1 Released (09 Sep 2019)

Monthly maintenance release INDI v1.8.1

Dialing in focus

4 months 3 weeks ago
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Dialing in focus #38544
What are the most reliable settings you've found with focusing?

I've been alternating between full field, auto select star, and manual star selection. I also typically have it set to redo focus every 60 minutes. But have experienced mixed results with all of them.

When experimenting with manually selecting a star, I have seen the focuser system go from .87 FWHM, to 5.0 FWHM. I can repeat the run on the same star over and over, and get wildly varying results. Sometimes it settles near the bottom end at .87, sometimes it settles in between around 1.4-1.5, and other times it will even get up to 5.0 and stop there. I realize some of this is due to seeing. But in almost every run it would get down low, then go back up quite a bit. Some runs it will settle at the bottom end. But because it's wildly inconsistent having the system refocus every 60 minutes causes nothing but headaches and lots of lost subs due to poor focus.

This past weekend I tried something different. I manually ran the focus module multiple times on a star I picked out, and stopped at the best FWHM result. Then I just left it there, and told it not to refocus during the session. For a smaller scope, this works really well, but larger scopes that are more effected by cooling through the night, you would still need to refocus.

What focuser settings are people getting the best consistent results with?

-Andrew
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4 months 3 weeks ago 4 months 3 weeks ago by Herrhausen.
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Dialing in focus #38547
Andrew,

I 100% share your experience. Ekos does a good job at measuring the stars but for whatever reason often does not settle at the optimal focuser position. Sometimes it settles way off. Normally I do run the autofocus procedure more than once, do additional measurements around what I deem the best focus position and after this validation I select the focuser position manually. I never use autofocus unattended.

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4 months 3 weeks ago
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Dialing in focus #38550
Andrew,
I have a Robofocus system driving a stock focuser on a Vixen/Orion 114ED F5.3 telescope (I bought this scope about 15 years back from Orion Telescopes). My Robofocus is set so that each tick moves the focuser about 0.0005” (half a thousands of inch) with a max travel of 5900 ticks. Over the past two years or so that I have been using KStars/Ekos I have mostly used the auto select star method. I have had good results with that method overall. Lately I have been trying out the full field focus with good results as well. In my system the focus module usually gives me a good type of V curve shape of focus points. I have attached a screen shot that shows a typical curve I get with full field focusing. I get a similar if not quite as good of shape with just using auto star select. As you can see my HFR is 0.95 pixels in this screenshot. This is quite good for my turbulent midwest skies. I usually have around 0.95 (low) to 1.2 or slightly more. The Focus Module has been consistent enough for me that I am able to run all night while refocusing every 60 to 90 minutes during a scheduled run. I usually set my step size to around 50 (or close, it is set at 70 in my screen shot). Perhaps looking at these settings in relation to your system may help.

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4 months 3 weeks ago
wjdrijfhout
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Dialing in focus #38559
Andrew,

I experience similar irreproducible results. Sometimes it works flawlessly, sometimes it just doesn't get there. In most cases where it does not work, focusing on the wrong star is the problem. It selects a nebulous object, a double-star, or just a way too big star. So rescue method 1 for me is manual selection of a small star.
Subjectively I seem to have slightly better results with polynomial vs iterative method.
But I'm also recording my focuser position at various outdoor temperatures and find that the relationship is linear and very predictable. So for my setup, a drop in temperature of 1 degree celsius equals focuser in by approximately 80 steps. Important to keep circumstances similar (system must be in equilibrium with environment and dew heaters need to be in same position.
I do not let the focuser run unattended, and if I'm in a hurry, or experience issues, I just set the focuser where it theoretically should be with pretty good results.

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4 months 3 weeks ago
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Dialing in focus #38560
I've seen those circumstances where it doesn't pick a star, it picks a bright spot in the nebula or galaxy, or it picks one that's too large. But the situation I described above was a controlled scenario where I picked the same star and couldn't get reproducible results. It would vary every time.

I did get a smooth curve every time similar to the above post, but I've also never changed the default settings. So I'm going to try some other settings on the next clear night. Looks to be a week out. But there definitely seems to be something wrong if I can't at least get within a predictable FWHM range even if seeing is off. But in my case, seeing was quite good, yet it still couldn't keep the FWHM low reliably.

-Andrew
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4 months 3 weeks ago 4 months 3 weeks ago by schwim.
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Dialing in focus #38562
I've run into similar problems with inconsistent measures. I sought to solve for variability by increasing the number of frames but this didn't seem to help. I tried pretty much all of the settings but didn't take a truly scientific analysis toward figuring out what worked best or why. Long story short, about the only thing I could find that behaved consistently was the inconsistent behaviour.

What really did help is using full-field mode which I resisted because Ekos doesn't seem to have the ability to use focus mode in cameras that support it. My hesitation was that it would be slow, and it is. However, by constraining it to 15%/50% I've found that I do generally get much more consistent behavior and HFR results far better than I'd seen in the past. Plus, it usually gets to best focus in 4-6 iterations where using subframes sometimes took 15-20 and multiple restarts. So, it may be slower in getting the frames, but the result of getting to focus is way faster. I'm guessing but I think full frame works better due to the averaging of HFR of the stars which has a stabilizing effect for the algorithms. I now let the focus module determine when I need focus, and for the most part things "just work". On occasion it does go wonky, and end up picking a focus that's far enough out that I can see the secondary mirror shadow. Fortunately this is rare and possibly related to bad seeing. I need to look into that at some point.

Details of my present config in case it's useful:

Optics: focal length 1700mm, f6.7
scale: 0.72 arcsec/pixel
step size: 2000 max travel: 5000 tolerance: 1% frames:1
Algorithm: polynomial Detection:gradient
Focus all filters w/ L as they're parfocal enough to get away with it.
No dark frames used, although this might help.

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4 months 2 weeks ago
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Dialing in focus #38639
I tried some new settings a couple nights ago (when I had a break in the clouds) that I was quite happy with. The first thing I corrected from previous sessions was to make the initial step larger than before. I have a Moonlite, and have found setting the step to 10 makes sure that it jumps far enough away from focus that it can truly find the sharpest point. When I had it set to 5 or even smaller, it would not move the focus far enough to find true focus at times. It would center around a false "tight" anomaly such as the seeing being better for that frame or two, and then not move far enough away from that to notice that is was getting better or worse. It is kind of hard to explain, but imagine this, if you have a single star and you take an image of it and get a half value of 2, then you step 2 steps closer focus and get a 2.05, so it moves two steps out and gets a 2.10, it can end up centering on 2, but if it moved 10 steps in first, it has a better chance of seeing actual change outside of atmospheric conditions, and might get a 1.95, which would then cause it to keep searching (correctly). Nothing magic about 10, it just worked well for me most of 2 different nights, so I am leaving it there now.

The next thing I changed was to do the full frame circle with the deleted circle center. I don't recall the percentage numbers, around 50% for the outside circle, and the inside one big enough to block the area around the nebula. The algorithm then selected about 8-10 stars and proceeded to focus. This seems to work really well as it uses the full area around the object to obtain focus without accidentally focusing on part of the object itself. I also used this method for 2 nights, and think I have found my new default settings. A caveat, doing the capture of that much data to focus with is a good deal slower than framing a single star, but I found it to be more accurate as well. I also do 2 frames per adjustment instead of the default 1.

Jim

12" pier with HDX110 using EQMod
ASI 1600 mono/color mains with ASI290MM in off-axis
ASI filter wheel
Moonlite focusers for the sharpening
AT115EDT and 8" Newt for the light
Fedora Linux, 100% INDI

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4 months 1 day ago
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Dialing in focus #39307
Thanks all for the valuable info in this thread. Last night I let Ekos autonomously control my Shoestring FCUSB motor focuser and got very bad results. Focus started off pretty well but 6 h later all stars were donuts. I was using Auto Focus on the entire FOV. I'll revise the settings with your tips and will try to get a configuration that works for me.


Clear skies, Wouter

ASI1600MM-Pro Cooled and filter wheel with a SkyWatcher Esprit 80 ED on a SkyWatcher HEQ5-Pro

ASI1600MM-Pro Cooled and filter wheel with an 8" TS Ritchey-Chrétien on a SkyWatcher EQ6-R

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4 months 4 hours ago
sterne-jaeger
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Dialing in focus #39345
Wouter,
maybe you should also revisit your settings about "max travel". From my experience, when the scope is too far out of focus, the focuser module recognises part of the donut as star and does nonsense.
I have made good experience in improving the focus quality when setting the initial step such that - starting from a good focus position - the focuser reached a position with a HFR of 4-7.
- Wolfgang
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4 months 4 hours ago
sterne-jaeger
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Dialing in focus #39346
Does anybody have experience here with focusing Pezval style refractors? My FSQ-85 shows the special behaviour that in out of focus positions, stars in the edges are elongated. Extra focally, the stars are tangentially elongated, intra-focally, the stars are radially elongated. Having round stars in the edges is a very good indicator for a perfect focus position.

Any ideas how to handle this?
- Wolfgang

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4 months 3 hours ago
wvreeven
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Dialing in focus #39347

sterne-jaeger wrote: Wouter,
maybe you should also revisit your settings about "max travel". From my experience, when the scope is too far out of focus, the focuser module recognises part of the donut as star and does nonsense.
I have made good experience in improving the focus quality when setting the initial step such that - starting from a good focus position - the focuser reached a position with a HFR of 4-7.
- Wolfgang


Thanks Wolfgang. I’ll give that a try as well.

ASI1600MM-Pro Cooled and filter wheel with a SkyWatcher Esprit 80 ED on a SkyWatcher HEQ5-Pro

ASI1600MM-Pro Cooled and filter wheel with an 8" TS Ritchey-Chrétien on a SkyWatcher EQ6-R

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