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Does an IR850 pass-filter limit the achievable resolution in reality

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#1 jesco_t

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Posted 26 February 2021 - 02:15 PM

I have an ZWO IR850 pass-filter that I use for shooting the moon. It helps a lot to stabilize the seeing compared to a simple UV/IR filter.

I am fine with the dimmer images, getting enough frames isn’t an issue since I am around 10ms even at f/20.

But I wonder it limits the achievable resolution in practical use? Simple optics tell me that the deeper I go into the IR regime, the lower the resolution will be. Comparing my lunar images to others on the net, I feel that mine are good but not as good as they could be.

Is it worthwhile to invest in an IR pass filter that’s not so far in the IR. There’s quite some options.

Thanks, Jesco

#2 Tom Glenn

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Posted 26 February 2021 - 02:27 PM

Yes it will limit the resolution, especially in small to moderate size scopes.  Assuming that you are using proper technique (perfect collimation and focus), then the two factors that limit resolution are the atmospheric seeing, and diffraction.  In smaller scopes, you can frequently be diffraction limited in good seeing, in which case your diffraction limit is defined by your aperture and wavelengths.  An 850nm filter will reduce your resolution by a factor of about 40% compared to a red filter, and over 50% for a green filter.  In very large scopes, you are less likely to be diffraction limited, and so the loss of resolution in IR will be less of a factor.  However, the 850nm filter is not recommended for lunar imaging except in extremely large instruments.  If you want an IR filter, I would stick with either a 610nm or 685nm filter.  But there are many options.  


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#3 jesco_t

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Posted 27 February 2021 - 04:07 PM

Hi Tom, thanks a lot for the explanation. I just placed an order for an IR642 from Astronomik. Can’t wait to try it out!

Cheers, Jesco

#4 j.gardavsky

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Posted 27 February 2021 - 05:27 PM

You can also try the 35nm wide H-Alpa filter,

I am sometimes using for visual on the Moon, and have had a nice contrast and detailed view on the albedos on Mars in 2020.

 

https://www.teleskop...tergewinde.html

 

Best,

JG


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