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DesertRat
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Re: CO vs. All That "Other Stuff"! new [Re: freestar8n]
      #5615680 - 01/10/13 02:52 PM

Frank,

The graph depicts 0.075wv rms low order spherical in each case. Not exactly 1/4wv PTV but close enough for depicting what happens. I could provide a graph with exactly 1/4wv PTV levels, but it would not show anything new. In my code I always input aberration levels as rms figures, but can force them to be the classic breakpoints.

The software is homebrewed with elements begged and borrowed, and under a constant state of development. I use it for example in creating aberrated PSF's used in deconvolution and for work I'm currently doing in digital microscopy. It has created high fidelity fresnel zone plates, as well as propagated wavefronts in the near and far fields. I've verified its accuracy with respect to almost exact solutions and on an optical bench. In my code I can focus on a speciman at different depths from a digitally captured hologram. For the PSF animation above I used a 2dfft size 4096x4096, you'll note how smooth the PSF appears there, it was not resampled. It shows a scope of 14" aperture operating at an efl of 10m in red light, and presents the PSF at approx 43 pixels per arcsecond. The code is working well at a number of levels, and I have a high confidence level in its power.

The conventions it uses are classical. Sorry. When I say an optic has 1/4wv PTV I'm using the classical idea of an optical path differenced from a gaussian sphere (albeit shifted) that would focus to a point. At this level the strehl is approximately 0.8. I do have Zernike code in development, mostly for depicting color coded wavefronts and interferograms in a graphical manner similar to some commercial programs.

In this forum I think it best to keep to conventional and classic descriptions of wavefront error. For a test scenario, call up an optician and say you want a mirror better than 1 wave.

Finally, it is true in the world of coherent imaging the MTF is never even calculated. It would'nt make sense. There you can have a complex multidimensional PSF and the transfer function is a full bodied OTF. Obviously in those circumstances phase plays a big part, in backyard astronomy not so much.

If you have a better way to graphically depict the effect of a CO please show us. A picture in this case really is better than a thousand words.


Everybody,

Astigmatism is different in how its responds to defocus in a practical sense. Since a real object like a planetary surface has structures with both horizontal and vertical components you might enhance one over the other. For imaging however, unless you use a non circularly symmetric kernel in a wavelet or for decon the results will dissapoint.

The basic point is that for a good telescope defocus is not a useful technique to tease out detail. You will be focussing anyway as you chase seeing and temperature effects. So in that case if you are seeing something better you have achieved best focus for that moment. A scope with significant aberrations is another story. CO is not an aberration, it only defines the entrance pupil, which could be any shape really.

The MTF is one of the best ways to depict what happens with a CO. I don't know of a better way to demonstrate the CO effect in a graphical presentation. One could show an idealized planetary surface with varying amounts of CO, its easy to do the calculations but its not easy to capture the real visual effect.

Glenn

Edited by DesertRat (01/10/13 03:44 PM)


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Alph
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Reged: 11/23/06

Loc: Melmac
Re: CO vs. All That "Other Stuff"! new [Re: DesertRat]
      #5615765 - 01/10/13 03:48 PM

Quote:

The MTF is one of the best ways to depict what happens with a CO. I don't know of a better way to demonstrate the CO effect in a graphical presentation


I would say that the MTF depicts some aspects of diffraction quite well. How does the MTF relate to what we can really see/resolve is a different story. Unfortunately the MTF requires extensive knowledge of complex complex math. I am not sure how many folks who discuss so passionately MTF on this forum have that knowledge. The most telling statement on the effects of central obstruction is the Meade's decision to discontinue the f/10 SCTs.

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DesertRat
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Re: CO vs. All That "Other Stuff"! new [Re: Alph]
      #5615834 - 01/10/13 04:25 PM

Alph,
Good point. As said earlier CO effects are often grossly overstated or the reverse in some cases.

How to understand and interpret the MTF is described in Suiter as well as countless sites dealing with photography. But anyone with a high school educaton can derive value from it, which is the level of the Suiter book.

Glenn


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Alph
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Re: CO vs. All That "Other Stuff"! new [Re: DesertRat]
      #5615848 - 01/10/13 04:37 PM

Quote:

How to understand and interpret the MTF is described in Suiter as well as countless sites dealing with photography.



To fully understand the MTF, you need to know a lot, otherwise you are taking it at face value as many folks on this forum do. The required math skills are well above high school level. The rigorous treatment is well above Engineer's math level. It is not a simple arithmetic or basic calculus.


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DesertRat
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Re: CO vs. All That "Other Stuff"! new [Re: Alph]
      #5615912 - 01/10/13 05:16 PM

Quote:

To fully understand the MTF ...




I never said fully!

Agreed the MTF should not be taken at face value. The sum total of what has been written about it here and other threads helps some. It is advisable to read Suiter or another source to understand it better. Its not perfect, but few things are. I don't believe I have seen a MTF here or in those other threads which convey invalid information.

To generate a MTF is not difficult, it involves creating an optical transfer function and calculating its effect on an idealized sequence. These operations involve transforms, but the whole thing can be done in a single page of script. Most engineers I've worked with do understand that level of math. But you don't need to know the details of how it is generated to derive some value from it.

If you or someone has a better way to relate this information let us know.

Or perform an experiment. Add an obstruction to a refractor or increase an existing obstruction and see its effect.

Glenn


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cn register 5
scholastic sledgehammer


Reged: 12/26/12

Re: CO vs. All That "Other Stuff"! new [Re: DesertRat]
      #5615937 - 01/10/13 05:41 PM

I'd like to see some "blind" experiments done where the person evaluating the image does not know if the scope has an obstruction, or how big it is.

I think that may be difficult to manage because as soon as you defocus the obstruction - or lack - is obvious. Maybe someone else has to focus and the evaluator only sees in focus images.

Chris


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DesertRat
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Re: CO vs. All That "Other Stuff"! [Re: cn register 5]
      #5615951 - 01/10/13 05:52 PM

Its not easy to evaluate CO experimentally, visually anyway.

The biggest problem with a largish obstruction for novices is placement of the eye at low powers.

We'll keep this thread going until we're sick of it. And come back later and the same issue will be under review!

Glenn


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Joe Cepleur
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Re: CO vs. All That "Other Stuff"! new [Re: DesertRat]
      #5617641 - 01/11/13 05:37 PM

Quote:

Joe,

Contrast is not just the difference between the brightest and darkest part of an image.

I think what people are referring to above has to do with the properties of the human visual system. It is not a linear system, so the mean level of brightness, the size of the exit pupil, magnification and many other things come into the picture.

It is a complicated problem. In your reading get hold of the definition of intensity, contrast (there are several) and gamma, and explore the visual system. That is if you want to.

You might start here:
http://en.wikipedia.org/wiki/Contrast_(vision)

Glenn




Thanks, Glenn;

I've begun. Looks as though I have my work cut out. It's astonishing how complex this all is, if one studies it in detail. I'm intrigued with the notion that different images, even sections of images with different frequencies of light or differently sized details, all can respond differently.

I suppose we study this sort of topic partly for the pleasure of understanding, and partly for its predictive powers? Presumably, if one understands these details well, one could imagine what kind of scope might be well suited to viewing a particular object, or might be a wise modification or purchase when upgrading.


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Starman1
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Reged: 06/24/03

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Re: CO vs. All That "Other Stuff"! new [Re: Joe Cepleur]
      #5617734 - 01/11/13 06:27 PM

When I first bought an SCT and left it outside in the shade on my roof for several hours, I started the night with a completely cooled scope at ambient temperature.
The scope had also just been collimated.
The seeing that night was exceptionally good.

The first image I had (at about 100X), the stars were all tiny little round points with a single diffraction ring. At 200X, the images were the same.
A few years later, after never having been able to duplicate that night, I finally got an SCT Cooler from Lymax. It made a profound difference in star image quality. I was back to the tiny stars on good nights.

I look through SCTs all the time where I observe (there are a lot of owners), and not 1 in 10 is collimated or even close to being cooled down.
I know from personal experience that the central obstruction isn't the biggest factor damaging good images. Nights with a properly cooled and collimated 8" SCT taught me that.

But if, in the field, only 1 in 10 users will ever think about cooling or collimation, is it any wonder this type of scope has a reputation for mushy optics? And refractors are considered superior, even in smaller sizes?

I have a Gregory-Maksutov Cassegrain, and I have learned a lot about cool-down from that instrument. Pretty much the ONLY way that instrument has good star images is if it's allowed to cool. I set it out, in the shade, 3-4 hours before I'm going to start observing. And when I do start observing, the images are excellent for the aperture. I think that 5" Mak takes as long as an 8" SCT to cool down.

So what's my point? SCT owners should worry less--a lot less--about the size of the secondary mirrors, and a lot MORE about cooling and collimation. Because, when everything is right, even the run of the mill scope does pretty darn well with image quality. If the larger secondary obstruction really damages the images (and I'm sure it does some)
it's a pittance compared to other factors.


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freestar8n
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Re: CO vs. All That "Other Stuff"! new [Re: DesertRat]
      #5618500 - 01/12/13 06:23 AM

OK - I asked about the software because I was looking for someone else who uses your convention - but if it's your own code then it makes sense to follow your convention. I don't think I can change your mind - but I will just say that my use of "1 wave of spherical aberration at minimum rms focus results in a Strehl of 0.8" is consistent both with classical aberration theory and every text on the subject I know - except for Suiter. I regard spherical aberration as an intrinsic flaw in the wavefront that has a well-defined quantity specified by the p^4 term, regardless of reference sphere, and when you combine it with varying amounts of defocus, you get different Strehl and MTF. But at this point I have no doubt you prefer Suiter's description.

If you buy a telescope that has a Strehl of 0.8, it probably has a combination of aberrations that includes both 3rd and 5th order spherical. But if the only aberration is 3rd order spherical, then it could be 1 wave and still work well - because a change in focus reduces its impact by a factor of four.

Regarding MTF and what is better - well my main point is that it is limited and misleading in a visual context. MTF is used in professional imaging and design, but that usually involves linear detectors rather than the eye. And there is virtually no professional astronomy done by staring into an eyepiece anymore, so visual performance wouldn't be published much.

I did look at Rutten/van Venrooij and was surprised that they do go into detail on the changing role of the visual system on the overall MTF. Just looking at a single MTF and leaving out the additional MTF of the detection process is an incomplete description - yet many of these threads imply MTF is rigorous and all-encompassing. It just isn't, but I would use it pedagogically to explain an actual observation at an eyepiece - i.e. I think it is ok to lead with an empirical result and use MTF to explain what's happening. But I would not use MTF in a predictive way to compare two systems - particularly if the object being studied is not specified.

I don't have strong feelings about the impact of CO, except I do adopt a more modern stance on optics/imaging that you need to specify the entire imaging system and what you are measuring before you can make a comparison. For certain double stars a CO may help - just as other apodizations with masks may help make the second star stand out.

Rutten/van Venrooij cite roughly 10-30% as historical values for how much can be tolerated in visual use. They also mention that based on contrast, 30% is approximately as bad as 1/4 wave wavefront error. But I view that as an amateur text, and it doesn't allude to the limitations and subtleties of MTF in a compound imaging system that a text on Fourier optics would.

Frank


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kfrederick
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Re: CO vs. All That "Other Stuff"! new [Re: freestar8n]
      #5618529 - 01/12/13 07:20 AM

I own two unobstructed telescopes one a 20inch f 8 for 4 years and a 17inch f 9 and I know there is a improvement in the image .One only needs to look at the HST images the central obstruction effects are easy to see .The effect on long focus newts is small but there I think a .3 CO =1/4 wave error . All good telescopes work great not saying anything bad on any design .

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Asbytec
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Re: CO vs. All That "Other Stuff"! new [Re: kfrederick]
      #5618656 - 01/12/13 09:25 AM

Frank, you always make an interesting argument. The MTF basically puts contrast onto the focal lane in a linear fashion in accord with diffraction. How we observe it is, apparently, non linear. But, with average vision, normal color perception, and normal contrast sensitivity most can expect to see pretty much the same thing. That is, pedagogically, at very small scales expect some higher resolution and at slightly larger scales expect the brighter rings to wash out some contrast. Of course, that can change from person to person or even between scotopic and photopic vision. But, on the focal plane just as in any linear imaging device, the MTF and PSF describe what's there well enough for amateur observing purposes, IMO.

I do understand how defocus can improve the image with some amount of LSA, but I cannot fathom how 1 wave can possibly produce a Strehl of 0.80. I am not saying your wrong, just not sure how that can be. Seems 1 wave SA would put so much light into the rings that peak intensity should be far less than 0.80 Strehl even at best focus. Maybe it depends on where you draw the reference sphere, I apply it at best focus not Gaussian focus.

Anyway, it does seem application of the MTF is important. If you're imaging a white picket fence from across the street, it should be no problem. Imaging fine print with a photo copier, maybe. Observing close, unequal doubles seems to obey the same contrast as planetary viewing because they are on the same scale. If it's not on the focal plane, it cannot be seen by anybody.


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maknewtnut
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Re: CO vs. All That "Other Stuff"! new [Re: Starman1]
      #5618932 - 01/12/13 12:01 PM

Quote:


So what's my point? SCT owners should worry less--a lot less--about the size of the secondary mirrors, and a lot MORE about cooling and collimation. Because, when everything is right, even the run of the mill scope does pretty darn well with image quality. If the larger secondary obstruction really damages the images (and I'm sure it does some)
it's a pittance compared to other factors.




Amen Don!


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Rick Woods
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Re: CO vs. All That "Other Stuff"! new [Re: maknewtnut]
      #5618984 - 01/12/13 12:29 PM

Quote:

Quote:


So what's my point? SCT owners should worry less--a lot less--about the size of the secondary mirrors, and a lot MORE about cooling and collimation. Because, when everything is right, even the run of the mill scope does pretty darn well with image quality. If the larger secondary obstruction really damages the images (and I'm sure it does some)
it's a pittance compared to other factors.




Amen Don!




That ought to be graven into a stone tablet somewhere, preceded by "Thou Shalt..." .


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shawnhar
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Re: CO vs. All That "Other Stuff"! new [Re: Rick Woods]
      #5619103 - 01/12/13 01:49 PM

In my neck of the woods, transparency and seeing seems to be more of a factor than the CO
With a 10" SCT, what level of seeing/transparency does it require to get to the point that the CO makes a difference?
It's one thing to look at all the math and plots based on a magical perfect scenario, but the real world in my back yard seems to throw me the biggest curve.


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maknewtnut
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Re: CO vs. All That "Other Stuff"! new [Re: shawnhar]
      #5619115 - 01/12/13 01:55 PM

One can't argue the physics of the MTF. However, execution of an optical system, combined with the ability of a system to approach it's potential is 'what it's all about'.

As for CO, IMHO it plays a bigger role in consistency of high mag performance than anything else. When seeing isn't ideal, larger obstructions seem to play a synergistic role in limiting a system's performance (ie, limiting it's potetial).


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Alph
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Re: CO vs. All That "Other Stuff"! new [Re: Asbytec]
      #5619156 - 01/12/13 02:16 PM

Quote:

How we observe it is, apparently, non linear. But, with average vision, normal color perception, and normal contrast sensitivity most can expect to see pretty much the same thing.




The human eye response to light intensity is approximately base 10 logarithmic. If you plotted modulation/contrast in a base 10 logarithmic scale, then the MTF graph would be mostly flat, and the difference between %30 and %50 obscuration would be barely noticeable. Maybe Rat could generate a couple of graphs for comparison in a base 10 logarithmic scale.


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pstarr
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Re: CO vs. All That "Other Stuff"! new [Re: maknewtnut]
      #5619181 - 01/12/13 02:30 PM

Quote:

One can't argue the physics of the MTF. However, execution of an optical system, combined with the ability of a system to approach it's potential is 'what it's all about'.

As for CO, IMHO it plays a bigger role in consistency of high mag performance than anything else. When seeing isn't ideal, larger obstructions seem to play a synergistic role in limiting a system's performance (ie, limiting it's potetial).




After owning several sct and trying to get high resolution planetary views, "execution" of these systems seemed like a good idea.


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Asbytec
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Re: CO vs. All That "Other Stuff"! new [Re: shawnhar]
      #5619184 - 01/12/13 02:31 PM

Well, sure, seeing trumps everything. But, your scope is always transferring contrast at all times according to the conditions its working with. That's what scopes do.

Remember, seeing can be modeled using the MTF, too. And each aberration, induced like seeing, cooling, collimation, and focus, are added to the MTF of the CO and inherent aberrations. So, each time one of those variables increased, the contrast transfer worsens. Even though one might be much more significant, one does not trump the other. Yes, seeing is significant (and complex), but each affect adds to the others.

In my neck of the woods, seeing is normally very good in a 6" aperture. With moderate temperatures, cooling is never a problem. And great seeing allows essentially perfect collimation and, best I can tell, perfect focus. Those are the jaw dropping moments when your scope is performing close to where it should be hampered only by the obstruction and inherent aberrations. So, induced aberrations are minimal at worst. In this case, and when doing high power, critical observing (such as very tight and unequal double stars and fine planetary detail) the CO matters somewhat.

Still, the affect is over relatively small scales for point sources (Airy disc scale) and a bit of contrast fall off on extended objects (first ring out to about 4x Raleigh) that can be seen under the best conditions.

For example, the Dawes limit for a 150mm clear aperture is 0.77" arc. My 150mm obstructed aperture has split 7 Tau at (as reported) 0.74" arc with a clear fall off in contrast between both stars just as MTF theory predicts (preliminary, anyway, higher spacial frequency above that of an unobstructed aperture.) It might go deeper, but I have not found a suitable pair to split below that (other than 72 Pegasi elongated at 0.57" arc.) If seeing were to "add" additional contrast transfer problems below some figure, it would never accomplish anything near that. I probably would not worry much about a CO, either.

(But, please don't ask if removing the CO will improve the image in bad seeing, cuz I dunno. Maybe. )


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Alph
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Reged: 11/23/06

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Re: CO vs. All That "Other Stuff"! new [Re: Asbytec]
      #5619204 - 01/12/13 02:46 PM

Quote:

For example, the Dawes limit for a 150mm clear aperture is 0.77" arc. My 150mm obstructed aperture has split 7 Tau at (as reported) 0.74" arc with a clear fall off in contrast between both stars just as MTF theory predicts (preliminary, anyway, higher spacial frequency above that of an unobstructed aperture.)



MTF applies to extended objects only. MTF does NOT describe point sources.

BTW The Dawes limit is off the MTF chart anyway.


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