Understanding the numbers

[Hans Tucker]

Does anyone have experience with understanding refractor objective specifications and can put in layman terms the following:

RMS 16 nm Lambda/40, V 97,4

[casstony]

If that means 1/40th wave RMS and 97.4 Strehl, it's a top quality objective. Maybe some context would help.

Mid tier manufacturers often present figures only for the best wavelength, while top tier manufacturers produce scopes with a high strehl ratio across a wide range of wavelengths.

[clive milne]

Optical quality ratings are so universally abused by opticians as to render them essentially meaningless unless derived from a test done by an independent 3rd party. Some questions to ask are; Was the test done using the full aperture (was the edge masked?)
Was the astigmatism term deleted? (a common practice believe it or not)
If the optic is a refractor... what is the performance of the objective from c - f
Was the strehl derived from an actual measure of the whole optic or an inferred figure of revolution derived from measurements taken over a small percentage of its surface.

Was the test structured to report the level of correction for a source at infinity... or have they conveniently removed the Z term and blown smoke over it under the guise of propriety 'testing technique' as one well known British manufacturer is known to do.

You get the gist I hope.....

[Hans Tucker]

It's a shame the IIS server crashed and all the previous responses were deleted..this thread was starting to kick off. As I wrote, before the crash, the numbers quoted are apparently from the test report of a Zeiss APQ 150 Refractor which is up for sale for the bargain price of $25K. Zeiss refractors are known for their quality of optics and some have reported that the performance of the APQ surpass AP & Takahashi but does that warrant a price tag twice that of the competitor of same size. I was curious about the strehl of 0.97 which is below that of the TOA-150 which apparently has a strehl of 0.99. Does the use of coatings affect the strehl value? Can a refractor with a lower strehl value out perform a refractor of a higher strehl? The late Thomas Back did not like to provide customers with the strehl of the scope he sold them because he believed the values would be mis-interpreted when customers inevitably compare their scopes.