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SGP and Auto-Focus

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#1 Doug N

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Posted 23 September 2019 - 11:17 AM

I used SGP for the first time last night and tried using the auto focus.  I expected problems and got them.  I solved most of them.  This one I think I know what is going on, but needed some help.  I took a snapshot of the the 'V-curve' during the auto focus and included it below.  Sometimes the right side of that curve was in flatter.

 

After I was all done I think I am seeing backlash in the focuser.  Is that correct, or is it something else.

 

Thanks.

 

 

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#2 jdupton

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Posted 23 September 2019 - 11:22 AM

Doug_N,

 

   Yes, backlash may do this but it could also be that you ran out of focus travel with the focuser.

 

   You can check for backlash by setting the focuser for larger focus steps. That may allow you to see what is beyond the flat portion of the curve. If you have reached end of travel, no amount of stepping will change the focus and the right side will remain flat forever during an auto-focus run.

 

 

John


Edited by jdupton, 23 September 2019 - 11:23 AM.


#3 Doug N

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Posted 23 September 2019 - 11:34 AM

I had tried various step sizes and all ran across the same pattern.  I don't think that I reached the end of the travel.  I least I can't justify it with the curve.  If I were at the end of the travel I don't think the right side would be flat for that many samples.  I will give your suggestion a try.



#4 dhaval

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Posted 23 September 2019 - 11:39 AM

I typically find this when there are clouds or bad seeing. How clear were the skies?

 

CS!



#5 jdupton

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Posted 23 September 2019 - 11:50 AM

Doug_N,

 

   I just noted in your equipment list (in your signature area) that you have a Newtonian. Is the focuser mounted "on top" of the OTA (toward the sky or "under" the OTA (closer to the mount)? Which way do the step counts increase -- when racking in or out? I could be that the focuser is slipping as it works against gravity. There may be a "slippery spot" in the focuser travel. If increasing step counts are working against gravity, that could also explain reaching a point where it can "lift" no more.

 

   Next time out you can try a couple of experiments. These are what I would try:

  • Manually take an image and let SGP determine the average HFR
  • Note the starting step count of the focuser.
  • Move the focuser all the way back to the zero count position.
  • Make the focuser GoTo back to the noted first step count.
  • Take another manual focus and again check the HFR
    If they are close, then there isn't any significant backlash.

     
  • Tell the focuser to move out well past the flat area step count.
    GoTo maybe the 24,000 step count region.
  • Start an Auto-Focus run from that point.
    If the process works from that direction, maybe you have a point in the focuser travel where it is slipping.

 

John



#6 jerahian

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Posted 23 September 2019 - 11:51 AM

I used SGP for the first time last night and tried using the auto focus.  I expected problems and got them.  I solved most of them.  This one I think I know what is going on, but needed some help.  I took a snapshot of the the 'V-curve' during the auto focus and included it below.  Sometimes the right side of that curve was in flatter.

 

After I was all done I think I am seeing backlash in the focuser.  Is that correct, or is it something else.

 

Thanks.

Looks like backlash to me as well.  I would set your backlash IN to 150 or 200 (the higher value won't hurt) and see how it goes.  I'm assuming the lower values are IN for your case (i.e. you haven't changed focuser direction in your config).

 

If you set it to 200, in the example above, the focuser would initially move to 22361, and then come back in to 22161 to clear out the backlash and start the AF routine.

 

GL&CS,

Ara


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#7 Doug N

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Posted 23 September 2019 - 11:54 AM

The humidity was pretty high.  Shoes were pretty wet by morning.  I thought the seeing was pretty good considering.  Probably category 4, maybe 3.



#8 Doug N

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Posted 23 September 2019 - 11:59 AM

No, I did not reverse the directions.

 

In addition this was not the Newtonian.  I was using a Takahashi 130 FSQ.  I didn't check, but I think it has an Optec focuser.

 

Thanks for the procedure.  Now I just need to wait for another clear night in Minnesota.



#9 ChrisWhite

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Posted 23 September 2019 - 06:52 PM

Most likely you are not analyzing enough stars from shot to shot. Pay attention to how many stars are being examined with each exposure. Some stars that were used in one frame may be out of parameter and rejected in the next.

The graph represents the measured results, not actually focus quality.

If the software is only using a few stars, you need to change your sampling settings.

Edited by ChrisWhite, 23 September 2019 - 06:52 PM.


#10 freestar8n

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Posted 23 September 2019 - 07:07 PM

It also looks like backlash to me - and for most any telescope and any autofocus system it is recommended to take the through-focus curve while pushing up against gravity.

 

In SGP the autofocus curve is always taken right to left - from high numbers to low numbers - and you need to make sure that direction is pushing the focuser up against gravity.  There are ways to change the direction in SGP and possibly in the focuser itself - but it tends to be very confusing.  Nonetheless - it needs to be set up right - and you can tell just by watching the procedure while looking at the focuser if it is working right.

 

At the start of the autofocus run the focuser will go to high numbers and overshoot the right side to take out backlash.  And watching the focuser you should see it drop downward with gravity as this happens.  Then it should unwind backlash and push up - then start taking the curve from right to left.

 

It takes some experimentation to get it all set up right - but after that it should just work.

 

It's better to have too much backlash compensation than too little.

 

Frank



#11 Jcwillis4

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Posted 24 September 2019 - 02:07 PM

+1 for backlash

 

I use the Tak FSQ106, and while it doesn't seem like a lot of backlash, once I add in the scope and the focus motor, I end up with a pretty decent amount of backlash.  I ended up setting 1000 steps of backlash compensation to make sure I kill it every time.  




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