What's in a Number

[Hans Tucker]

Can you visually see the difference between a refractor that has a Strehl ratio of 0.95 compared to refractors that have Strehl ratio of 0.99 or 0.998?

[Leo.G]

I don't know if the following link may help answer your question Hans, it's beyond the capabilities of my pathetic brain (any more than 2 syllable words are beyond my comprehension) but it is an interesting read on cloudy nights discussing much the same and a couple of other interesting links:


https://www.cloudynights.com/topic/860203-strehl-ratio-overall-vs-by-wavelength/


https://www.cloudynights.com/topic/864600-095-vs-higher-strehl/



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


I only have achromat refractors, I'm more concerned about street light ratios than Strehl ratios.

[gregbradley]

You would probably notice it more in images.

Greg.

[By.Jove]

The first issue is whether you’re looking at a monochromatic (probably green) strehl measurement or polychromatic (visible spectrum) in which case the figure is invariably worse. From memory there is a detailed analysis I’ve seen suggested refractors - including APOs - cannot exceed strehl 0.95.

AFAIK you need a reflector (Newtonian) to achieve better.

Then there is this page.
https://www.telescope-optics.net/commercial_telescopes.htm

[Peter Ward]

In company with a good friend who I’d classify as one of the most skilled planetary observers in Oz we did just such a test.

Comparing two excellent refractors (.97 vs .94 ) it was extremely difficult to separate them visually but at silly magnifications (read 1200x) the higher strehl unit simply ran out of light rather than showing any image degradation (as was seen in the .94).

Even then you had to wait for fleeting moments of excellent seeing.

Where a higher strehl really comes into play is because it gives you more of a buffer from seeing perturbations when imaging. Systems that are barely diffraction limited instantly fall below that threshold with any seeing disturbances and often fail to deliver tight stellar profiles as a result.