Thread: Strehl ratio
View Single Post
Old 02-09-2019, 04:07 PM
gregbradley's Avatar
Registered User

gregbradley is offline
Join Date: Feb 2006
Location: Sydney
Posts: 16,419
Originally Posted by Peter Ward View Post
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.

Spot on Peter.

Visual specialists for sure can tell the subtle differences between optics and splitting double stars etc, different shades and details on Saturn and Jupiter. Warm colours, neutral colours, false colour on the edges of the moon.

Imagers though are far more tough on optics. A highly precise and well designed optic stands out. It punches above its size in clarity and detail.
Stars are smaller and tighter, sharp details are able to be coaxed out. There are no spurious colours and rings around stars.

Visually a high end optic gives a wow factor at the eyepiece.

The most obvious difference with high end optics is whether the stars are round in the corners or not, whether there is false colour around the brighter stars and how sharp is the detail in the image.

I also totally agree with the need for high end mechanicals. This is where some earlier FSQ EDX models fell over. There were too many having issues with the focuser who used large heavy cameras, filter wheels and guiding gear. Luckily the ones I had were good.

A good Feathertouch focuser or AP focuser is the closest you can get to a guarantee you won't have these issues. Typically the cheaper scopes tend to have this as a weak spot although it seems to have improved a lot over the last several years as Chinese made scopes have improved and improved. Nice FPL 53 triplet optics have come down in price dramatically in the last several years. Also there is a supply of Canon Optron FPL53 triplets that make there way into several brands as well.

Reply With Quote