Hi Jason,

As Nou suggests it looks like you are moving the focuser too far from the focus point. So what value to set Initial Step Size to? There are a couple of ways:

1. Set it to the Critical Focus Zone of your Optical Train. The CFZ module will calculate it for your optical train but you'll need to supply 1 parameter which is how far in microns the focal plane moves when the focuser moves 1 tick. Its the highlighted field:


For refractors you can move the draw tube in and out, measure how far it moves with callipers / ruler and note down how many ticks the focuser moved. This will give you the number of microns / tick. For other telescopes where you are moving the primary or secondary mirror you'll need to investigate how to measure this with the telescope specs. If you are struggling with this if you send details of your telescope and focuser I'll see if I can figure it out from the online specs (some manufacturers publish the required info, but some don't unfortunately).

So in the screenshot, if you measured 1 tick to be equal to 1 micron then your CFZ would be 39 steps so try setting the Initial Step Size to that to start with. Once its working you can tweak to optimise it.

2. Start at focus and move the focuser (in or out it doesn't matter) a few ticks and take a frame. Keep repeating. What you are looking for is a step size that makes a difference to the HFR (displayed at the bottom of the v-curve) but doesn't go so far as to get bad values in the HFR. Once you get too far away from optimum focus star detection breaks down and the HFR will be incorrect. So you are after the sweet spot between moving enough to get different values for the HFR but not so far as to start to get star detection breaking down. When you have worked out a range from optimum focus to where star detection starts to break down then you have the operating range for Autofocus. I would suggest aiming to get around 10 datapoints so 5 either side of optimum focus. So divide your previously calculated range by 4 and that's your step size.

Backlash
You have 50 ticks in AF Overscan. Maybe that's fine. Can't tell. There's help in the manual for sorting out backlash if you're unsure. It will need to be sorted out in order for focus to work.

There are some other settings that you have that could probably be optimised but lets get the basic focus working before fiddling with those.

Hi Bryan,

With that equipment your focus is going to pretty sensitive to many things, like ambient temperature, start point, filter, seeing, etc. If you get problems again feel free to post on the forum. There may be some things we can do to make Focus more resilient.

Thank you John, Nou.

I took into account your remarks, and I believe I reached a non-textbook-orthodox-solution, at very least, it seems persistant through various autofocus procedures with different filters. Nevertheless, I'll continue studing it since this is much deeper as I thought, considering John's remarks.

I measured microns/tick and it is 0,7636 microns/tick. My resolution is 0,96 arcsec/pixel with a 203/800 Newtonian using ASI1600MM (sensor with pixels of 3,8microns and 4656x3520) I believe I also introduced wavelengths by filter (roughly)

I'm sorry but I skept what advisor propose me: 18 ticks with x5 multiplier. I went to 75 ticks initial step with No OverScan and no multiplier because I changed the algorithm to use to Polynomial and increased tolerance to 10% (instead of 5%, I tested with 20% and 10%, I can imagine the lower the better focused image) Seeing and temperature difference of the primary and ambient is something that should be seriously considered. I could say before that it was important, now I believe is critical!

You can see all parameters below. Like I said, solution is not orthodox (I ignored advisor) but it seems to work so far.

I'll keep testing and improving my knowledge!
BTW: Among the screenshots there is the solution autofocus found for L, Ha, Oiii & Sii. Next step I'll try the "Offset Generator" to put more time on imaging and less on focusing.



Many thanks for your advices!

Hi everybody,

I am discouraged ! I install the last update of Kstars, after reading the messages on this post. I made tries this afternoon in simulation mode. It seems AF was running well. I made tries on the real sky this night. No way !!!!! No real V-curve, no reproductibility !!! No stars detection I see on the screen !

My setup: ED80 with reducer 0.85, ASI183MM, Focuser DIY based on Robert Brown model. It runs well with NINA !
step size=7,9 micron. I start the autofocus near the focus point, made with Bahtinov.

How is it possible to have this v-curve with two point at the opposite with the same HRF ?

The internal plate solver crashes.

Hi Jason,

Glad things are starting to work for you!

The 18 ticks is probably too small for the step size. I normally would use a tolerance of 1 in the CFZ screen to get the tick size which would turn the 18 into 54 ticks. Its just a starting point and if 75 works for you then fine.

I suspect the Advisor is suggesting the 0.33 tolerance which is why you got 18 ticks. I'll look into changing that.

I would recommend using Linear 1 Pass as I personally believe it to be more accurate than Polynomial but obviously its your choice.

Let us know how things go!

Hi Morelli,

Sorry you're having these struggles.

So first off, did you do anything with my previous recommendations here?
www.indilib.org/forum/ekos/13911-kstars-...autofocus.html#96295

However, looking at your recent log most of the time you don't have any stars which is why focus is failing. Sometimes you detect 1 or 2 stars hence the odd shaped curves but mostly no stars. I didn't look into plate solving, but that won't work either without any stars.

If you see an HFR = -1 that usually means no stars were detected.

So why no stars?

I see you started out using a Gain = 0. Then you went to Gain = 270 but still failed to detect any stars, then to Gain = 450 with the same result. You tried 1s and 2s exposures. Just googling your camera ASI183mm pro, looks like unity gain is around 120 so probably makes sense to use that.

Given you have a widefield with the SW 80ED, I would expect you to be detecting literally hundreds of stars.

I can't see anything obvious in the log but the first problem to solve is why no stars. Were any stars visible in the fitsviewer with these images?

Either no stars were visible (clouds, not pointing at the sky, etc) or star detection wasn't working for some reason.

Did you keep any of the focus frames? If so, can you send me a couple.

Hello John,

about plate solve, it was just that I had not installed astronomy.net nor Astap, on my new laptop ! Just a remark, Watney was installed but every plate solve it says :The Watney Solver needs at least 300 stars. Adjusting keepNum to 300. and fails.

I'm rather absent-minded and not very rigorous. So, gain at 0 or no right filter it's unfortunately usual. At least, with good gain, good filter, good sky, yes I could see stars on the viewers when Ekos said no stars ! Moreover,I have imaged quickly NGC869 during this night.

I obtain a good focus one time, according the Bahtinov, the judge. Next, the focus passes by a good focus, but it goes on and fails. Or it stops with success but stars are big ballon !

And the v-curve is very strange, with two measures wtih same HDR on very different positions. It is not a v-curve as those I send you from NINA.
With Linear 1 pass, the v-curve is rather flat. With polynomial it is better.

Again, I don't undstand why the focus goes on and on as stars get bigger and bigger. I expect the process stop and go back.

I setup step size=7,9, range move focuser=0-10000. No backlash then backlash > 0.

I'll try to post next time, snapshots of the process and I 'll check the developer option about focus.

Thank for your help.

Can you post a screenshot of the SEP parameters screen for the focus profile you're using 1-focus-default please. Its this screen...

Hi Morelli,

This is the default SEP parameters used by the Alignment module (not Focus).

If you bring up parameters from the Alignment module, you get specific screens for Alignment, whilst if you bring up parameters from say Focus you'll get to the ones used by Focus.

Can you repost the 1-focus-default parameters (which is what you were using on your last (bad) focus session).

Thanks.

OK, I understand.I setup Keepnum to 300, but the issue occurs still. I retreive a message about watney.it says it is valuable for .Net write in C#. So, I get is ride off. Intenal, Astrometry.net or Astap run perfectely.

Hi Morelli,

Thats still the SEP profile used by Alignment; I need the one used by Focus.