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Parfocal Filters and Refocusing

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#1 ESzczesniak

ESzczesniak

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Posted 18 September 2020 - 08:05 AM

The forecast appears much more favorable in my area than it has been for several days and I'm getting ready to image my second narrowband target, the Heart Nebula.

 

I am using an ASI1600MM with the ZWO 31 mm unmounted filter set, run through ASIAIR Pro.  The narrowband filters are listed as parfocal on the ZWO website.

 

I imagine best practice is to focus between filter changes?  I have 6.5-7 hours of time off my battery, so was planning on 3 x 6 hours nights for image acquisition.  I'd prefer to average image rotation, seeing conditions, etc Ha, OIII, and SII each of those nights.  But I won't be babysitting the scope to refocus.  ASIAIR does not yet have autofocus for filter changes.  

 

So after all my rambling, should I really just do a single filter a night or will it not be that big an issue with focus changes between filter changes?



#2 AtmosFearIC

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Posted 18 September 2020 - 08:35 AM

You really should be refocusing not only for filter changes but also for temperature change.
When I was imaging with my refractor I refocused every 1.25°C drop. With my RH200 I refocus every 3°C, I could do it less but it also has such a tiny focus zone. With my 10” F/10 I refocus every 90 minutes.

#3 ESzczesniak

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Posted 18 September 2020 - 09:32 AM

You really should be refocusing not only for filter changes but also for temperature change.
When I was imaging with my refractor I refocused every 1.25°C drop. With my RH200 I refocus every 3°C, I could do it less but it also has such a tiny focus zone. With my 10” F/10 I refocus every 90 minutes.

For better or worse, I'm aware that I should be doing time and temperature based refocusing.  Once ASIAIR Pro adds the auto focus, I will be doing so.  But unfortunately at this time in life with a full time job, a toddler, and a family; I can't afford to babysit the scope for the whole night.  I try to cool the scope down as much as possible so it doesn't change as much during the night and then accept the reality of life.  Luckily, I have a fairly short refractor and temperature swings overnight are typically about 5F at this time of year.  So typically thermal contracture should be about 0.03 mm, in the worse case scenario 0.09 mm (for my scope specifically).  I know everything matters with focus, but it's either that or don't image at the moment.

 

And I guess I already knew the answer to my question.  I shouldn't accept "parfocal" as truly parfocal.  I should make sure the filter is focused as best as possible and stick with that filter until I can focus again.  


Edited by ESzczesniak, 18 September 2020 - 09:33 AM.



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