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Orion Optics v Skywatcher!

philsail1

By philsail1
in Member Equipment Reviews

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John

John

Posted
Posted

With a Strehl value around 99% and PV wavefront aprox 1/10PV wavefront I guess its time to order the Ultra Grade OO 24" f4 mirror set[/] c/w Hilux Coating. Price at just £6,355.00 :undecided:

Can anything actually have 100% Strehl - or is that impossible ?

John

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Milamber

Milamber

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No, you can't. As I said, the Strehl rating is for an *element*, not a system. It would be same as saying... oh, I dunno, maybe "I have three cars, a Ferrarri and two Clios. The Ferrari does 10 miles to the gallon, the Clios do 55 miles to the gallon. So, the Ferrarri really does 40 miles to the gallon."

Arthur

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Milamber

Milamber

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Whatever Gaz - but it's meaningless and indefensible... whoever is doing the comparison. One can quote values for individual components in a closed system but to give a value for the whole entity and to call that value a Strehl Ratio displays an inherent misunderstanding of the rating system itself and an inability to understand the meaning of that system of measurement. It's also misleading too :undecided:

Arthur

PS - I would doubt that the values you have seen were for the complete systems, care to give an example?

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GazOC

GazOC

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Posted

The point I was trying to make was that the Strehl is the percentage of light in the Airy disk vs. the "perfect" mirror/ lens. Any figure modified by a central obstruction is still "meaningful" as long as you bear in mind that fact. I'd also appriecate that if you are going to insult me you don't stick a smilie on the end of the sentence, it keeps things clearer IMHO.

You can pick your way though this thread Arthur, it has both sides of the "argument".

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/2252325/page/0/view/collapsed/sb/9/o/all/fpart/1/vc/1

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John

John

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Despite the slight "frisson" to them, the past few posts have been interesting - thanks Gaz and Arthur :undecided:

Personally I think it would be very useful to know the combined performance of the whole optical system, more useful in fact than the stats for just one componant in it. Perhaps Strehl is not the most useful way of expressing this ?.

This is also one of the reasons that I find it difficult to understand OO's philosophy with regard to the telescopes they sell - why put mirrors that are made to a high standard in a mediocre optical tube ?.

John

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GazOC

GazOC

Posted
Posted

You're right John, its not the best way to measure the quality of an optical system (I don't think I said it was? :) ) but its still a decent way to measure the quality in a system as long as you either allow for the effects of the secondary/ CO or just compare "like for like" scopes.

Nevermind, I'm sure most people understood the point I was trying to make.... :undecided::)

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Milamber

Milamber

Posted
Posted
I'd also appriecate that if you are going to insult me you don't stick a smilie on the end of the sentence, it keeps things clearer IMHO.

I'll try to remember that sir, when I do want to insult you.

You can pick your way though this thread Arthur, it has both sides of the "argument".

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/2252325/page/0/view/collapsed/sb/9/o/all/fpart/1/vc/1

I think not - as you say, it's just another arguement and I have other plans for today. However:

"Strehl ratio ... is the ratio of peak diffraction intensities of an aberrated vs. perfect wavefront."

"Strehl ratio is an expression of the amount of light contained within the Airy disk as a percentage of the theoretical maximum that would be contained within the disk with a perfect optical system."

"The [strehll] ratio of the intensity at the Gaussian image point (the origin of the reference sphere is the point of maximum intensity in the observation plane) in the presence of aberration, divided by the intensity that would be obtained if no aberration were present, is called the Strehl ratio. "

"The Strehl Ratiois the amount of light that the optics put in the center of the Airy disc, divided by the amount of light that would have been put there, if the optics were perfect. "

In this case, (see above), it looks like the evidence is against me though, and so I have to say that you were correct and I was not. I still think it is misleading to do things this way as it allows too many variables to be introduced into the equation but there you go.

Arthur

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barkis

barkis

Posted
Posted

Why don't we just revert back to the Foucault test with the Coude mask, or the Everest test with a stick across the mirror's diameter with pins stuck in at the zones. Throw the Zygo in the bin, and get back to some normaliity.

Ron. :mrgreen:

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lunator

lunator

Posted
Posted

I have been following this thread with interest being an owner of 2 OO scopes and a skywatcher.

I do like my ST120 it is a great scope.

Last night I spent 2 hours collimating the OMC250 and once this was done the scope was also properly cooled.

I live in SW London and the sky is never great but last night at around Midnight I was observing STF2900AB using the 6 &5mm orthoscopic eyepieces. This gives mags of x375 & x450.

The secondary is about 0.65" from the primary and it was clearly visible during times of good seeing.

I hope this shows that even from a VERY average site it is worth having the best mirror you can get and spend time making sure all the optics are set up as well as you can, it's a pain but it is worth it.

Cheers

Ian

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ngc2403

ngc2403

Posted
Posted

very good guys i like this sort of just about light hearted battle,

something i was thinking about the other day was the strehl ratio of APO's and reflectors.

Apo's have a strehl and no CO;- strehl = quality,

Newt's have a strehl for the main mirror & CO;- strehl * diffraction of CO = Quality,

etc

so take a telescope with a strehl of 0.80 and a CO diameter of 20% of the primary, from the equation that gives, Light In the first order after the CO is;

I = [1-(v)2]2

Where; v is the ratio of the obstruction / mirror diameter.

; I illumination of the first order

by multiplying the I and the strehl the quality of any telescope could be compared.

ally

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philsail1

philsail1

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Just how could any telescope manufacturer give (a layman) an accurate guide as to the visual performance of a given telescope, when there appear to be so many variables involved!

I think I'll go along with "wingman," and trust the MK1 eyeball test!

Regards,

philsail1

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Milamber

Milamber

Posted
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My point, how can you do anything else? The manufacturer has no idea what EP you are going to use, so with the best will in the world any test results will be arbitrary to say the least. The 22" figure was tested with a Foucalt thing and *nobody* that looked through it complained about odd shapes!

The eyes have it I say glasses-slip.gif

Arthur peace.gif

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