×

INDI Library v2.0.6 is Released (02 Feb 2024)

Bi-monthly release with minor bug fixes and improvements

Temperature and Altitude (residual) focus compensation

  • Posts: 398
  • Thank you received: 117
Thanks for the BL report; I'll definitely check mine for comparison. What focal ratio are you operating at? It would seem that 84 counts on an EAF shouldn't produce that visible of an impact unless you're operating at low f ratio....but it's an interesting data point....so thanks for commenting!

About poly focus alg, we totally agree. I can't use it at all.... In that sense, the linear alg is head & shoulders above the others. I do think there's still room for improvement however, even given how much better it already is.
Last edit: 3 years 2 months ago by Doug S.
3 years 2 months ago #66276

Please Log in or Create an account to join the conversation.

  • Posts: 1009
  • Thank you received: 133

That's on a Sharpstar 140, F/6.5. So not fast at all. 1 step corresponds to 2.8μ movement, so 84 steps is 0.235mm. That is a R&P focuser, maybe it just has a coarse gear ratio.
IIRC, around 30-40 steps of that BL are internal of the EAF, based on various forum posts here and there.
3 years 2 months ago #66277

Please Log in or Create an account to join the conversation.

  • Posts: 398
  • Thank you received: 117
I'm going to need to think about what you've said. Using the CFZ formula from here (www.goldastro.com/goldfocus/ncfz.php), it seems you should not be able to tell any difference in 20um of change if already well positioned in the CFZ. Your CFZ size for 1, 1.5, 2, and 3 arcsecs seeing (10% tolerance) is 42um, 63um, 84um, and 126um respectively. I guess the real question is how did you arrive at 2.8um / step for your system? We're both using EAF. My Celestron focuser thread pitch is 750um/rev (0.13um/step). You've calculated 21x coarser threads (seems hard to believe, but I couldn't find the thread pitch for your focuser to confirm). Were you forced to not use the 10/1 fine focus on your setup, or have I screwed up somewhere in the calc?
3 years 2 months ago #66278

Please Log in or Create an account to join the conversation.

  • Posts: 1009
  • Thank you received: 133
Doug,

I'm indeed not using the 10:1 fine focus. The reason is that those are friction-driven(*), and, like crayfords, are sensitive to slip. One can usually hold/fix the coarse knob on the opposite side and then turn the fine knob, and will be able to move it. I think I even read somewhere in a manual (maybe from the Pegasus? don't remember) to explicitly not use the fine gear for motor focusers.
And if the CFZ is around 50μ, a step width of 0.13μ sound like clear oversampling, isn't it? :D

(*) at least those that I know; they use an outer cup, an inner axis, and three steel balls as "gear". Don't know if there are others that have a real planetary gear for this.
3 years 2 months ago #66280

Please Log in or Create an account to join the conversation.

  • Posts: 398
  • Thank you received: 117
Now that you mention the 10/1 avoidance, I think I read that too somewhere. Still, that's a big thread. 0.13um for my setup is more resolution than I need, but I preferred this to the Celestron offering which was only ~1/5th (or less) the resolution. I wasn't thinking about HFR instability on V curve fit when I did this.....but regardless, I'm still happy with the EAF (or will be until I measure the BL).
3 years 2 months ago #66282

Please Log in or Create an account to join the conversation.

  • Posts: 398
  • Thank you received: 117
I can now confirm DerPit's backlash comment. My EAF has tested today @ 95+/-5 counts of backlash. I definitely didn't expect that much! This has likely added spread in my dataset, reducing sensitivity, but even so, the story is pretty clear. It should be easy to tune and get a tighter dataset from here forward. I guess focuser BL tuning should be considered mandatory for low f-ratio systems!
3 years 2 months ago #66286

Please Log in or Create an account to join the conversation.

  • Posts: 398
  • Thank you received: 117
@DerPit: Re: "One thing that keeps me from 'blindly' adjusting focus based on temperature is focuser backlash. How do you handle this, especially how did you measure it (assuming you do have some)? "

I wanted to return to this question just for a moment (and thank you for helping me to get my BL sorted out). I've measured my BL now and set it in the INDI control. I've also verified reversal behavior is as sensitive as expected. For very short f-ratio setups, I can see that BL tuning is very important (but less important as f-ratio increases). Using the INDI BL control settings, it seems BL can be well managed. Am I missing something more important from your question?
3 years 2 months ago #66370

Please Log in or Create an account to join the conversation.

  • Posts: 1009
  • Thank you received: 133
No, I think things are sorted out fine. Especially, for you the effect of the BL might be much less than for me, as you use the 10:1 fine control. It seems most of the BL (also mine?) is intrinsic to the EAF. In itself a good thing IMO, as then it very confined and one should be able to reproducibly correct it.
Maybe just one note: Watch out for drifts in your focus position that could indicate issues from creep using the 10:1 drive.
Cheers,

Pit
3 years 2 months ago #66375

Please Log in or Create an account to join the conversation.

  • Posts: 398
  • Thank you received: 117
Hey Pit, FWIW, I'm not using the 10/1 either. The EAF instructions for the Celestron were to remove that part of the focuser assembly and use the main focus shaft. So, all should be well, although I will watch to see if I need to be more careful about how I position before powering down so I won't screw up my focuser dataset going forward. Hopefully my dataset will tighten up a bit now and give my elevation residuals a better fit.
The following user(s) said Thank You: Peter Sütterlin
3 years 2 months ago #66376

Please Log in or Create an account to join the conversation.

  • Posts: 398
  • Thank you received: 117
I have been busy implementing what I'm now calling Adaptive Focus Control (AFC) in kstars/ekos in the last few weeks, and I'm happy to report that a version of this is now running! I have tested this both on a laptop Ubuntu setup (with remote INDI on Pi4), and also on a standalone Pi4 development env. My test focuser (ASI EAF) is absolute, but all currently supported focusers should work. I've tested this with real HW, but only off-sky thus far (cold box conditions). A multi-night on-sky test is planned in early March.

This message is a heads-up for anyone interested in Autofocus "seed" start positions or "between exposure" focus updates to turn on verbose level focus logging to collect needed temperature and elevation focus sensitivity characterization data (already implemented - see OP for details). That data, and a reasonably well-behaved focuser (possibly with backlash setting if needed?), is all that will be required to take advantage of the new AFC features.

I could use some beta test feedback from a volunteer or two who have their own kstars/ekos development environments. I am already testing very short f-ratio (f/2.2), but could use feedback from longer setups (f/4, f/7 preferred, but even f/10 would be of interest). The new code is compatible with all current focus capabilities (e.g scheduler switches for autofocus constraints, etc.). If interested, please send me a private message and I'll forward the new files and build / test instructions. The integration is just a simple replacement of a few existing files plus 2 new files. Hopefully, after some beta test feedback, this can be merged with the kstars/ekos baseline in the not too distant future. Stay tuned!...

Cheers, Doug
The following user(s) said Thank You: Jasem Mutlaq, Ken Self, Andrew Burwell
3 years 1 month ago #67520

Please Log in or Create an account to join the conversation.

F7.5 here, would be happy to test this.
3 years 1 month ago #67528

Please Log in or Create an account to join the conversation.

  • Posts: 269
  • Thank you received: 53
I'm happy to give it a try. I am F5.5 with an Optec TCF focuser. I'm interested to see how it goes with a non-linear temperature compensation, between images corrections and altitude correction. I can compare with the built-in linear, continuous corections.
The Optec has a temperature probe. Can I read the temperature from that probe and does the temperature property need a specific name to be usable?
3 years 1 month ago #67532

Please Log in or Create an account to join the conversation.

Time to create page: 0.629 seconds