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For those with focus issues

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"I'm going to perform a test of the backlash by moving it outwards for a number of steps and inwards."

Do you mean "backlash", "slippage" or "missing steps"? Slippage and missed steps is going to be the much bigger problem than backlash. With a heavy camera, both can become a massive problem, especially if your telescope is nearly vertical.


"My EAF is attached straight to the main axis of the focuser instead of the single knob and here I have a question about the number of initial steps. Some of you write that they set initial steps of 200-500 ticks, while others write that they use 20-50 ticks. What does it depend on? I have read all the thread and nowhere is mentioned whether the stepper motor should be attached to the main axis or to the 10:1 reducer axis. The ratio of 200-500:20-50 suggests that the bigger values are related to the reducer axis."


Depends on 1) your code (how many steps does it allow), 2) on the torque your stepper motor can deliver (if attached to the main axis, you will probably require microsteps to achieve precise focus resolution. Microstepping reduces the available torque and will likely result in missed steps, especially when the telescope is nearly vertical and has to lift the camera and everything attached to it, so I have my focuser (also controlled by Arduino) attached to the reducer axis to minimize the need for microstepping.
Last edit: 1 year 3 months ago by Jose Corazon.
1 year 3 months ago #89271

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Replied by Doug S on topic For those with focus issues

Hi Martha, you might want to read my comments (multiple) on page 3 of this post:

indilib.org/forum/ekos/11668-new-focus-a....html?start=24#82726

Basically, you shouldn't need to guess the step size. It should ideally be calculated from your CFZ size, EAF/focuser drawtube travel, and a couple of user selectable parameters. See the formula for New CFZ as defined in the post above. Then you'll want to know (or measure) your focuser travel per revolution and match this up to the EAF counts/rev. Set your step size to be somewhere between 1/2 and 1 CFZ. You don't want to jump over the CFZ, and if you set it too small, you'll just waste time during focus jumping within the same CFZ. As for how the EAF is mounted, If I remember correctly from the user manual, you should not mount on the 10:1. The counts/revs is already plenty high enough to do fine focus work as is. Expect backlash on the order of 85 counts (That's about where mine was when I tested, and it was confirmed by others in the forum with EAF).... Good luck & CS, Doug S
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Last edit: 1 year 3 months ago by Doug S.
1 year 3 months ago #89272

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Replied by John on topic For those with focus issues

Hi Martha,

If you haven't already, I'd suggest you have a look at the focus section of the kstars manual...
docs.kde.org/trunk5/en/kstars/kstars/tool-ekos.html#ekos-focus

It discusses the various algorithms and by all means give them all a go, but I'd suggest Linear 1 Pass (although since I wrote it I'm obviously biased!)

There is a section on "How to Setup for an Autofocus Run" which goes through some good starting values for various parameters and the trade-offs to consider. If you find anything unclear, wrong or missing let me know and I'll update the document.

Good luck and let us know how you get on.

John.
1 year 3 months ago #89274

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Replied by Martha on topic For those with focus issues

@Jose, @Doug and @John,

Thank you for your answers. in the beginning I decided to fillow the Doug's advice, especially that I understand all the NCFZ rule.

1. I measured the full travel of the focuser tube using a caliper and it's 94.4 mm. In the meantime the EAF performed 25,992 steps.
2. I found out that 1 step/tick mens 3.631886734 µm, please correct me if I'm wrong.
3. I didn't find whether for the diffraction limit I should use the Dawes limit (120/D) or the Rayleigh limit (140/D) so I took an average value 130/D.
4. I solved the equation for the 102mm F/ refractor and for everage seeing that is usually around 2 arcsec and my NCFZ is 42 µm.
5. I found on the 2nd page of the mentionet thread that the Step Size should be equal to 1/2 of the NCFZ. Should I understand that if 1 µm means 0.275338983 of the step/tick then the 1/2 of the NCFZ is 21 * 0.275338983 = 5.783512841
-> 6 steps ???

Edit.
Sorry, my fault, I have F/7 OTA now, not F/5. That means the NCFZ = 82.33984917, half of it is 41.16992458 and the Step Size is 11.33568517 -> 11
/Edit.

I cannot believe that so small movement may change something.

I really want to understand it, so Doug, please help me.

Martha
Last edit: 1 year 3 months ago by Martha.
1 year 3 months ago #89281

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Replied by Martha on topic For those with focus issues

I made a quick test. I assumed that the Step Size comes from not 1/2 NCFZ, but straight from the full value of the NCFZ, so the Step Size is 26. If the linear algorithm moves the tube owtwards for 10 x Steps Size that is 260 steps, then the tube moves outwards less than 1mm. And that should be OK, as my calculation says that 1mm means 275.3390 steps.

But my question is still going: if the proper sampling begins after coming back, inwards, at the position of -5 steps, so is it able to find the focus within less than 1mm? (-5 steps, +5 steps)?

Martha.
1 year 3 months ago #89282

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Replied by Doug S on topic For those with focus issues

Hi Martha, Point #2 as calculated is technically correct... however, I don't believe it (from a sanity check perspective).

From my calcs, your CFZ is 71.1 um @ 10% focus tolerance, and 87.1 um @ 15% tolerance. This seems right to me for f/7. If you're getting 2 arcsec seeing, I'd constrain to 10% tolerance or better...you don't want to waste good seeing with poor focus!

Now, about that travel measurement.... I'm having a hard time believing the measurement (unless I didn't understand what you said). I would have expected you to have at least 3-4 hundred motor counts for a CFZ that size, but you've measured 94400 um / 25992 motor counts or 3.632 um/motor count (only 0.27 motor counts/um). Are you sure you did that measurement right? You might want to measure again (or tell me what I didn't understand). In the end, I'd expect to find for f/7 something on the order of 300-400 motor counts per CFZ, and a corresponding step size of 250 or so (educated guess). How about a 2nd measurement of your focuser travel and EAF motor counts. Make sure you push one direction with the EAF for at least 100 counts (to negate backlash), and then start your test in the same direction, noting your starting EAF motor count, travel distance, and final motor count. I'd be curious to hear the result. Cheers, Doug
1 year 3 months ago #89284

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I think that value isn't unreasonable for an EAF directly connected to the focuser shaft. Mine (Sharpstar AL-140PH) has 2.8μ steps (coming from my BL measurements, using a micrometer gauge). And yes, 20 steps give a clear difference in HFR (I do have quite good seeing).
1 year 3 months ago #89286

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Replied by Doug S on topic For those with focus issues

Hi Peter...ok, that means that the focuser has a pretty coarse thread pitch. I'm not used to that. Celestron has a typical thread pitch of 0.75um/rev, leading to a 0.13um/EAF step, so I get many more steps in my CFZ. Just wondering out loud, I wonder at the measured low counts/CFZ, maybe the 10:1 gear would have been an interesting test to get more sensitivity? I'm not recommending anything other than what EAF recommends, but it does make me wonder. Martha can measure and confirm (or not) if she has about what you have. As long as it works... Cheers, Doug
1 year 3 months ago #89287

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Replied by Martha on topic For those with focus issues

Hi Doug,

Unfortunatelly, there is very cloudy and rainy outside, so I perform my measurements and calculations at home. I MUST be abolutely sure what am I doing before I go outside. That's why I ask for support. All my calculations are made in the Excell, so it's easy to verify it and to change the values quickly.

The EAF has a great feature that it detects a too high torque and it stops then. So I know where is its real zero position and where is its real end position. But I didn't ralize till now that if I moved the tube to the zero position it didn't mean that the count ended with zero, but i.e. 147 instead. Requesting the zero several times moves it slightly, step by step more inwards (76, 42, 1) while I can hear the movement and finally it stops at zero, so I cannot heare the work of the gear. As I wrote earlier, I measured the maximum extrusion using the vernier caliper. I read a rough distance on a scale engraved on the tube after 10,000 steps and based on that I calculated a rough number of steps needed to reach the end point that is around 95 mm. I requested the Ekos to perform the full extrusion and then I found out that I have to add some steps in Ekos/Focuser tab. I set the step as 25 and was adding the value to the calculated number by pressing the "arrow up" button. This way I found earlier that the end position was 25,992 steps.

As I wrote, the EAF stops automaticaly, when the torque is too high. It ends the travel at the end of the toothed treadmill and confirms it by double beep. I positioned the EAF and the focuser at the zero position and requested the Ekos to move it to a 27,000 that is over its capability. Now it stopped at 25,703 steps, maybe there is the backlash invlved. So I repeated the scheme: moved to the zero (it stopped exactly at zero and I can see that there is no gap) and again to the 27,000 - now it stopped at 25,822 steps. And again: 25,805. And once again: 25,803. As you can see, the number of steps is not trustworthy too much, unless you physically force the zero position. Now I can say that all the path takes the EAF ~25,800 steps. It doesn't change too much in the final values: 1 step means 3.658914729 µm while 1 µm means 0.273305085 step.

I assumed 5% of the tolerance and 2 arcsec seeing, but I can assume now the 10%, as you wrote, at least for the beginning. What about the diffraction limit? Can I use the average 130/D value?

I found another my fault in the calculation of the total seeing: I forgot to multuply the diffraction limit by 0.2... Ooops.

Now the NCFZ is 80.18745577 µm. So, even for the full value of the NCFZ the Step Size is 22 steps, because 1 µm meas 0.273305085 step. If I understand it properly, Step Size = NCFZ (or 1/2 NCFZ) * 0.273305085 steps = ~22 (or ~11).



The top row in the picture is the example from the website.

Our calculations are similat till this point. I think we calculate different the Step Size. If I multiply the NCFZ by 3.658914729 that is the movement caused by 1 step expressed in µm, then I get 293.399063 that is very close the result expected by you. But If I follow the proprtion below, I get a the value of 22:



Doug, where is the main fault?

Martha
Last edit: 1 year 3 months ago by Martha.
1 year 3 months ago #89289
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Replied by Doug S on topic For those with focus issues

Hi Martha, The diffraction limit doesn't really apply because you are dominated by seeing which is larger. You'll likely never get seeing at the diffraction limit, but if you believe you might, you can just adjust your seeing in the calculation to the diffraction limit (making the CFZ smaller...~16+ cnts). About tolerance, I'd say that if you started with 5% tolerance, keep that (it's a tighter criteria for CFZ). I just didn't want you to use 15% tolerance if you think you're going to get 2 arcsec seeing. Better to avoid sloppy tolerance when the seeing is good.

Ok, it looks like your CFZ fits in less than 20 steps. That's a pretty small window compared to the available focuser travel! You're going to be pretty sensitive to donuts in the focus routine until you get a handle on where the CFZ is initially. That's could also make your choice of algorithm a bit trickier depending on how repeatable your setup is. If it's reasonably repeatable, linear or 2nd gen linear should be favored. If not, possibly polynomial will work better. That's a separate question however.

Your post started with a question about what to set the step size to. Now you know. Your CFZ is ~20 steps. If you use ~15 steps as your step size, you'll not jump over the CFZ with each iteration, and you also won't waste a lot of time hunting within the same CFZ. The issue that I see for you is finding that small range in such a large travel window. Others could probably advise you better on how to start the hunt, but you might need to initially loosen that EAF setscrew while you find infinite focus, and then reset the screw and then run the focus routine (only a 1 time event...after that it should hopefully be routine to find it). Take note of your EAF motor counts (in case you need to restore in the future). Your first focus is likely going to be the hardest!
1 year 3 months ago #89290

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Hej Doug,

I'd say that isn't uncommon for standard rack&pinion focusers. And there's no real issue with that, IMO. That is why we use a stepper motor that has no problem doing single step accuracy. The 10:1 is for "clumsy fingers", but always has the chance of inherent slip, so it's really not recommended if you want reproducible focus-by-number. There's no gain of accuracy in having 'big numbers' when using a stepper motor.

As for the low 0.75μ/rev, that sounds to me like it is the movement of the main mirror, not the movement of the focus position, so that would get a magnification of about 10 from the secondary re-imaging, depending on your type of optics. Would still lead to quite large numbers, but that is really just focuser specific. A Crayford likely also has smaller step sizes than a R&P.

Cheers & Happy New Year!
1 year 3 months ago #89293

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I run my refractor (140mm, F/6.5) with a step size of 20, and the Linear 1P algorithm. The NCFZ for my setup/site is 31μ, or 11 steps. The chose step size stems from the first linear implementation that did the second run at half step size. I didn't change that when switching to L1P, but at least for me the interpolation it does usually leads to very good results.

As you mention, it is somewhat crucial to have a good start point, even without the donut problem (refractors don't have that). I think the (L1P) algorithm has improved in that respect, still it is (IMO) highly recommended to do an initial manual focus (I use a bright star and video mode). Doesn't have to be extremely accurate. And for automated AF, definitely use filter offsets if you use a filter wheel.
1 year 3 months ago #89294

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