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Auto Focus Offsets problem

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#1 Jon2070

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Posted 20 August 2019 - 08:57 AM

Last night I attempted to find the focal offsets for each of my filters, L,R,G,B,Ha,OIII and SII and I got strange results…

 

My method was to use a Bahtinov mask to get L pretty much perfect and then I ran auto focus for each filter concurrently in the order above, each time noting the position given, which is in the table below. The temperature readout from the sensor on the focuser stayed at 12.5 degrees Celsius. I have also added the offsets. As you can see they are all over the place.

 

I have a feeling that backlash may be the issue, I don’t know how to counteract this though, I have read several posts and am none the wiser. I have included screenshots of my settings in SGP.

 

Another thought is that my method may not have been the best, should I have focused L each time for every filter? For example L and then R, 4 times in a row?

Attached Thumbnails

  • Autofocus Settings.jpg
  • Filter Offsets.jpg
  • Focuser Settings.jpg


#2 OldManSky

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Posted 20 August 2019 - 10:41 AM

It's likely backlash.

I wasn't familiar with Lakeside's motor focuser, so I looked it up -- they're using a relatively small stepper with an attached gearbox, which almost certainly has some backlash, thus your results.

I'll let SGP users chime in on dealing with backlash in SGP...I use NINA, and it has a feature to measure focuser backlash and account for it.

There may be a function in the Lakeside driver to account for backlash as well -- on their web page I found "...When in manual mode it is possible to set a backlash value."


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#3 premk19

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Posted 20 August 2019 - 02:14 PM

What's the resolution of the focus motor? It's possible that all values are within the CFZ for your scope. If that's the case you're totally fine.


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#4 Jon2070

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Posted 20 August 2019 - 04:08 PM

What's the resolution of the focus motor? It's possible that all values are within the CFZ for your scope. If that's the case you're totally fine.

4000 steps per revolution. What is CFZ?

 

I forgot to mention in the original post that sometimes when I have auto focused the focus is off slightly, which is another reason I thought that it might be backlash.


Edited by Jon2070, 20 August 2019 - 04:09 PM.


#5 OldManSky

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Posted 20 August 2019 - 07:45 PM

4000 steps per revolution. What is CFZ?

 

I forgot to mention in the original post that sometimes when I have auto focused the focus is off slightly, which is another reason I thought that it might be backlash.

CFZ is "Critical Focus Zone."  Think of it as kind of a "depth of field" measurement if you're familiar with general photography terms.  If your focus point lands anywhere within the CFZ, you're "in focus."  The size of the CFZ depends on the focal ratio of your scope, the wavelength of light you're interested in, etc.

Here's a formula to calculate it:

 

http://www.goldastro...ldfocus/cfz.php

 

So, basically, you'd calculate your CFZ for, say, green light.  Let's say it comes out to 2mm.  Then using your 4k steps per rev on your focuser, and the distance your focus tube moves per rev in mm, you can figure out how many mm per step movement your motor produces, and how many steps you have inside your CFZ.  If you get (for example) 0.4mm per focuser step, then you have 5 steps inside your CFZ -- which means a variance of +- 2.5 steps in your offsets means absolutely nothing, you'll be in the CFZ (and in focus) whether you use the offset or not.

 

Make sense? :)


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#6 premk19

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Posted 20 August 2019 - 08:39 PM

I forgot to mention in the original post that sometimes when I have auto focused the focus is off slightly, which is another reason I thought that it might be backlash.


That certainly sounds like backlash. Does the lakeside focuser software have backlash compensation?

You can estimate backlash this way - make the focuser move to a position, say 5000 steps. Make a mark on the drawtube (A). Now move to another point, say 7000 steps and note drawtube position (B).

Go back to 5000 and see if the focuser can hit point A again precisely. If it doesn't, see how many steps it takes to get there, and that's the number of steps of backlash that needs to be compensated.
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#7 Jon2070

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Posted 23 August 2019 - 01:43 PM

CFZ is "Critical Focus Zone."  Think of it as kind of a "depth of field" measurement if you're familiar with general photography terms.  If your focus point lands anywhere within the CFZ, you're "in focus."  The size of the CFZ depends on the focal ratio of your scope, the wavelength of light you're interested in, etc.

Here's a formula to calculate it:

 

http://www.goldastro...ldfocus/cfz.php

 

So, basically, you'd calculate your CFZ for, say, green light.  Let's say it comes out to 2mm.  Then using your 4k steps per rev on your focuser, and the distance your focus tube moves per rev in mm, you can figure out how many mm per step movement your motor produces, and how many steps you have inside your CFZ.  If you get (for example) 0.4mm per focuser step, then you have 5 steps inside your CFZ -- which means a variance of +- 2.5 steps in your offsets means absolutely nothing, you'll be in the CFZ (and in focus) whether you use the offset or not.

 

Make sense? smile.gif

Thank you for the explanation, yes that makes sense.

 

That certainly sounds like backlash. Does the lakeside focuser software have backlash compensation?

You can estimate backlash this way - make the focuser move to a position, say 5000 steps. Make a mark on the drawtube (A). Now move to another point, say 7000 steps and note drawtube position (B).

Go back to 5000 and see if the focuser can hit point A again precisely. If it doesn't, see how many steps it takes to get there, and that's the number of steps of backlash that needs to be compensated.

I'm not sure, going into settings, it won't open 'Config' without the focuser connected, it will be Sunday (weather permitting) before I go to the observatory again, would the backlash compensation in SGP not supersede anything in the lakeside software? Thank you for the method, I will give it a try. 



#8 premk19

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Posted 23 August 2019 - 02:06 PM

Yes, you can use backlash compensation in SGP too. Either works.
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#9 Jon2070

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Posted 24 August 2019 - 05:18 AM

Yes, you can use backlash compensation in SGP too. Either works.

I posted on the Main Sequence Software/SGP forum as well, in the past the replies have been a bit slow compared to CN, although this time I got some quick replies. It seems that a value between 100 and 200 works, direction for me will be IN as it is a refractor. Hopefully will be able to test it tomorrow if the forecast holds.



#10 1DegreeN

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Posted 24 August 2019 - 08:02 PM

I have a similar setup with the Lakeside Astro focuser on a WO FLT110 scope. Each step of the motor moves my focuser 4.5 microns and the theoretical CFZ is between 70 and 100 microns depending on wavelength i.e. around 15 to 25 steps. I have backlash set up in SGP but my attempts to determine filter offsets have been as unclear as yours, even though I always get a very nice V curve in SGP's autofocus. There seem to be so many factors at play here that you probably need to repeat the test 100 times to get a statistically significant estimate of the offsets. 

 

Another way might be to set up a sequence with one frame of a few seconds for each filter, run AF once (or use the mask) to determine best focus and note the focus position, disable autofocus and then  move the focuser in increments of e.g. 5 or 10 steps over a range that straddles the initial best focus position and run the sequence each time. Then use SGP or another tool (I'd use PixInsight) to measure the HFD or FWHM of each captured frame. Construct a table of FWHM vs focus position for each filter and if you see a clear minimum in FWHM for each filter then that may give a clearer indication of what the offsets are. Its helpful to include the focuser position and ambient temp in the filename template.


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