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Temperature controlled automated re focus- any free software?

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#1 pyrasanth

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Posted 02 March 2021 - 12:39 PM

I have a ZWO EAF focuser with the temperature probe fitted.

 

I've noticed that changes of a little over 1 degree can significantly change the focus of my RASA enough to be noticed.

 

I understand that focus adjustments due to contraction or expansion of the OTA are extremely linear so easy to compensate for by temperature.

 

The ZWO driver has a temperature compensation feature and the probe appears to be very sensitive.

 

Is there any software that will allow capture of the focus points vs temperature automatically or will I need to focus at any given temperature change & plot the results to work out if I truly see a linear data set?

 

I looked at T-comp- it appears to be a very expensive piece of "shareware" quality software at $50.00 which is quite expensive for a utility.

 

Does anybody know if there is anything free that will do the job or am I just better in trying to plot my own graph and feed the change per degree into the ZWO focuser at perhaps interval measurements of 2-3 degrees?

 

Please let me know your thoughts.



#2 han.k

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Posted 03 March 2021 - 01:55 PM

CCDCiel (free program, Windows, Linux, Mac) can do that. And I guess Nina probably can do that also.

 

For my APO, I set the software to  refocus after every 0.5 degree Celsius difference. 

 

Earlier I tried to use  find a compensation factor temperature to -> position , but my focuser has some slippage and I also suspect the temperature to focus position is not a linear process with a lot of delays in it, so first and second order elements. 

 

CCDCiel will report after each image how much temperature difference is left before the next auto focus is due

 

Han

 

CCDCiel setting

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#3 Der_Pit

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Posted 03 March 2021 - 04:47 PM

Well, I thought the point was not to do an AF run, but - based on earlier calibration runs - blindly change the focus, based (only) on temperature.

 

There's currently a project for this in kstars, see this thread.  The idea - as I understand - is to analyze the AF runs from the logfiles, to derive the temperature change slope, and then use that to correct the focus between exposures.  It's in the early stages though, and you need to compile yourself from the sources....



#4 han.k

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Posted 04 March 2021 - 12:44 AM

The problem is that the relation is not always linear. The rate of temperature change influences  the correction required due to delayed transmission of the temperature in the different telescope parts. Have a look to the graphs " temperature -> focus position" at the end of this FocusMax presentation

 

 

https://www.focusmax...ng FocusMax.pdf

 

as found at:

https://www.focusmax.../documents.html

 

 



#5 Bob_Stan

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Posted 04 March 2021 - 02:20 PM

I am no expert, but after looking into the possibilities of temperature compensated focusing with an SCT I decided it isn't worth it.  The temperatures you are sensing may not reflect the accurate temperature changes at your mirror (if using an SCT or Newtonian).  I found it much easier to keep exposures at a reasonable range and refocus after a number of exposures.



#6 DuncanM

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Posted 04 March 2021 - 07:08 PM

I have a ZWO EAF focuser with the temperature probe fitted.

 

I've noticed that changes of a little over 1 degree can significantly change the focus of my RASA enough to be noticed.

 

I understand that focus adjustments due to contraction or expansion of the OTA are extremely linear so easy to compensate for by temperature.

 

The ZWO driver has a temperature compensation feature and the probe appears to be very sensitive.

 

Is there any software that will allow capture of the focus points vs temperature automatically or will I need to focus at any given temperature change & plot the results to work out if I truly see a linear data set?

 

I looked at T-comp- it appears to be a very expensive piece of "shareware" quality software at $50.00 which is quite expensive for a utility.

 

Does anybody know if there is anything free that will do the job or am I just better in trying to plot my own graph and feed the change per degree into the ZWO focuser at perhaps interval measurements of 2-3 degrees?

 

Please let me know your thoughts.

The EAF has an ASCOM driver. What software do you use now?

 

You can calculate the coefficient of expansion using your tube dimensions, or you quickly calculate it by doing an autofocus, and note the focuser position and temp, then refocus after the temp falls by several degrees and note the temp and focus position again. The thing is that the OTA will have some degree of depth of field (confusion zone = CZ) and as long as the focuser keeps the focus point in the CZ you're OK. However, unlike a crayford you probably have some image shift when you refocus, so it's probably best just to run an autofocus between subs, when the temp drops by a set amount or some given number of subs.




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