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Adding 1/3 of Filter Thickness to Back Focus

Astro Buer

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Newbie question...

I’m looking at the Skywatcher Espirit 80 ED.

With the flattener (using it with the supplied M48 spacer) the back focus distance is 55mm.

My current spacer setup for my ZWO ASI1600 is 56mm.

I’m using Astrodon filters which are 3mm, so adding 1/3 of the filter thickness would give me a back focus distance for the ED 80 flattener of 56mm?

If this is correct I will be bang on, obviously... but just wanted to ask other members if this it is crucial to add the extra distance?

*hoping you all say yes so I don’t have to change the spacers!!*


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Yes, your assumptions are correct. The 55mm stated for the ED80 is the spacing in air. The filters will move the focus point a further 1mm away from the flattener. So 56mm is the right spacing with 3mm thick filters. ?


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