Thread: Strehl ratio
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Old 01-09-2019, 07:16 PM
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Atmos (Colin)
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Quote:
Originally Posted by Ukastronomer View Post
It still isn't real life though is it

ten years ago I had an argument with camera club members who swore blind that different cameras/pixels, dxo marks, lenses made so much difference I set a challenge

As many people as you like take their cameras out together on to Bodmin Moore, we all set up same place, same iso, same f ratio, same shutter speed, and I will print the same images to A3+

NO ONE could tell any difference between a Canon?Nikon/Pentax, zoom or prime

I 100% ignore Strehl and any other, because unless you are compatring a 1k scope with a 100 scope etc, etc no one will "see" any difference between scopes of similar size/type/cost

Real world, I don't live in a lab
A lot of it comes down to whether or not you're pushing any system towards its limits, that's when the differences start to become noticeable. A perfect example of this can be when comparing two CFF reflectors which are of the same quality and price; a 400mm F/20 Classical Cassegrain and a 400mm F/8 RC. The focal ratios themselves don't make any difference per se but when viewing planets there will be a significant difference in fine contrast due to the secondary mirror size differences and this will show up in the Strehl of the two systems.

The larger secondary (usually around 50%) on the RC drops the Strehl as it takes a lot of energy from the centroid and deposits it in the airy disk. Someone who is better familiar with the terminology can correct me but I think I got the terms correct

Another example would be with a 150mm F/5 achro, it could have a Strehl of .98 in green due to having a very smooth optical surface which would be confirmable with a green filter on planetary contrast BUT such a refractor would have a polychromatic Strehl of .5 making it terrible for planetary viewing without any kind of filter.
If you wanted to do some comparisons with the same size, focal ratio and kinda same price, compare this against a 150mm F/5 newtonian. It could have a Strehl of .92 (polychromatic or otherwise) and there is no way you cannot tell me that you wouldn't see a difference between those two
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