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About parfocal filters

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#1 mgutierrez

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Posted 21 June 2021 - 02:37 AM

Hi all,

 

does it make sense to use parfocal filters (in a filter wheel) with achromatic or even doublet refractors? By definition (most on achromatic), blue, red and green light waves focus at different focal distance. So, is it meaningful to use parfocal filters on such scopes if I would always need to refocus?

 

kfoxiaa7n8xnphme.png

 

thanks!



#2 happylimpet

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Posted 21 June 2021 - 04:36 AM

Not really, though the offsets will be small, so if thats advantageous, then its worth it.



#3 mgutierrez

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Posted 21 June 2021 - 04:56 AM

Not really, though the offsets will be small, so if thats advantageous, then its worth it.

Thanks for the reply. But you, for example; do you have a cfw and parfocal filters? do you refocus when changing let's say from red to green?



#4 happylimpet

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Posted 21 June 2021 - 05:07 AM

Thanks for the reply. But you, for example; do you have a cfw and parfocal filters? do you refocus when changing let's say from red to green?

I have parfocal filters (mostly!) but also a newtonian, so the OTA doesnt induce any focus shift.

 

I use NINA which allows for focus offsets between filters, but i mostly use baader filters with the same thickness, so its only a couple which need offsets (like my ZWO 850nm longpass).

 

I dont refocus when changing between filters i know to be parfocal - this is basically all my Baaders, so RGB, 610 longpass, Ha, SII, clear and OIII. Most, now I think of it!



#5 mgutierrez

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Posted 21 June 2021 - 05:16 AM

I have parfocal filters (mostly!) but also a newtonian, so the OTA doesnt induce any focus shift.

 

I use NINA which allows for focus offsets between filters, but i mostly use baader filters with the same thickness, so its only a couple which need offsets (like my ZWO 850nm longpass).

 

I dont refocus when changing between filters i know to be parfocal - this is basically all my Baaders, so RGB, 610 longpass, Ha, SII, clear and OIII. Most, now I think of it!

great, thanks for the info. Yes, I guess that using a newtonian changes everything since RGB light focus point do not change. Thanks for your help!

 

m


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