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Space Forum / Astronomy / November 2004Contrast Vs RayTracing (Telescopes)
Hello all, Can a Raytracing software program also evaluate Contrast on (telescope) images? What are the existing softwares that can evaluate Contrast? How good are these softwares? Thanks and "Clear Skies" Mohib.  SignatureLook for the HILAL (Crescent Moon) -------> ) It is one of the Most BEAUTIFUL of Creations -------> ) Then offer an INTENSE PRAYER to the ONE CREATOR, OF ALL -------> ) Surely, ALL SINCERE PRAYERS ARE ACCEPTED -------> ) Homepage: http://Hilal-Sighting.Com -------> ) >Hello all, > >Can a Raytracing software program also evaluate Contrast on (telescope) >images? >What are the existing softwares that can evaluate Contrast? >How good are these softwares? Simple raytracing software can't tell you much about contrast. However, reasonably sophisticated applications can calculate the MTF (modulation transfer function) of an optical system, which is the primary metric for contrast. I use OSLO for optical design and analysis. This is a very full-featured program, although the learning curve is pretty steep (especially if you have little experience with optical modeling). A free version is available, limited to 10 optical surfaces (which is nearly always enough for analyzing telescope optics). http://www.sinopt.com/ _________________________________________________ Chris L Peterson Cloudbait Observatory http://www.cloudbait.com Hello Chris, Thanks for the input about OSLO. I will download and try it out. Do you know if OSLO can also give "spot diagrams"? More generally, can Spot Diagrams also give an indication of contrast? Mohib. > >Hello all, > > [quoted text clipped - 18 lines] > Cloudbait Observatory > http://www.cloudbait.com >Hello Chris, > [quoted text clipped - 3 lines] > >Mohib. OSLO can generate spot diagrams, but they are not very useful for assessing contrast. _________________________________________________ Chris L Peterson Cloudbait Observatory http://www.cloudbait.com > OSLO can generate spot diagrams, but they are not very useful for assessing > contrast. [quoted text clipped - 4 lines] > Cloudbait Observatory > http://www.cloudbait.com Spot diagrams are of almost no use for diffraction limited optics such as a telescope. If it's not diffraction limited then there isn't a great deal of point worrying about contrast. Use wavefront anaysis instead. Contrast is very difficult to quantify. One can easily (if you have a decent computer program) calculate MTF but the does not tell you what happens to stray light. The best way to maximize contrast is to carefully use baffles wherever required. To those of you who wish to do non sequential ray tracing to determine stray light that is fine. However, unless you know all the scattering and reflectivity properties of the materials being used, and the exact geometry, this is of limited use. There is no substitiute for a well designed system. Hi Mohib, I'll give you a lifetime of free access to my optical design program ODP (www.westcoastengineering.com) including free on-demand access to the full source code and unlimited user-support in return for you taking sci.optics off you posting list of crescent moon sighting times. It is not that I have anything particularly against your religious beliefs. I am kind of a druid in that I see God in nature rather than in a church represented by a priest,rabbi,etc. but every time you post it here it stirs up so many people, bringing out the best (or worst) from Uncle Al and others. In-your-face religion in this country is certainly allowed by our freedom of speech and religious belief and I would not have it any other way but it tends to offend a whole lot of people and I think you must admit that your posts are not "purely" scientific or astronomical. We humans have had calculators and computers for over 100 years which could accurately predict (within a fraction of a second) when the moon is "new", "crescent" etc. You are correct that it being a crescent and seeing it are different things and until we can accurately predict weather, smoke and dust (several hundred more years at least), it will be hard to predict the sighting thing. This is a serious offer. No charge at all. Just no more humongous posts that only a scholar interested in the topic could read. ODP is very like ACCOS-V and HEXAGON (ex Hughes Aircraft Code) with many significant additions and enhancements suggested by 25+ years as an optical designer and good ideas from ZEMAX, OSLO and Code-V including (Contrast Calculations) of Geometrical MTF (for non-diffraction limited systems) and Diffraction MTF (for systems with less than one wave of OPD error. It does pretty much everything and I'll gladly add anything reasonable to ODP if you take me up on my offer. Sincerely, Jim Klein West Coast Engineering >Hello Chris, > [quoted text clipped - 34 lines] >> Cloudbait Observatory >> http://www.cloudbait.com "Dr. Mohib. N. Durrani" <mdurrani@verizon.net> wrote in message news:<OLxod.4838 > More generally, can Spot Diagrams also give an indication of contrast? > More generally, can Spot Diagrams also give an indication of contrast? Yes it can. It is fairly simple with pure aberrations: spherical, coma, astigmatism and defocus. There are simple relations between the blur size in units of Airy disc diameter, p-v error and RMS error for each aberration. Blur size determines the RMS error, which directly determines Strehl ratio. The Strehl directly determines average contrast loss over the range of MTF frequencies. For instance, 0.95 Strehl means 0.05 average contrast loss. It can also be done with combined aberrations, assuming separate aberration contributions are known. Strehl ratio for each separate aberration is, in fact, an MTF degradation factor. So, for instance, 0.90 Strehl for spherical aberration and 0.95 Strehl for astigmatism would result in 0.855 "combined" Strehl, indicating 14.5% average contrast loss over the MTF frequencies due to these two aberrations. In the same manner contrast consequences can be indicated for local surface deformations, roughness, etc., as long as their RMS error is known. This still doesn't give you insight in how much is any particular frequency affected. Also, there are other important factors influencing contrast to consider in practical applications, as mentioned. But it does give you good general idea through a single number you can arrive at starting from the blur size. Vlad >> More generally, can Spot Diagrams also give an indication of contrast? > [quoted text clipped - 5 lines] >contrast loss over the range of MTF frequencies. For instance, 0.95 >Strehl means 0.05 average contrast loss. It can be more complicated for refracting systems though; even misleading if the weight of the wavelengths are not factored in. Take a 6" f'15 ish achromat that is corrected to null e and C, with F overcorrected much futher than a typical C-F lens in that class. The C through F spot size will be almost twice the size of that of the C-F lens, yet the overall contrast on bright extended objects would be slighlty better than the C-F corrected lens (unless observing bright objects with strong blue features), since more of the weighted visual spectum is actually in focus. The down side is the orange tint to anything that ought to look white. Dan > >... The Strehl directly determines average > >contrast loss over the range of MTF frequencies. For instance, 0.95 > >Strehl means 0.05 average contrast loss. > > It can be more complicated for refracting systems though; even > misleading if the weight of the wavelengths are not factored in. It is more complicated for systems with uncorrected chromatism. It wouldn't be "misleading if the weight of the wavelengths are not factored in", it would be simply an error. > Take a 6" f'15 ish achromat that is corrected to null e and > C, with F overcorrected much futher than a typical C-F lens in that [quoted text clipped - 3 lines] > observing bright objects with strong blue features), since more of > the weighted visual spectum is actually in focus. Sure, it all boils down to the amount of energy thrown out of the Airy disc (in case of chromatism, combined with eye sensitivities). Blur size is mainly irrelevant, generally speaking. But if there is established relationship between the blur size and energy loss - such as the example of pure aberrations - then it is there - period. Nothing wrong or misleading about it, as long as it is clearly stated (which I have done, haven't I?). What you may have missed is that I wasn't talking about blur size in general, but about blurs caused by specific aberrations. While blur size in general is pretty much meaningless contrast wise (it is energy that counts), blur size for any specific pure aberration is not. Vlad >. What you may have missed is that I wasn't talking >about blur size in general, but about blurs caused by specific >aberrations. While blur size in general is pretty much >meaningless contrast wise (it is energy that counts), blur size for >any specific pure aberration is not. Vlad, The poster did ask about spot diagrams in general and its possible indications of contrast loss and my comment was simply an additional example of what one may not necessarily infer contrastwise from studying spot diagrams. Dan > Vlad, > > The poster did ask about spot diagrams in general and its possible > indications of contrast loss and my comment was simply an additional > example of what one may not necessarily infer contrastwise from > studying spot diagrams. Dan, This begins to look like "who said what" match, but what the poster asked was "can" spot diagrams indicate contrast level. As a matter of fact, they can. Not always, not always easy, but they can. My post was specific on when and how. You're "playing innocent" now :), but what actually triggered my response to you was that you said that what I posted can be "misleading". It only can be misleading if generalized as "blur size does indicate contrast loss - period" which is not what I said. >>> More generally, can Spot Diagrams also give an indication of contrast? >> [quoted text clipped - 18 lines] > >Dan The correct scheme for properly weighting the wavelengths when computing the approximate Strehl Ratio from RMS OPD is given in both the CODE-V manual and in the OPD manual. A free copy of the ODP manual is available at www.westcoastengineering.com. No strings attached. When the exact Strehl is computed, the wavelengths and their relative spectral weights are automatically folded into the calculation when the polychromatic PSF (Point Spread Function) is computed. I guess one could also generate a psuedo Geometrical PSF from the Spot Diagram via some gridding system at the image plane for systems that are far from diffraction limited, say > 1.0 wave p-v OPD and then compare its peak intensity to that of a diffraction limited Airy Function but I'd have some difficulty calling it a "Strehl Ratio". It might be an interesting way of putting a metric on a system which is so intrinsically aberratied that the diffraction based PSF is difficult to perform due to wavefront sampling problems (too large a ray and transform grid). Jim Klein > "Dr. Mohib. N. Durrani" <mdurrani@verizon.net> wrote in message news:<OLxod.4838 > [quoted text clipped - 7 lines] > contrast loss over the range of MTF frequencies. For instance, 0.95 > Strehl means 0.05 average contrast loss. Not for a diffraction limited system such as a well corrected telescope. > It can also be done with combined aberrations, assuming separate > aberration contributions are known. Strehl ratio for each separate [quoted text clipped - 3 lines] > "combined" Strehl, indicating 14.5% average contrast loss over the MTF > frequencies due to these two aberrations. For a well corrected system completely untrue. You need to use wavefront addition which includes phase information. See my posting above. > Vlad >> "Dr. Mohib. N. Durrani" <mdurrani@verizon.net> wrote in message news:<OLxod.4838 >> [quoted text clipped - 7 lines] >> contrast loss over the range of MTF frequencies. For instance, 0.95 >> Strehl means 0.05 average contrast loss. The Strehl ratio is the ratio of the peak intensity of the diffraction based point spread function of an aberrated optical system to the peak intensity of the diffraction based point spread function of the same optical system (F/#, aperture,wavelength, obscruration (shape and size), aperture (shape and size), etc. with the OPD (wavefront error) set to and assumed to be zero (no aberrations). It is related to contrast loss but is is not as simple as the above explaination except for very small RMS OPD errors. ODP (my program) will calculate both Strehl's (exact and the RMS OPD approximation) for both poly and mono-chromatic systems, exactly, unlike many other commercial optical design programs. So will the DEMO. Not bad for $49.00 including a year of user support and the option to get full program source code ($5.00 per CD) for you of your company to protect your hefty investment. Jim Klein West Coast Engineering www.westcoastengineering.com >Not for a diffraction limited system such as a well corrected >telescope. [quoted text clipped - 12 lines] > >> Vlad Unless the optics are really bad, seeing usually dominates. I find that most of the loss in contrast comes from scattering from dust, coatings and poor sky baffles. Unless one has the proper surface preparation, the shallow angle scattering decreases contrast in a bright sky. (use preparation B not H) Newtons should have an extended tube to help reduce scattered light and maybe a light trap opposite of the eyepiece dan >>>"Dr. Mohib. N. Durrani" <mdurrani@verizon.net> wrote in message news:<OLxod.4838 >>> [quoted text clipped - 47 lines] >> >>>Vlad I was assuming clean optics, optimal baffling and no atmosphere. Usually, in the optics inductry, Strehl means what I wrote about it. Jim >Unless the optics are really bad, seeing usually dominates. I find that >most of the loss in contrast comes from scattering from dust, coatings [quoted text clipped - 57 lines] >>> >>>>Vlad > The Strehl ratio is the ratio of the peak intensity of the diffraction > based point spread function of an aberrated optical system to the peak > intensity of the diffraction based point spread function of the same > optical system (F/#, aperture,wavelength, obscruration (shape and > size), aperture (shape and size), etc. with the OPD (wavefront error) > set to and assumed to be zero (no aberrations). Strehl ratio also directly implies average contrast loss over all spatial frequencies. Strictly talking, it does it only when the phase contrast function is zero (for symmetrical aberrations), but it pretty much stands as an average's average for asymmetrical aberrations as well (where contrast vary with detail orientation). Btw, being normalized, Strehl is independent of aperture and obscuration; it is exclusively intended to measure wavefront quality. Also, F# has no consequence on it. Even if it wouldn't be normalized, it is a peak intensity, therefore, determined by the wave amplitude at that point - something that doesn't change with F#. > It is related to contrast loss but is is not as simple as the above > explaination except for very small RMS OPD errors. Well, it is as simple in the concept given, and not only for "very small errors". Remember, it wasn't about calculating exact values, it was about indicating contrast loss. You can check it out with your software. As long as we can come to the RMS/Strehl starting with a blur size, it does give very good indication of contrast loss. > ODP (my program) will calculate both Strehl's (exact and the RMS OPD > approximation) for both poly and mono-chromatic systems, exactly, > unlike many other commercial optical design programs. So will the > DEMO. Sounds interesting. Will check it out tomorrow... Vlad >> The Strehl ratio is the ratio of the peak intensity of the diffraction >> based point spread function of an aberrated optical system to the peak [quoted text clipped - 16 lines] >therefore, determined by the wave amplitude at that point - something >that doesn't change with F#. Yes, I mentioned that because I have had it suggested, by some really senior NASA guys, that the aperture shape between the ideal system and the aberrated system could be different, something that is quite wrong. In ODP, we simply create a ray PSF, then zero out the OPD map and do another PSF and then ratio the peak values. This does ignore centroid position shift due to asymmetric aberrations but then, that is like an LOS shift. >> It is related to contrast loss but is is not as simple as the above >> explaination except for very small RMS OPD errors. [quoted text clipped - 12 lines] >> >Sounds interesting. Will check it out tomorrow... I think you will find it useful. As always, email any comments and suggestions for refinement. I'm always looking for ways to improve the computational accuracy and flexibility of OPD. Jim >Vlad West Coast Engineering <westcoastengineering@westcoastengineering.com> wrote: >>> The Strehl ratio is the ratio of the peak intensity of the diffraction >>> based point spread function of an aberrated optical system to the peak [quoted text clipped - 23 lines] > >In ODP, we simply create a ray not ray but real meaning that it includes aberrations in the form of OPDs >PSF, then zero out the OPD map and do >another PSF and then ratio the peak values. This does ignore centroid [quoted text clipped - 25 lines] >> >>Vlad > >... Strehl ratio for each separate > > aberration is, in fact, [quoted text clipped - 5 lines] > For a well corrected system completely untrue. You need to use > wavefront addition which includes phase information. Can you elaborate on "wavefront addition which includes phase information"? Vlad > Simple raytracing software can't tell you much about > contrast. However, reasonably sophisticated applications can > calculate the MTF (modulation transfer function) of an optical > system, which is the primary metric for contrast. I think all the salient points have been mentioned by other posters, but maybe a summary won't hurt. The "diffraction MTF" is indeed the primary metric for contrast, and any of the "professional grade" programs will calculate it *for perfect optical surfaces*. However, in the real world, optical imperfections and especially such things as dirt and dust on the optics will greatly degrade contrast. (Not to mention the atmospheric "seeing" as someone pointed out.) I believe some optical modelling programs allow for such things in a semi-empirical way, but I'm not sure how reliable they are. At a minimum, you would have to have a very good description of at least the statistical properties of the contaminating particles or imperfections. One thing is for sure, though: the contrast you achieve will be *no better* than what your favorite program predicts for perfect optics. In practical terms, contrast is maximized by having an unobstructed light path (i.e., no secondary mirror), as few optical surfaces as possible, and very clean surfaces with a minimum of scratches or other imperfections. SignatureSteve Willner Phone 617-495-7123 swillner@cfa.harvard.edu Cambridge, MA 02138 USA (Please email your reply if you want to be sure I see it; include a valid Reply-To address to receive an acknowledgement. Commercial email may be sent to your ISP.) |
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