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What should a SGP Focus curve look like?

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#1 magsterone

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Posted 23 October 2020 - 09:39 PM

Out of curiosity how should the v-curve look for focusing with sgp look like?  As I write it seems oxymoronic but should it be a strict “v” or a slightly rounded “v” which sounds like a “u”.  Mine are more rounded at the bottom. Just asking. Be safe.



#2 jerahian

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Posted 23 October 2020 - 09:50 PM

Out of curiosity how should the v-curve look for focusing with sgp look like?  As I write it seems oxymoronic but should it be a strict “v” or a slightly rounded “v” which sounds like a “u”.  Mine are more rounded at the bottom. Just asking. Be safe.

 

Rounded at the bottom, not a sharp V, but when all is said and done, should look more like a V than a U.



#3 cmassa

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Posted 23 October 2020 - 10:09 PM

Here's mine from a few minutes ago. My telescope focuses VERY well. smile.gif

 

Chris

 

 

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#4 KNak

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Posted 24 October 2020 - 12:56 AM

They are fitting a quadratic, so one could interpret that as how it "should" look, which is pretty accurate for me (C11 with stock focuser). 



#5 magsterone

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Posted 25 October 2020 - 11:16 AM

Thanks for the information, be safe.



#6 Richoff

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Posted 04 November 2020 - 05:23 PM

@ Chris - I wish I could get anything close to the curve you posted. Over the last 2 years of imaging with auto focus, my HFR focus points meander all over the screen, and SGP usually gives up after three runs, yet my subs are all tack sharp with HFR in the 2-3 range. I've tried adjusting backlash settings, and turned it completely off. I'm not complaining, just wondering why I can't get a "curve", when the pics are actually in focus. Anyone have some idea's?

Cheers



#7 t-ara-fan

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Posted 04 November 2020 - 06:40 PM

 my HFR focus points meander all over the screen

How big are your "steps" between focus points?  Obviously it varies with scope, f-ratio, and focus motor.  I use 25-75 depending on my setup.

 

If you don't have big enough steps to get out of focus, you can't get a V curve.

 

There is a method to calculate the step size to use.  But a quicker way is to just move your focuser until you are a little out of focus, while noting how many steps that was.  Use 25% of that number as your step size.  
 



#8 Richoff

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Posted 04 November 2020 - 09:55 PM

How big are your "steps" between focus points?  Obviously it varies with scope, f-ratio, and focus motor.  I use 25-75 depending on my setup.

 

If you don't have big enough steps to get out of focus, you can't get a V curve.

 

There is a method to calculate the step size to use.  But a quicker way is to just move your focuser until you are a little out of focus, while noting how many steps that was.  Use 25% of that number as your step size.  
 

Thx t-ara-fan, I just checked, and I have it set at 20 step size on 7 data points... I'll give it a go with a 40 step size and see what happens.

Cheers



#9 cmassa

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Posted 04 November 2020 - 10:05 PM

Thx t-ara-fan, I just checked, and I have it set at 20 step size on 7 data points... I'll give it a go with a 40 step size and see what happens.

Cheers

Adjust your step size until the max HFR is 3-4x the min HFR on the graph. With a refractor you should not have much trouble getting a good curve. Telescopes with central obstructions can be a bit trickier. Also, make you put in enough backlash compensation. There really isn't a perfect number for backlash compensation as long as it's enough, more than you need won't cause a problem. If not enough, you'll typically get a flatter right side of the graph than the left.

 

Chris



#10 Richoff

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Posted 05 November 2020 - 03:17 PM

Adjust your step size until the max HFR is 3-4x the min HFR on the graph. With a refractor you should not have much trouble getting a good curve. Telescopes with central obstructions can be a bit trickier. Also, make you put in enough backlash compensation. There really isn't a perfect number for backlash compensation as long as it's enough, more than you need won't cause a problem. If not enough, you'll typically get a flatter right side of the graph than the left.

 

Chris

Thanks for the info cmassa. I'm not with the Rig right now to check, but I seem to remember seeing; being able to set backlash to either a positive or negative setting??



#11 cmassa

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Posted 05 November 2020 - 03:59 PM

Usually you would select 'IN' for backlash compensation but it will depend on how your focuser is set up. The focuser should be lifting the camera as the numbers get smaller. With it set to 'IN' it will move further out and then lift it back up to the desired focus position so it is completely supported by the mechanism. You just need a number of steps that will completely take up the backlash. If your number is too big, it doesn't really matter.

 

Chris



#12 Richoff

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Posted 05 November 2020 - 08:35 PM

Thx again Chris, for the additional info. I often wondered about which way the steps should increment... I think there's a setting to reverse that anyway. I'll give it all a go when the skies clear.

Regards



#13 Raginar

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Posted 05 November 2020 - 09:11 PM

Increase your steps if you're seeing a U.  It needs to move farther in/out to generate the V.




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