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Strehl ratio question

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#26 gnowellsct



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Posted 28 June 2020 - 04:08 PM

No spacers. Do you see spacers in your ED/apo's? I've never seen any.


Refractors can have a turned edge. Would you accept a Strehl ratio of a mirror that only measured 95% of the aperture?



Actually that is an interesting question.  Is it not the case that many, many Newt mirrors have a tad of TDE and that masking it would improve the Strehl?


Greg N

#27 BillP



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Posted 28 June 2020 - 04:29 PM

The Strehl number of a telescope is how close the telescope is to the perfect design for (that) telescope.  Obstruction has nothing to do with it.  An SCT, if perfectly executed, would have a Strehl of 1.0.  Same way with a Newt.

True that obstruction does not have anything to do with it.  But one needs to look at the strehl in context.  If used for assessing the optical precision of a component of the scope, which is most often true, then fine.  However, there is absolutely nothing wrong or incorrect about wanting to determine exactly how far the point spread function (PSF) deviates from ideal, i.e., the strehl, for the entire system with all components interacting.  This is actually IMO a little more meaningful as it will be telling me what the PSF will be at the eyepiece so how much energy is going into the diffraction rings instead of the spurious disk of the airy pattern.  Frankly, I could care less about individual components.  So if a manufacturer tells me the strehl of the main mirror on an SCT that is just one link in the chain.  Would really need to know how precise the corrector and secondary are also, and the CO size, to come to an idea of the overall impact in the entire system.  I mean when I am using any telescope, I am never using just one component.  Would be a much nicer test report for me if every assembled scope was tested as a complete operating system as that is the only way it is used.  So when purchasing a scope, factoring the impacts of any CO is important.  PSF and MTF for the system at the eyepiece for each scope that comes off the production line would be a nice report.

Edited by BillP, 28 June 2020 - 04:30 PM.

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#28 SeattleScott


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Posted 29 June 2020 - 04:48 PM

I only have one scope with a test report, from the vendor, I believe done in red wavelength if I remember right. So instead of being 1/8th to 1/9th pv it is probably more like 1/6th to 1/7th. Maybe a bit better than typical mass produced stuff. But it delivers sharp views so I am happy with it. I don’t really think of it as a premium mirror so much as a high quality one. But it is good enough I don’t have an urge to refigure it. It is sharper than the mass produced reflector it replaced.

Likewise I replaced a 100ED with a Vixen ED103S. No idea what the Strehl of either is. But the Vixen provides sharper views.

In both cases, I replaced relatively cheap scopes with somewhat higher end options. In both cases the higher quality scopes deliver better optical and mechanical quality, so money well spent in my opinion. The reflector is no ZOC. The Vixen is excellent but probably not quite as good as a new SVX or TEC or Tak. Not that I have compared it to a premium Apo. For me it is more about the brand reputation than a test report. I mean ZOC doesn’t give test reports. Doesn’t keep people from buying them.


Edited by SeattleScott, 29 June 2020 - 04:49 PM.

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#29 vahe


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Posted 30 June 2020 - 02:54 PM

If you really want to know what an obstruction can do to the image there is a simple test that allows you to see it for yourself.


First and foremost set aside all numbers, take a refractor, preferably an APO and a high quality obstructed scope, examine the surface of the full Moon at high power, there should be no shadows and decide for yourself which scope is able to deliver a clean and detailed image.


I have done this test many times, only an APO will pass this test, no obstructed instrument comes even close.



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