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Darren Drake
Carpal Tunnel
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Reged: 10/09/02

Loc: Chicagoland
Re: Does paracorr add any SA? new [Re: Starman1]
      #5843568 - 05/06/13 01:17 PM

I would like to see bench testing done with and without a paracorr in place for a relatively fast mirror. 1/100 wave difference is the same as no effect and I don't believe this is the case with faster mirrors. The Telescopes Eyepieces Astrographs book doesn't get real specific on this. It doesn't even mention the paracorr by name and says it has a mag of 1.11X instead of 1.15. There is also no detailed spot diagram I suspect since the specs aren't avaialble.

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DAVIDG
Post Laureate
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Reged: 12/02/04

Loc: Hockessin, De
Re: Does paracorr add any SA? new [Re: Starman1]
      #5843647 - 05/06/13 01:47 PM

The Paracorr adds more then 1/100 of wave error since the top of yellow shaded area on the Televue graphs is the diffraction limited cut off of 1/4 wave and the y-axis is linear. So for each graph one can divide that area up and determine what the scale would be in fractions of a wave. So the Paracorr is adding about 1/10 of wave or more of SA depending on what the f-artio of the mirror it is used on. For example if you look at the curve for the F/3.5 mirror the diffraction limited area starts at just a little under 3 on the graph so that is 1/4 wave and the Paracorr data starts at about 1.3 or so, which would translate into roughly an 1/8 wave of under correction. http://www.televue.com/engine/TV3b_page.asp?id=61&Tab=_con
I believe it has been discussed here, a number of years ago, of designing an astrograph using at that time the Paracorr which today is the Type-1 version in which the primary was made hyberbolic to correct for the under correction introduce by the Paracorr. I thought that Televue might have sugguested this as well.
The older Type-1 unit added more undercorrection then the new Type-2 units.

- Dave


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mark cowan
Vendor (Veritas Optics)
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Reged: 06/03/05

Loc: salem, OR
Re: Does paracorr add any SA? new [Re: Darren Drake]
      #5843768 - 05/06/13 02:51 PM

Quote:

Very interesting. So the paracorr adds undercorrection. I wonder if any opticians who KNEW that the mirror was gonna be used with a SIPS in place has ever intentionally added a slight amount of overcorrection as a result.




You might just discuss the amount of undercorrection the original Paracorr introduced with Al himself. That idea was on the table but I passed. However anybody else who wanted to probably could. More interesting however is to build hyperboloidal mirrors to a conic designed into a Paracorr III that would also get flatter field...

Dave is correct here regarding type I vs type II, and indeed it adds more for faster mirrors, basically the price you pay for the coma reduction.

NB: 1/100th wave was re. the Ethos on-axis SA design amount, not the Paracorr I or II and not the combination of the two. In practice (of course) the optics aren't manufactured at design levels, SFAIK, but the newer designs are intended to function better with the coma correction at fast f/ratios than older ones could. A lot of it has to do with how the converging beam intersects the elements of the EP as it approaches the focal plane, and it ain't simple.

Best,
Mark


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Starman1
Vendor (EyepiecesEtc.com)
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Reged: 06/24/03

Loc: Los Angeles
Re: Does paracorr add any SA? new [Re: DAVIDG]
      #5843846 - 05/06/13 03:42 PM

Quote:

The Paracorr adds more then 1/100 of wave error since the top of yellow shaded area on the Televue graphs is the diffraction limited cut off of 1/4 wave and the y-axis is linear. So for each graph one can divide that area up and determine what the scale would be in fractions of a wave. So the Paracorr is adding about 1/10 of wave or more of SA depending on what the f-artio of the mirror it is used on. For example if you look at the curve for the F/3.5 mirror the diffraction limited area starts at just a little under 3 on the graph so that is 1/4 wave and the Paracorr data starts at about 1.3 or so, which would translate into roughly an 1/8 wave of under correction. http://www.televue.com/engine/TV3b_page.asp?id=61&Tab=_con
I believe it has been discussed here, a number of years ago, of designing an astrograph using at that time the Paracorr which today is the Type-1 version in which the primary was made hyberbolic to correct for the under correction introduce by the Paracorr. I thought that Televue might have sugguested this as well.
The older Type-1 unit added more undercorrection then the new Type-2 units.

- Dave



The yellow shaded area is the size of the Airy disk, not 1/4 wave of aberration.


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DAVIDG
Post Laureate
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Reged: 12/02/04

Loc: Hockessin, De
Re: Does paracorr add any SA? new [Re: Starman1]
      #5843979 - 05/06/13 04:46 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


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Starman1
Vendor (EyepiecesEtc.com)
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Reged: 06/24/03

Loc: Los Angeles
Re: Does paracorr add any SA? new [Re: DAVIDG]
      #5844048 - 05/06/13 05:26 PM

Quote:

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.


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jpcannavo
scholastic sledgehammer
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Reged: 02/21/05

Loc: Ex NYCer, Now in Denver CO!
Re: Does paracorr add any SA? new [Re: Starman1]
      #5844496 - 05/06/13 09:47 PM

Quote:


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.opticsinfobase.org/josa/abstract.cfm?uri=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


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Achernar
Postmaster
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Reged: 02/25/06

Loc: Mobile, Alabama, USA
Re: Does paracorr add any SA? new [Re: Darren Drake]
      #5845105 - 05/07/13 08:38 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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