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INDI Library v1.9.8 Released (29 Sep 2022)

Bi-monthly INDI Library released with new drivers and bug fixes.

New Focus Algorithm in Ekos

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Thank you, thank you, thank you!!! Several of us have been saying for almost two years about the linear auto focus always choosing the wrong focus point. I always had to go back in and move it back to the pre-calculated (from the first pass) position which was ALWAYS better than the spot EKOS stopped at. I baffled me why it go through the trouble to calculated a perfect focus spit, then choose a spit just inside of it. I image with a C11 w/ the ZWO EAF and the initial calculated point always produced a better focus point.

What I found worked was to let EKOS run through the first pass, and then I would move the focuser outward a ways (to take up the backlash) and then move it inward to the calculated spot. It worked great - but was basically manual.

With the C11 EdgeHD and ZWO EAF it was important that the final focus movement was INWARD. Otherwise I would end up with some mirror tilt.

iOptron CEM120 EC2 and CEM25P
Celestron C11 EdgeHD and William Optics Star71
ASI 1600MM Pro, ASI 462MC
Moonlight Litecrawler (C11) and Motorfocus (WOStar71)
LodeStar X2 and ZWO OAG
Nextdome, AAG Cloudwatcher
MacMini
The following user(s) said Thank You: R Dan Nafe
7 months 4 days ago #82671

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Concerning the CFZ discussion, to determine the microns per motor step one could simply measure the max. drawtube travel and the corresponding step delta and divide those figures. So no need to know the drawtube screw pitch and motor scale.

The linear 1-pass algo (in principle any focus algo) then could have 3 additional (optional) input fields: "Microns per motor step", "Critical focus zone in micron" and "Desired step size in CFZ units". The latter can default to 0.5, but the user can overwrite it (using 1, for instance, or whatever).

The calculated step size would then be "Desired step size in CFZ units" x "CFZ in micron" / "Microns per motor step". Alternatively, the user can ignore those additional input fields and enter the step size manually as it is today.
254/1450 mm Newton, 130/730 mm APO, 70/336 mm APO, EQ6-R, ZWO EAF focusers, DeepSkyPro 2600c, ASI183MM, Intel NUC 8i5, Ubuntu 22.04
7 months 2 days ago #82696

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Replied by John on topic New Focus Algorithm in Ekos

Hi Doug and Bernd,

I started looking at the CFZ. Some observations:
1. Whilst I easily found the steps / revolution of my focuser, I can’t find the thread pitch of my SW draw tube. I think it has a 1:11 gearing but that may be incorrect so I’ll have to measure movement per focuser step as Bernd suggests. Since I don’t have anything other than a ruler to measure this the result won’t be very accurate, but I’ll give it a go.
2. Bernd suggests 3 parameters to capture which is of course possible.
3. In addition some decisions would need to be made on:
a) Seeing. Would need to capture a value from the user for astronomical seeing or total seeing. Could calculate the diffraction component for the telescope based on filter (wavelength of light being used) but this may be overkill.
b) Tolerance %. The article quotes many different values from a few % to a few 10%. Based on the formula this has a sqrt(tolerance) impact on NCFZ.

I have to say that this looks like it will require quite a bit of knowledge from the user to fill in the correct data to get a reasonable answer for NCFZ and then step size. For those that know this info the GoldFocus website provides a calculator to do this so I’m not convinced putting it in EKOS would provide much benefit.

Just my initial thoughts.
7 months 1 day ago #82703

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Replied by Doug S on topic New Focus Algorithm in Ekos

Hi John, non-professional sites typically have seeing in the 2.5 to 3.0 (or worse) category. Folks love to claim they have sub 1 arcsecond seeing in their backyards, but well....you know how that goes. Anyway, I'd suggest defaulting to 3 and then users could change it if they want. You could also default tolerance to 10% (which should be fine).

Remember that only very fast telescopes (f/2 - f/5) will be particularly sensitive to the values. Those with f/7 to f/10 systems have such large CFZs that it's not going to matter much. About the thread pitch and motor spec, I was being technically precise, but not implying that folks need to locate that info. The vast majority of people will simply run some reasonably large number of motor counts and measure the millimeters of focuser travel. If you allow entry of those numbers in the GUI (measured mm travel and motor steps for that travel) the alg could solve for microns/step internally to avoid that confusion for users. Cheers Doug
RASA11, Celestron CGX-L, ASI183mc Pro, 60mm guider + ASI290mm mini, ASI EAF focuser, PPB, Rpi4-4Gb+SSD, Powered USB3 hub, hardwire Ethernet.
Last edit: 7 months 1 day ago by Doug S.
7 months 1 day ago #82710

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I don't know about this technical talk, but all I would love is to be able to use autofocus without having to go back in manually and move the focuser back out to the calculated spot - which is ALWAYS better than the final focus point that EKOS chooses. My whole routine has been to run autofocus, then manually move the focuser OUT about 800 points on the ZWO EAF (to prevent mirror tilt), and then back in to the previously calculated focus point. I image at 7,000 ft in central New Mexico, with humidity levels in the single digits. The difference between these two focus points is very noticeable and frustrating that EKOS aways chooses the worse point. At least it is consistent in that regard.

iOptron CEM120 EC2 and CEM25P
Celestron C11 EdgeHD and William Optics Star71
ASI 1600MM Pro, ASI 462MC
Moonlight Litecrawler (C11) and Motorfocus (WOStar71)
LodeStar X2 and ZWO OAG
Nextdome, AAG Cloudwatcher
MacMini
7 months 1 day ago #82711

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Replied by Alfred on topic New Focus Algorithm in Ekos

Peter, why don't you make use of the "backlash" functionality in the focuser tab? Once activated, you only need to manually set the point of best focus and Ekos will take care of the rest, i.e. move the focuser further out and back in automatically.
7 months 19 hours ago #82715

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Since f/ratio is known, "Critical focus zone in micron" could be calculated automatically (also with ccd-corrected formulae since we know sensor pixel size too). So only needed parameters are "Microns per motor step" and "Step size (CFZ units)". For "microns per motor step", I think the best approach for non technical users is to provide them with two fields: "maximum focuser travel (mm)" and "maximum steps (for total focuser trave)" and let the code do the division. This suggests the users to measure the max travel and not, to say, the needed steps to travel 1 cm, which is useful to reduce impact of inevitable measure error.

My 10 cents.
Matteo
7 months 16 hours ago #82716

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Good point. So the focus routine can collect the needed data from the mount (f#) and camera (px size) sections.
254/1450 mm Newton, 130/730 mm APO, 70/336 mm APO, EQ6-R, ZWO EAF focusers, DeepSkyPro 2600c, ASI183MM, Intel NUC 8i5, Ubuntu 22.04
7 months 15 hours ago #82719

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Alfred, I have done that, and it takes care of the focuser's backlash. But what I was talking about is the slight primary mirror tilt I get when the focuser moves it outward. I have found that my images suffer from very small tilt when the focuser ends it's movement outward. Fortunately EKOS ends it's movement INWARD, which is great, but it overshoots the ideal calculated spot forcing me to backup and move the focuser outward to the best focuser position. That adds the tilt. So I have to first go outward quite a ways, and then back inward to straighten the primary.

iOptron CEM120 EC2 and CEM25P
Celestron C11 EdgeHD and William Optics Star71
ASI 1600MM Pro, ASI 462MC
Moonlight Litecrawler (C11) and Motorfocus (WOStar71)
LodeStar X2 and ZWO OAG
Nextdome, AAG Cloudwatcher
MacMini
7 months 10 hours ago #82724

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Replied by Doug S on topic New Focus Algorithm in Ekos

Matteo and Bernd, Both of you lost me on your comments about pixel size. Camera pixel size doesn't enter the equation. Microns per motor step is the derived quantity we're trying to find, and that is found by running the focuser some predefined number of counts, and then measuring the drawtube travel. An alternative method would be to know the pitch of the focuser screw thread (a linear distance per revolution) and then factor the counts per revolution of the focus motor. Given that the average Joe observer is going to most easily move the focus motor by some number of counts (say 1000), and then measure the millimeters of focuser drawtube travel, the GUI could allow for those fields and then convert to microns per motor count. The rest is just the formula as given by the critical focus zone web link (which includes f-ratio, aperture size, a user preference for focus tolerance, and an estimate of seeing). See here:

www.goldastro.com/goldfocus/ncfz.php

CS Doug
RASA11, Celestron CGX-L, ASI183mc Pro, 60mm guider + ASI290mm mini, ASI EAF focuser, PPB, Rpi4-4Gb+SSD, Powered USB3 hub, hardwire Ethernet.
7 months 5 hours ago #82726

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Replied by Alfred on topic New Focus Algorithm in Ekos

Peter,

with backlash activated, the focuser never ends its movement outward. It always moves further out and back in, just like what you do manually. This should prevent your primary mirror from tilting.

If you find focuser position 60212 to be the point of best focus during the first run, just hit the stop button, set 60212 manually (in the "desired absolute focus position" field), hit the "go to an absolute focus position" button and you're fine.

At least this is how it works here. I can hear the motor "overshoot" and "come back in" on out movements.
Last edit: 6 months 4 weeks ago by Alfred.
6 months 4 weeks ago #82732

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Doug, my English is propbably bad. I didn't say pixel size is necessarily part of the equation: given that you can find CFZ equations that take pixel size into consideration when sensors are involved, I only said that, if needed we know pixel size; the whole point was to say that CFZ should be calculated automatically by code, not entered by the user (maybe you already said that and I missed it). If that equation is better than another one, I can't say and I won't debate about it.
About microns per motor step, we agree. Again, I was only saying that user should be encouraged to measure the focuser travel over the highest possible number of steps, to minimize the inevitable measure error.

Matteo
6 months 4 weeks ago #82738

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