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Focus drift

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#1 T~Stew

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Posted 04 February 2024 - 02:24 AM

I've heard one or two mentions about focus drift during the eclipse. I'm wondering for those who have shot a prior eclipse how much of an issue it is. I know to have my gear out and acclimated. I'm not sure that much heat will enter the optics with a solar filter in front. Then things perhaps cool again during the full... is there any real issue with cooling during this time?

 

I'd like to minimize fuss as much as possible around totality, to enjoy it myself, so hoping for a final focus check maybe 30 minutes or more prior and then hands off. Is it worth wrapping my lenses in reflectix or anything? I'll likely have two dslr/lenses on one GEM tracking the whole event, and perhaps an Ha solar scope on another (not worried about totality with that one as it'll be Ha only). Trying to do my overthinking well in advance. grin.gif



#2 ch-viladrich

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Posted 04 February 2024 - 10:43 AM

Yes indeed, focusing drift with temperature makes can be quite significant. For example, on a Mak 127 f/12, I had a 0.5 mm focus drift during the last 20 min before C2 (TSE. 2016 and 2017).

 

So it is better the re-focus 10 min, or even better 5 min before C2.



#3 T~Stew

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Posted 04 February 2024 - 07:24 PM

Yes indeed, focusing drift with temperature makes can be quite significant. For example, on a Mak 127 f/12, I had a 0.5 mm focus drift during the last 20 min before C2 (TSE. 2016 and 2017).

 

So it is better the re-focus 10 min, or even better 5 min before C2.

Do you think perhaps wrapping my lenses in reflectix would minimize this?

I have a sufficient quantity, I bought a roll for my SCT so I can take it outside without having to worry about temperature acclimation. That wasn't for focus shift but for thermal currents, so I am not sure if this technique would apply to the eclipse. But if there would be a way to avoid refocusing close to totality it would be worthwhile to pursue.



#4 ch-viladrich

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Posted 05 February 2024 - 06:38 AM

Do you think perhaps wrapping my lenses in reflectix would minimize this?

I have a sufficient quantity, I bought a roll for my SCT so I can take it outside without having to worry about temperature acclimation. That wasn't for focus shift but for thermal currents, so I am not sure if this technique would apply to the eclipse. But if there would be a way to avoid refocusing close to totality it would be worthwhile to pursue.

In fact, the data I gave was with the OTA wrapped in two safety blankets :

http://astrosurf.com...nate/Mak127.JPG 

 

This is probably better than nothing. I'll do it again for next eclipe. Still, this is not enough.

 

This being said, it could be than some optics are more "thermally" stable than others. Maybe have a black or a white OTA makes a difference. In any case, to be on the safe side, I focused until the last 5-10 min or so during the last eclipses I observed (Takahashi FSQ 106, Mak 127 f/12, Sigma 150-600).

 

BTW, I also used a safety blancket for solar imaging with the C14 years ago .


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