Thread: Strehl ratio
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Old 02-09-2019, 11:26 AM
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Peter Ward
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Peter Ward is offline
Join Date: Dec 2007
Location: The Shire
Posts: 7,139
I've owned and used many 'scopes over the years: C8's, C9.25", C11, Questar 3.5, Questar 7, Meade 10SCT, Takahashi FSQ106, Takahashi FSQ85, AP130, AP130GTX, AP155EDF, AP 305RHA, AP92, GSO RC8, GSO RC10, William Optics 132, William Optics 80mm, William Optics 90mm, RCOS12.5, RCOS14, Alluna RC16 and Alluna RC20.

While it has only been in more recent times that I have been privy to the test data of these instruments....I can say without doubt the higher the Strehl value, the better the results I've seen or managed to achieve.

However, there is more to an imaging system than just the optical quality. Those optics need to be mechanically and thermally stable, and I've found without a rigid focus mechanism you are always chasing your tail, particularly with larger sensors, keeping the entire field in focus.

Strehl counts for naught if the mechanicals won't hold stable focus! Then there is the mount....(won't go there just now)

The good news is manufacturers than actually take the time to actually certify their optics (though some manufacturers, who will remain nameless, have proved to have zero cred, their certificates should have been issued on toilet paper) also tend to spend time making the other mechanical elements of an optical tube assembly worthy of their optics.

I have read with interest heroic accounts on IIS where users have literally re-built OTA's replacing all but the optics, yet still have residual problems, despite the optics likely having an acceptable Strehl.

So it is not all about the optics, but all things being equal there and getting back to the point at hand...does a high Strehl produce better images?

I think unequivocally yes. (taken to it's absurd conclusion, comparing 0.2 Strehl and 0.99 Strehl system, the results are black and would be reasonable to assume there are shades of grey in between )

If a system is marginal and say has a Strehl of 0.8, then ANY seeing disturbance will drop the system below "diffraction limited" . A high quality system with say a Strehl of 0.98, can absorb significantly higher disturbances from seeing before it hits the diffraction limited stop.
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