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

Bi-monthly release with minor bug fixes and improvements

New Focus Algorithm in Ekos

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Agreed. Seems that the first pass calculation is really good... then it should move way out (to eliminate backlash) and then back to that point.

One additional thing that seems to happen to me... When it currently moves way out, then in, it doesn't wait for the focuser to hit that "out" position. Which means it doesn't always eliminate the backlash. Perhaps you could put a small pause after it moves out... before it moves back in?

Does that make sense?

Ron
1 year 11 months ago #82621

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Replied by John on topic New Focus Algorithm in Ekos

Hi Ron,

Thanks for the reply. So what happens with Linear is that whenever it moves outward it does the "backlash hokey-cokey" by moving "5 * Step Size" further outward than needed, then when that completes, it moves inward by "5 * Step Size". Step Size does get changed by the algorithm but it will never be bigger than the Initial Step Size field on screen. So, for example, if Initial Step Size = 40, then it will correct for 200 ticks of backlash... if you have, say 300 ticks of backlash, then 100 will be uncorrected.

With Linear 1 Pass there will be 2 ways to deal with backlash:
1. Same as Linear (if on-screen backlash field is set to zero).
2. User entered backlash into on-screen backlash field. The hokey-cokey will be use this value. So in the above example, if your real backlash is, say 100, then set the field to 200 and you'll be fine.

Hopefully this would fix your issue.

John.
Last edit: 1 year 10 months ago by John.
1 year 11 months ago #82623

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Hi John,

I greatly appreciate your approach since I face the same issues with my EAFs. Only "linear" works, but not very reliable in the 2nd pass.

Question about the standard deviation weighting of the fit: The standard deviation will be calculated from all HFR values obtained from the field, correct? The HFR distribution depends (1) on the distribution of the star brightnesses in the field and (2) on local effects (seeing) on individual HFR values. So (1) will be nearly constant for a given focus field, but (2) varies and is a measure for the "quality" of that particular focus frame. Or am I making a mistake here?

Nevertheless, I would be very happy to test your routine!

CS, Bernd
1 year 10 months ago #82631

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Replied by John on topic New Focus Algorithm in Ekos

Hi Bernd,

Thanks for replying. On the standard deviation:
1. If single star mode is selected, the SD option is disabled.
2. For multi-star... the stars go through an extraction process to find suitable candidates (I haven't changed this). There are several options but if you use the default SEP there are many parameters to control this, for example removing clipped stars and those below a certain brightness. After this process has run there will be a set of stars that meet the criteria and each has its HFR calculated. I then calculate the SD of this set.

The star set will vary from frame to frame and there is some existing work in Linear to try and use the common subset of stars for consistency. I am trying to use this in Linear 1 Pass, but there is still some work to do. Variation in brightness of the stars should not affect the HFRs too much (assuming the extraction and processing parameters are set reasonably).

The variation in HFRs will depend on focus effects, seeing, etc. but I think its a reasonable measure of "quality" for the focus point. What I've observed on my equipment is a trend whereby SD increases with distance from optimum focus but then certain points deviate from this with transient effects like seeing, wind gusts, etc etc.

As to whether applying weighting makes enough of a difference to the calculated optimum focus point to be noticeable, I don't know. We'll just have to give it a try.
1 year 10 months ago #82633

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Thanks for the explanations, and your project sounds really promising, I'm very curious!

Regards, Bernd
1 year 10 months ago #82636

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John,

Thank you. I understand the process... but in my case, when it does the "backlash hokey-cokey" it moves out then in so quickly, that I don't think the focuser has reached the full "out" setting. I think it would benefit from a move out, pause 1 second, move in... to assure that it getting the full effect.

Thanks,

Ron
1 year 10 months ago #82639

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Replied by Doug S on topic New Focus Algorithm in Ekos

Hi Ron,

A couple thoughts for you. I like the approach, it has a BIG advantage over the current linear alg in that it doesn't "pull up short" which has previously been described by prior posts on the subject.

My only suggestion is to consider whether you want to make the same mistake Linear Alg made in the Mechanics tab. Picking an "Initial Step", and "Out Step Multiple" has created a lot of unnecessary confusion for folks. In reality, the "proper" way to define the step size and out step multiple is to calculate those parameters from other system info as it relates to CFZ. Knowing the focus drawtube screw pitch and motor scale would ensure the correct step-size is used. Alternatively, not knowing these parameters creates the conditions for step-size exceeding the CFZ (jumps over), or too small (wasting focus time). This is completely avoidable. You might want to consider at least having an option to calculate the step size, correlating to CFZ, motor scale, and focuser thread pitch size. See the following info which might help:

www.goldastro.com/goldfocus/ncfz.php

Cheers, and good luck, Doug S
1 year 10 months ago #82642

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Doug,

Thanks for the thoughts. I have read this article and find it interesting. I guess that I have used "trial and error" for my different scopes and focusers.

Like others, I find that the "one pass" does a pretty good job at finding the best focus. The second pass process, while theoretically valuable, does little to improve the focus. In fact, it usually was the cause of creating an error, aborting and restarting the process.

I'm looking forward to using John's new process!

Ron
1 year 10 months ago #82644

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Replied by John on topic New Focus Algorithm in Ekos

Thanks Ron, I'll take a look. I see you have a Celestron Focus Motor and ZWO EAF. Do you think both have this issue?
1 year 10 months ago #82645

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Replied by John on topic New Focus Algorithm in Ekos

Thanks Doug, I'll take a look at this.
1 year 10 months ago #82646

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John,

I only use the Celestron focuser with the linear method of my Celestron 8HD Edge. I'm not sure that I would describe it as an issue with the mechanics... the current process just doesn't give the focuser time to complete the "out" movement before it issues an "in" command. It may be working fine and I'm simply experiencing what Doug is describing. However, when I'm sitting at the scope it sounds like it starts reversing before it finishes the out movement.

This is one of those situations where I would think it wouldn't hurt to have a very short (1 sec) pause between commands.

BTW... I'm not fond of the Celestron focuser. I sometimes experience connection issues with INDI... seems unstable. I've ordered an ZWO EAF to replace it on the 8HD Edge, but have been waiting several weeks for the back order to clear.

The ZWO EAF on my refractor works perfectly with the polynomial process.

Thanks for your effort!

Ron
1 year 10 months ago #82647

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Replied by Jim on topic New Focus Algorithm in Ekos

Would it be possible to have the system determine what the step size should be in some way each session? The starting step for determining the final step size could even be calculated from the user's equipment configuration. Store this value and use it as the seed for the next time an auto-configure of step size is run.
The following user(s) said Thank You: R Dan Nafe
1 year 10 months ago #82655

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