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Equipment Discussions >> Cats & Casses

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yonkrz
scholastic sledgehammer


Reged: 11/16/06

Loc: SW Minnesota
Re: just for FUN.....anyone care to speculate? new [Re: Dwight J]
      #5612825 - 01/08/13 10:24 PM

Pretty awsome Dwight,a lot bigger than i was expecting,thanks for the pic.

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freestar8n
Post Laureate
*****

Reged: 10/12/07

Re: just for FUN.....anyone care to speculate? new [Re: TG]
      #5613093 - 01/09/13 02:34 AM

Quote:

I own a C11HD with virtually identical inside/outside diffraction patterns and it wouldn't surprise me if it has better than 1/30 RMS optics (= 0.96 Strehl), which is what Roddier testing my erstwhile C9.25 showed me. It's "real Strehl" may be 0.80 but then it's an 11" scope and where do I find an unobstructed 11" scope in a compact package?




The "real" Strehl is what it is and somehow factoring in CO would not be a real Strehl. Strehl has two magic properties:

1) For a given optical system (with or without CO) the peak irradiance only depends on the RMS wavefront error (including defocus) - for small amounts of aberration. The nature and profile of the wavefront error doesn't matter.

2) In addition, as you increase the error, the energy will shift from the central peak to the rings in the same way - regardless of the type of aberration. This means that not only is the height of the central peak determined by RMS alone, but the entire diffraction pattern and MTF. This is all an approximation assuming small RMS.

In contrast, when you change CO, you lose light in the central peak, and the central peak narrows as energy goes into the rings - which themselves shift. The MTF and PSF are now different from the effect of aberration alone, and you can't compare them directly.

Frank


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Asbytec
Guy in a furry hat
*****

Reged: 08/08/07

Loc: La Union, PI
Re: just for FUN.....anyone care to speculate? new [Re: freestar8n]
      #5613113 - 01/09/13 03:41 AM

Frank, you are correct and really what I think Tanveer was driving at using the term "real Strehl." Maybe it's better said to be nominal or system Strehl. In any case the lower peak intensity is a Strehl-like number defining peak intensity.

It can be approximated using Strehl(nominal) = I * Strehl (aberrant.) Where intensity is approximated using I = (1 - o^2)^2. So a scope with 0.95 aberrant Strehl can have a "diffraction limited" peak intensity of 80%, which I am sure Jon was driving at earlier. It's not as good as a refractor where it's peak intensity equals it's aberrant Strehl.

Anyway, in accord with the MTF (such as it is) the main difference between an aberration and CO is the size of the Airy disc: 1.22 Lambda/D in an unobstructed scope and 1.11 Lambda/D with a 0.3D obstruction. This accounts for the large shift of the curve to the right near maximum spacial frequencies (which are correspondingly higher in an obstructed scope, more so than an unobstructed same aperture.) The rings are correspondingly brighter with an obstruction, but the central disc is smaller allowing better resolution smaller that Raleigh's and Dawes limits (defined using a refractor.) The rings do not shift significantly in radius, unless one takes into account human vision. But, on the focal plane, the radii should be very similar, save for the first minimum.


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freestar8n
Post Laureate
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Reged: 10/12/07

Re: just for FUN.....anyone care to speculate? new [Re: Asbytec]
      #5613133 - 01/09/13 04:25 AM

Quote:

Maybe it's better said to be nominal or system Strehl. In any case the lower peak intensity is a Strehl-like number defining peak intensity.





Ugh - no. I would not associate it with Strehl at all - the poor guy. You could compare two clouds based on their heights and call it a Strehl - or refer to the Strehl of the stock market also based on the peaks achieved on different days.

When you have a small amount of aberration - both Strehl and RMS characterize the entire effect of that aberration - magically - with a single number. When you compare one CO to another, there is no such magic single number that describes the impact of the change on the optical system.

So - I would avoid using "Strehl" at all.

Also - when you only have aberration involved, there is no change in throughput to deal with and you don't have to worry about re-normalizing the peak intensity due to the change in area. When you add a CO, you change the throughput - so not only has the entire MTF and PSF changed in unusual ways, but the total energy is changed also.

I guess you could say one system has a peak irradiance of X watts/cm^2 and the other has Y watts/cm^2. And then describe the peak width and the heights of all the rings. That's a lot more stuff than the single Strehl or RMS value that works for a single system with different but small amount of arbitrary aberration.

Frank


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Asbytec
Guy in a furry hat
*****

Reged: 08/08/07

Loc: La Union, PI
Re: just for FUN.....anyone care to speculate? new [Re: freestar8n]
      #5613176 - 01/09/13 06:12 AM

Yea, it's a good point. Normally, I just refer to the final diffraction product as peak intensity since that incorporates both the CO effects (aberration and obscuration) and wavefront. Using Strehl loosely creates some confusion.

So, what was the original question? Care to speculate on a C8 and Q7? Well, let say the Q7 has a Strehl of 0.98 and a CO of 33%, while the C8 has a Strehl of 0.95 and a CO of 37%. I don't know their actual figures, but then again we're just speculating here.

Q7: I = (1 - 0.37^2)^2 ~ 0.79. (for both obstruction and added diffraction). Maximum intensity with aberration (approximated) 0.79 * 0.98 ~ 78% peak luminance, or something similar. All approximations.

C8: I = (1 - 0.37^2)^2 ~ 0.75. (for both obstruction and added diffraction). Maximum intensity with aberration (approximated) 0.75 * 0.95 ~ 71%.

Between the two and based solely on these conditions - and for fun - so far the Q wins. Now, add an inch of aperture. Can you make that call with approximately 8% difference in final peak intensity (and marginal reduction in ring intensity?) On star images, maybe. On planets, I'd speculate it's too close to call in one sitting.

Sometimes I think these comparisons come about because they are so close. Someone probably sits down and crunches the numbers to find a very closely matched pair then asks which one is best in this forum. Truth is, they are very close with the Q offering a slight improvement in mid contrast (more refractor like with a smaller CO and higher Strehl) and the C8's improvement is at higher resolution and the inch of added aperture might make up some mid range contrast advantage of the Q7. Anyway, its the same old argument wrapped in a different condition.

Next time, make it tricky using a Newt with a spherical wavefront and spider vanes. ()


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Slow Astronomer
member


Reged: 05/01/10

Loc: Milky Way Galaxy
Re: just for FUN.....anyone care to speculate? new [Re: Dwight J]
      #5614351 - 01/09/13 07:30 PM

All I know is a Q7 owner was constantly amazed at the "better view" of DSO's through my Orion ST120 than through his Q7! We used the same high quality eyepieces, swapping them back and forth. We finally concluded it was the difference in contrast we were seeing. We suspect the CO in the Q7 and the difference in focal length were the cause.

BTW - we did get a killer view of Uranus thru the Q7.

Clear skies, Dave


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GlennLeDrew
Postmaster
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: just for FUN.....anyone care to speculate? new [Re: Slow Astronomer]
      #5614461 - 01/09/13 08:25 PM

Dave,
What kinds of DSOs appeared better in the 4.7" achro vs the 7" Mak?

I could buy larger 'fuzzies' being easier to see in the smaller scope due to the ability to achieve both a larger exit pupil and field of view. But such things as globular clusters and compact planetaries will definitely be better seen in the larger scope.

Another question. As regards the difference in contrast, can you supply some details? Was this gauged at same exit pupil diameter? Or at same magnification? On the same target? Was it considered over a range of scale, from arcsecond level planetary to tens of arcminute DSO to full field scattering/veiling glare?

One of the biggest errors made when comparing instruments is not eliminating all variables which will affect results of the test. And this becomes ever more difficult with an increase in disparity between objective aperture.

To compare optical quality and contrast transfer, the most meaningful results accrue when the exit pupils are identical. For one thing, the image surface brightness will be the same (less the differences in transmission efficiency), which allows the eye to more reliably assess contrast differences. And it's the exit pupil diameter which controls the point at which aberrations become visible, due to it being the normalizing factor for all apertures.


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Slow Astronomer
member


Reged: 05/01/10

Loc: Milky Way Galaxy
Re: just for FUN.....anyone care to speculate? new [Re: GlennLeDrew]
      #5615591 - 01/10/13 01:56 PM

Glenn, I just want to make it clear that I'm not advocating the ST120 is better than the Q7, it was just startling to see the difference in contrast. The background through the ST120 just seemed to be "darker".

The tight planetaries (NGC2392 & NGC 7662) were definitely better in the Q7 as the FL allowed higher magnification. We also got a nice view of Uranus but that's off topic.

On globs, open clusters, galaxies & M42 our gut reaction to the view was definitely "wow" compared to the Q7. Again the "darker" background in the ST120 seemed to make them pop. List = NGC 869, 884 & M31, 35, 37, 38, 44.

We didn't compare exit pupils - but I did posit this difference to the Q7 owner at the next club meeting. We did compare like targets and try to match some magnifications. We were both using high quality EP's (TV Delos, Pan, Nagler & ES100) It was a casual observing session, not a critical side-by-side comparison of the two scopes.

We had expected mediocre viewing conditions due to the latent local light dome and moisture in the atmosphere but conditions really improved as the night went on. We were able to see Orion's shield and other fainter stars with the naked eye. Something that doesn't happen too often around here. Clear skies,

Dave

P.S. It was amusing that the Q7 owner, an experienced AA, spent most of his time on my ST120 with his jaw hanging open!


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GlennLeDrew
Postmaster
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: just for FUN.....anyone care to speculate? [Re: Slow Astronomer]
      #5615652 - 01/10/13 02:38 PM

Dave,
Regarding the hanging jaw syndrome, I know what you mean. An experienced observer forgot all about his fine 4" APO when I let him spend some time with my home-made 60mm right-angle bino. With the 13mm Ethos eyepieces delivering 20.8X and 4.7 degree field, and the Ultrablock filters installed, he said he'd never seen better views of the large nebulosities in Cygnus.


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