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Does paracorr add any SA?

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#26 Starman1

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Posted 06 May 2013 - 04:26 PM

When the image is larger then the Airy disk it is no longer diffraction limited by definition. So when one has exactly 1/4 wave optics they produce an image in which all the light just fits inside the Airy disk. So this is the upper bounds of the shaded yellow area. So one can relate the theoretical diameter of the spot formed when using a Paracorr which in this case is labeled Airy disk radius to the wave front and estimate the amount of spherical aberration the Paracorr is generating.

- Dave

I disagree with that analysis.
1/4 wave of SA in the wavefront modifies the MTF and changes the image in a detectable way in the star test and on the test bench.

The Airy disk is determined by the aperture and f/ratio of the scope, and would be no different if the scope had ZERO spherical aberration.

The idea that 1/4 wave is somehow "diffraction-limited" or essentially produces a perfect image no one can tell apart from perfection is a historical, experiential, analysis by 18th and 19th century observers. 1/4 wave SA is very noticeably different on an MTF graph from perfect optics.

So your assumption the Airy disc size represents 1/4 wave of SA is incorrect. It represents theoretical perfection. The wavefront error that produces visible errors in the star image can be much smaller than 1/4 wave, per Suiter, and Aberrator, though the aberrations that throw light out of the Airy disk into the diffraction rings will reduce the size of the disk rather than increase it.

Though, I agree, if the SA added by the coma corrector is additive to that already present in the optics, it's not ideal. But it is obvious from the graphs the addition will be very small. I can vouch for that, since I DO know how to perform a star test (and I usually see all kinds of aberrations in scopes, the two most common of which are astigmatism and spherical aberration), and the With/Without tests using the Paracorr II display no visible axial changes at all in my f/5 scope.
Of course I cannot extrapolate those results to f/3, but I can speak about f/3.6 and a Paracorr I, where the improvement in star images was profound. And that aperture was always seeing limited anyway.

There are many things to be concerned with in optics, but the added SA from a Paracorr isn't one of them.
But don't take my word for it--call up TeleVue and ask.

#27 jpcannavo

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Posted 06 May 2013 - 08:47 PM

But it is obvious from the graphs the addition will be very small. I can vouch for that, since I DO know how to perform a star test (and I usually see all kinds of aberrations in scopes, the two most common of which are astigmatism and spherical aberration), and the With/Without tests using the Paracorr II display no visible axial changes at all in my f/5 scope.


The star test is sufficiently sensitive that if ain't revealing an aberration to an experienced practitioner, the presence or absence of said aberration is of academic interest only.

http://www.opticsinf...ri=josa-50-1-21

Also agree with Don in pointing out what may be some questionable interpretations of the Televue diagrams, perhaps involving a confusion of two entirely distinct phenomena.

A spot diagram is a purely geometric optic ray tracing that assumes light to be moving in simple straight ideal lines. As such, a spot diagram for a perfect optical system generates an infinitesimal point, while for an aberrated system it generates some blurred - i.e. a non-point - spot. But, in contrast to this geometric optic idealization, the physical optics of the situation calls for an Airy disk (wave nature of light, diffraction etc) from the perfect system - which is not an infinitesimal point at all. Now, recognizing this as something of an apples and oranges mix, the size of the Airy disk is typically included in spot diagrams to provide a metric of sorts to the depiction of blur size. (Note that in the TV website the more usual depiction is essentially mapped onto a graph - spot size and Airy disk on y axis etc - but the relevant distinctions are the same) Moreover, associated with this deliberate mixing of apples with oranges is a rough rule of thumb, whereby optical systems generating blur sizes no larger than the Airy disk are sometimes taken as being well corrected with respect to the aberration in question (coma, chromatic aberration, astigmatism etc.). But a quantification of wavefront optical quality (i.e. strehl, MTF, wavefront error) is by no means some simple (i.e. independent of the specific aberration/aberrations in question) function of the ratio of blur size to Airy disk size. So, for example, the deviation from perfect strehl for a blur size exactly equal in size to the size of the Airy disk is not the same when the aberration in play is astigmatism than when it is spherical aberration - a fact that is by no means evident from a mere inspection of said blur and Airy disk depictions, and instead calls for a far more complex level of analysis.
Joe

#28 Achernar

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Posted 07 May 2013 - 07:38 AM

Tried that with my 15-inch F/4.5, which does have slight spherical abberation. I can see no change insie and outside of focus with and without the Paracorr. If it introduces spherical abberation of its own, I haven't seen any sign of it.

Taras






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