strehl and diameter as comparison?

[derherr65]

Would using strehl and diameter as a comparison of mirrors be accurate? For example would a 0.5 strehl 11.314" diameter mirror give the same view as a 1 strehl 8" dia mirror?

[sixela]

No. The very least you would want to know is the contrast transfer function of the two mirrors, and that's not just one number. And then you would get a multitude of answers - one for each spatial frequency. And even then, some people would say you really need the point spread function (2D) to really "know" things.

You just can't reduce reality to a single number.

The point is that many different kinds of error can cause a Strehl to go from 1.0 to 0.5, and each evil you pick does slightly different damage to different kinds of features.

[xboxdevil]

I hate to see unanswered questions, I'm not sure that I know the answer but at least I can bump this back up for you.

If it’s valid to say that Light gathering x strehl = transmitted light to focuser

Then we have transmitted light = (Pi x r^2) x strehl

So for the larger mirror we have (Pi x 5.657^2) x 0.5 = 50

And for the smaller mirror we have (Pi x 4^2) x 1.0 = 50

So all things being equal then I think the answer to your question would be yes. However, all things are not equal as it is unlikely that the rest of the optical train would be neutral. I think in practice you would need to adjust for central obstructions, different f ratio’s etc., but let’s no overcomplicate things for now.

I would think that the larger mirror would give a more detailed view but with less contrast than the smaller mirror.

I may of course be talking complete rubbish, so hopefully there will be someone along soon who won’t be making it up as they go along :-)

[derekm]

A Strehl ratio of 0.5 would be a pretty poor mirror. See:-

http://www.rfroyce.com/standards.htm

1/4 wave, which is Rayleigh limit is 0.82 Strehl and 1/3 wave is 0.71, so 0.5........ .

[FraserClarke]

Strehl ratio has nothing to do with light transmission. It's a measure of the optical quality of the mirror -- it's the ratio of the peak of the actual PSF to a perfect theoretical PSF (i.e. a Strehl of 1.0 is a "perfect" mirror). A Strehl of 0.5 means that the peak of the star image is only 50% of the theoretical maximum. The total amount of light is still the same -- just spread over a larger area.

It's rather hard to compare two mirrors with such different sizes and strehls. You'd need to define a "better" metric; i.e. best contrast/faintest galaxy/faintest star/etc... As Derek says though, a 0.5 Strehl mirror would be a very poor mirror.

PSF == Point Spread Function; what the image of a star looks like, basically.

[sixela]

If it’s valid to say that Light gathering x strehl = transmitted light to focuser

But it's not. The light still makes it to the focuser, but if the Strehl ratio is low it's just smeared out on places far from where the source is supposed to form an image. In other words, it won't give you a dim image, it'll give you a blurry image.