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Autofocus, temperature compensation, and dew heaters

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#1 Tom K

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Posted 14 February 2019 - 01:33 AM

Greetings

 

I have a question regarding temperature compensation to trigger autofocusing. If I am not mistaken, one of the reasons to do temperature compensation is related to the contraction of the glass in the lens more than expansion or contraction of the OTA. Of course, I may be totally incorrect here.

 

If I have dew heaters keeping the lenses warm, but my temp sensor is monitoring the ambient air temperature, won’t the temp compensation triggered focuser be performed even though the lens temp may not have changed anywhere near what the change is at the focuser?

 

Thanks

 

Tom



#2 james7ca

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Posted 14 February 2019 - 03:06 AM

I think most of the focus shift will happen because of changes in the length of the optical tube. The temperature may also affect the lens but I'd expect that the issue there would be temperature instability (or rapid change) which could temporarily distort the lens elements (resulting in poor quality images).

 

In any case, you'll have to calibrate the temperature compensated focusing which means you'll be measuring the response from the entire system (lens elements, tube, focuser).

 

In any case, I know that Tele Vue says that they improved the lens cell on their NP101is and NP127is scopes to reduce focus drift with temperature changes, so it can be a complex set of factors. Also, since the NP scopes use a pair of widely spaced elements the temperature-shifted focus actually moves backwards from that of a typical doublet or triplet. What I mean is that the focus behaves as if the tube has gotten longer with decreasing temperature (the opposite of what you would expect from simple tube contraction).



#3 Tom K

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Posted 14 February 2019 - 09:00 AM

Thanks - I am not sure where I got the idea that most of the effect was in the glass.   I did some poking around and for my ED80 aluminum tube a 10 degree F change would change the length about 104 microns (per Engineering toolbox website).   According to the Wilmslow Astro calculator my CFZ is about 100 Microns.   If I were to guess that I was in the midpoint of my CFZ at perfect focus at the outset then I would then be out of it.   

 

I stand corrected - thanks!

 

I have two empty slots on my dew heater controller - perhaps I could add two bands to the tube itself to regulate its temperature!  lol.gif



#4 mranch

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Posted 11 March 2019 - 01:39 PM

Hello Everyone,

I've been looking into temperature comp. focusers lately. What are some of the better brands and models? Thanks

Bob



#5 Tom K

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Posted 18 March 2019 - 08:31 AM

I now have two Moonlite focusers - one for my SCT and one for the refractor.   Both are great and both use the PnPFocus Arduino controller that I made.




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