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Space Forum / Amateur Astronomy / November 2004Notorious "SEMI-APO" term used again.
7450 From: dhennig2 <dhennig2@t...> Date: Tue Nov 23, 2004 1:00am Subject: Re: 92mm Fluorite - clarification on definitions? Something here is really frustrating me. First, Tom, congratulations on getting a fine scope. The shots through it are most interesting... Now - it's the term "Semi-Apo" being used for this fluorite scope. I recall many months ago [about 2-1/2 years to be exact] that Vic Maris of Stellarvue was roasted fiercely by several parties eluding that his 102EDT f/6.1 triplet refractor could be dubbed semi-apo. There were rants all over SAA, the SV Yahoo! group, and elsewhere, scathing SV for using such a term. Interestingly enough, it was also a time when Tom Back got everyone to think of scopes in terms of either APO or achro performance, based on their color performance and spot behavior throughout the spectrum. There was a LOT of talk back and forth regarding this, then a hot topic amoungst refractor afficiandos. Why now is the term Semi-Apo being allowed to pervade the marketing once again, and what right does the likes of WO and BO [and for that matter TMB] have to use this "iffy" term now, when not even 3 years ago it was considered a false marketing term that did not mean anything? I just find it very interesting that all of a sudden, this seems okay for everyone with a Chinese-made optic. It seems very hypocritical to me. Some of the members here are ones who did the initial stuff against Stellarvue [SV] for this, and now its okay. Not fair! I suggest that someone here [Tom?] pin this issue down tight, else risk flooding the refractor marketplace with seemingly meaningless terms. Darren. ====================================================================== Darren, I do understand you NOW. But, one question - were you was at that time? You suppported "semi" camp? ;>) It was not fair in a past, it is not fair now and it will be not fair in a future. Note, that WO already used this notorious term to increase selling of their plain achromat (Mrgrez-I) and they pioneered such apporach to color correction "improvement" and hidden internal aperture stop. It was then used in BO prototype 102F/6 - to hide poor edge performance and improve color correction. One of SV scope also was made with internal color correction improver and another one with "special" formula - notorious MV cut filter inside of it's objective. What will be the next trick to foolish the public, which still believe in magic in optics - in magic from peoples, who don't know the optics! VD Valery, Please make Roland his eyepieces and stop trying to stir up meaningless controversy. I don't think that 3X color corection improvement (which is what the TMB design on the new scope called the Burgess/TMB Semi-APO provides) is out of the question to be discussed as a semi-apo. I know what Vic Maris sold. Thomas Back has designed a few lenses that have color correction far better than the scope in question from StellarVue. The color correction on the scope Vic called an EDT was nowhere near the level we are discussing here. In that case, it was a triplet, but it did not have much more that achromat correction, due to the design and glasses used. The term semi-apo would not really have applied there, and the use of the term "EDT" was unfortunate, as it did not use ED glass. In this case the term semi-apo is correct, as it does provide enough improvement over a well-corrected achromat (3X) to make the term have merit. Anyone looking through this scope would readily see a marked color correction improvement. I really think that if you choose to address anyone on this subject, address Thomas Back. He chose the name, and I think he might have an idea of the difference between an achromat and an APO. He has designed at least a few of them that are in happy customer's hands. Thanks, Tom Davis > 7450 From: dhennig2 <dhennig2@t...> > Date: Tue Nov 23, 2004 1:00am [quoted text clipped - 57 lines] > > VD > Valery, > > Please make Roland his eyepieces and stop trying to stir > up meaningless controversy. 1. Eyepieces were not stopped. In fact a lot were made and waiting the papers form local customs. We have very good manufacturing rate, especially if consider the fact, that two of three lenses in this design have very steep curves and can be made only one by one, not in blocks. If consider their very small diameters, then you can imagine how difficult to make them. And I do not stir up the water here - some other companies did and doing now. > I don't think that 3X color corection improvement (which is >what the TMB design on the new scope called the Burgess/TMB [quoted text clipped - 14 lines] > looking through this scope would readily see a marked > color correction improvement. Did I said, that 3x color correction improvement is not a "semi-apo" ??? Indeed, even 2x correction can be called as "semi-apo". However, I still have to see a real spot diagram for that 102mm F/6 doublet vs C-F achromat of the same size and F/D to conclude if this is really semi-apo. Note, that even this is not convinceable! If a scope has 3x smaller spot diagrams (in diameter), this absolutely does not mean, that this design has 3x better color correction. Untill I see a graph of longitudinal color aberrations, I can't say, that this objective has 3x better color correction. I also know, that it is quite difficult to find glasses for _doublet_, which will deliver 3x better color correction and will not use real ED glass. This may be even impossible. I should remind - 3x better color correction means, that longitudinal color aberration is 3x shorter, not 3x smaller spots!!!! I should remind also, that Meade ED apos have only about 4x better color correction, than ordinary achromat. But they use true ED glass FPL51 with matched crown-flint KF-3. Not one doublet can deliver 3x better color correction, than achromat being F/6 and use only short-flint as ubnormal glass. Even if some combinations are possible, they have very steep surfaces and very sensitive to collimation and lenses decentering. Therefore, I think, that that SEMI-APO in question does not has 3x shorter secondary spectrum and just has 3x smaller spot diagrams - in other words - the next in a row trick. > I really think that if you choose to address anyone on this > subject, address Thomas Back. He chose the name, and I > think he might have an idea of the difference between an > achromat and an APO. He has designed at least a few of > them that are in happy customer's hands. Tom, were did you saw, that I adressed these poits to YOU??? You just speak instead of Bill and Tom Back. VD >I should remind also, that Meade ED apos have only about >4x better color correction, than ordinary achromat. And in that case, it's too bad the term "semi-apochromat" isn't applied to them. If the telesopes under discussion do deliver the claimed 3x improvement, they would belong in the same category. If they didn't, of course, that would be another issue, but if it is possible for a flint glass to be well-matched to fluorite, allowing something competitive to an ED glass lens to be made, I would fault the people who call ED scopes just plain apochromatic, rather than the makers of the telescope under discussion, for abuse of terminology. John Savard http://home.ecn.ab.ca/~jsavard/index.html >Valery, > [quoted text clipped - 25 lines] > >Thanks, Tom Davis Once again, marketing takes over and old arguments seem to "vanish." Why don't you refractor makers simply tell the rest of us what the colour correction is? That way, we'd have a way of automatically comparing all scopes? >Why don't you refractor makers simply tell the rest of >us what the colour correction is? That way, we'd have >a way of automatically comparing all scopes? Because it's not at all clear how you'd *measure* colour correction; it's not really a single number. I think it's at least a four-dimensional function, maybe five; you want the shape of the point spread function at multiple wavelengths at multiple distances from the optical axis, and I'm not completely sure that you can ignore the angle of the image within the field of view. [on the other hand you're only losing one dimension in this confusing function when looking at reflective optics; but the other N-1 dimensions kick in nastily if you're trying to compare different multi-mirror wide-field designs, or designing things to compensate for coma in short-focus reflectors. I'm not sure why those debates are so much less loud than the refractor one. Central obstructions, which are entirely a matter of the _shape_ of the PSF, are I suppose one of the other major debates] You could compute "assuming perfect manufacturing and neglecting diffraction, the diameter of the disc encompassing 90% of the energy in the PSF is below [mumble] arcseconds between 350nm and 700nm"; you could probably compensate for known imperfections in manufacturing; but people would still debate whether 90% or 95% is the right energy to use, and ask whether it's better for the widest PSF to be at 450 or at 650 nanometres. It's intrinsically multi-dimensional, and there's no good ordering on such things. Hence debate. Tom >>It's intrinsically multi-dimensional, and there's no good ordering on such things. Hence debate. Tom>> Hey Tom, how about joining the ATM design group on Yahoo: http://groups.yahoo.com/group/ATM_Optics_Software/ Rolando >>Why don't you refractor makers simply tell the rest of >>us what the colour correction is? That way, we'd have [quoted text clipped - 28 lines] > >Tom How about starting with the visual? Lets say given the average human's colour perception this ( ) measurement will mean no in-focus colour on Venus? Seems pretty straight forward. As for manufacturing variances, the apo mfgs won't agree there ever ARE any, except with competitor's lines. They will NEVER answer you - there is $$$$$$$$$$ at stake. > >>Why don't you refractor makers simply tell the rest of > >>us what the colour correction is? That way, we'd have [quoted text clipped - 34 lines] > As for manufacturing variances, the apo mfgs won't agree there > ever ARE any, except with competitor's lines. > Because it's not at all clear how you'd *measure* colour correction; > it's not really a single number. I think it's at least a > four-dimensional function, maybe five; you want the shape of the point > spread function at multiple wavelengths at multiple distances from the > optical axis, and I'm not completely sure that you can ignore the > angle of the image within the field of view. I think most of us would settle for the longitudinal chromatic aberration, C to f (or whatever the usual standard is). One part in 2,000? In 4,000? In 8,000? In 16,000? That sort of thing. No, it would not capture the shape of the focal length vs wavelength curve, and it says nothing about the spherochromatism, but it's one piece of information we don't often get for anything beyond an achromat. (I'm willing to stipulate that any achro will get in the neighborhood of 1 part in 2,000.) When people talk color correction, I think they usually mean on-axis and geometrically, so that we can ignore diffraction effects and the off-axis distortions. Of course, I agree that color correction has, in its entirety, many dimensions. But I also think it's somewhat disingenous to suggest that that is why manufacturers do not give longitudinal CA in parts, say. I think they don't do it because it leads to arguments and returns. I suspect that most people wouldn't lose their cool, but it only takes a few before manufacturers just get fed up with being helpful. Brian Tung <brian@isi.edu> The Astronomy Corner at http://astro.isi.edu/ Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/ The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/ My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt >> Because it's not at all clear how you'd *measure* colour correction; >> it's not really a single number. I think it's at least a >> four-dimensional function, maybe five; you want the shape of the point >> spread function at multiple wavelengths at multiple distances from the >> optical axis, and I'm not completely sure that you can ignore the >> angle of the image within the field of view. >I think most of us would settle for the longitudinal chromatic aberration, >C to f (or whatever the usual standard is). One part in 2,000? In 4,000? >In 8,000? In 16,000? That sort of thing. Precisely. The maximum distance of the focal point from that for the helium d line over that range clearly distinguishes a three-element fluorite lens from a crown and flint achromat, and it would make it very clear where a given lens falls on the continuum between those two designs. It might not be definitive in ranking ED two-element lenses among themselves, but it would definitely make it clear what broad class of telescope a given refractor belongs to. John Savard http://home.ecn.ab.ca/~jsavard/index.html >>> Because it's not at all clear how you'd *measure* colour correction; >>> it's not really a single number. I think it's at least a [quoted text clipped - 17 lines] >themselves, but it would definitely make it clear what broad class of >telescope a given refractor belongs to. And would give more credence to the idea that there is a "sub-class" below apos and above achros that should perhaps have it's own name. The issue is _not_ creating a workable standard comparative. The _issue_ is commercialists and their practices! The _issue_ is fraud. Face up to it once in your life. > > Because it's not at all clear how you'd *measure* colour correction; > > it's not really a single number. I think it's at least a [quoted text clipped - 27 lines] > The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/ > My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt > The issue is _not_ creating a workable standard comparative. > The _issue_ is commercialists and their practices! > The _issue_ is fraud. > > Face up to it once in your life. Hi Jerry. Brian Tung <brian@isi.edu> The Astronomy Corner at http://astro.isi.edu/ Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/ The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/ My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt As usual you ignore the issue. You have an excellent technical mind but your values sometimes derail it all. Just an observation. Jerry's aunt. > > The issue is _not_ creating a workable standard comparative. > > The _issue_ is commercialists and their practices! [quoted text clipped - 9 lines] > The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/ > My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt > >Why don't you refractor makers simply tell the rest of > >us what the colour correction is? That way, we'd have > >a way of automatically comparing all scopes? > > Because it's not at all clear how you'd *measure* colour correction; > it's not really a single number. Preposterosis! It's totally clear to me and to others, and has been for decades (centuries?). M. > I think it's at least a > four-dimensional function, maybe five; you want the shape of the point [quoted text clipped - 23 lines] > > Tom > Preposterosis! It's totally clear to me and to others, and has > been for decades (centuries?). > M. What has? That you have preposterosis? > >Why don't you refractor makers simply tell the rest of > >us what the colour correction is? That way, we'd have [quoted text clipped - 26 lines] > It's intrinsically multi-dimensional, and there's no good ordering on > such things. Baloney. > Hence debate. Hence obfuscation, ambivalence, commercialistic gobble wobble ............. since there's money at stake! The Chinese will settle it all for you. Good Chapter-7 luck. Mark > Tom >Now - it's the term "Semi-Apo" being used for this fluorite scope. Well, at least it _is_ a fluorite scope. And it's better than calling a telescope with a two-element lens just plain "apochromatic". If new materials allow a two-element lens to be constructed with such closely matched dispersions as to equal the performance of an apochromatic lens, then I suppose that has to be accepted. But if the performance is much better than ordinary achromatic doublets, but yet significantly short of that of a three-element fluorite lens, I would think that "semi-apochromatic" is appropriate and accurate, and much to be preferred over calling such telescopes just "apochromatic" in advertising, as has happened. John Savard http://home.ecn.ab.ca/~jsavard/index.html >>Now - it's the term "Semi-Apo" being used for this fluorite scope. > [quoted text clipped - 12 lines] >much to be preferred over calling such telescopes just "apochromatic" in >advertising, as has happened. One caveat; Semi-Apo, if adopted as a term should not include scopes ranging from a Pronto (high colour error) to say an Orion ED80 (low colour error) because it almost implies they are similar, and they are not. This is why a rating system for basic colour error is needed. What we do NOT need are manufacturers (William Optics, for eg) using the term, "apo" in any way to imply that there is insignificant colour in their products. Semi-apo could become as abused now as wavefront correction figures were in the 1980s. -Rich "RichA" <none@none.com> wrote in message > What we do NOT need are manufacturers (William Optics, for eg) using > the term, "apo" in any way to imply that there is insignificant colour > in their products. Unless, of course, they are refering to a WO product that does have excellent color correction. Ed T. I'm very troubled by this whole discussion. How many years has it been going on? This has long-ceased to have anything to do with science. I have the solution. Let the National Georgraphic define and set standards. ie., somebody outside of the system. Because that is how this is going to end, in any event. Knowing this, you-all might want to run and jump on the board to protect your turf and your fame/advertising dollars. A curse on all your houses! Mileva > 7450 From: dhennig2 <dhennig2@t...> > Date: Tue Nov 23, 2004 1:00am [quoted text clipped - 57 lines] > > VD |
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