Thread: F-ratio myth
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Old 20-02-2018, 07:34 PM
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Peter Ward
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Peter Ward is offline
 
Join Date: Dec 2007
Location: The Shire
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F-ratio myth

I have been puzzled by recent posts about Fast optics (eg F3.8) having the ability to capture data in less time than a slower optic (eg F8.0) .

This old chestnut was well addressed by Stan Moore....and I still have to agree with him.

Aperture is king.

Having recently dusted off my FSQ and its delightful F5 optics, I can say with certainty it does not gather data very quickly...its humble four inch aperture is not exactly a light bucket.

At F5.5 I have another optic I like to use (with five inches of aperture) which demonstrably gathers photons faster than my FSQ....despite being half an f-stop “slower”

I suspect the f-ratio myth persists as a carry-over from photographic lenses....which vary aperture at a constant focal length.

...not something you’d do with a telescope optic, unless you want to purposely reduce the aperture with a field-stop, then wonder when you removed the stop, why things got brighter!

This is not to say fast telescope optics with fine pixels is a folly.

Far from it!

Mechanically smaller (hence less taxing on the mount) plus they give good sampling and a wider field compared to the same aperture in a longer focal length. The big acreage CCD’s required to get the same field are not cheap! (or the filters/field correctors)

...but if you think more flux will fall into a smaller pipe...regardless of the F-ratio...

As they said in “The Castle”....tell ‘em their dreamin’.
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