David Cotterell Toronto, Ontario "If an observer actually sees an object, there is no point in referring to a formula to find out whether he ought to see it; and if he fails to detect it, no formula will ensure his success." - W.H. Steavenson 8" f/15.5 TEC Maksutov - 16" f/5 Teeter/Zambuto Dob - TEC 140 - AT 65EDQ APO Refractor - Astro-Physics Mach 1 GTO Mount - iOptron ZEQ25 mount - Discmount DM6 - Canon 60Da
Stellarvue SV70ED, UA DwarfStar, Oberwerk / AstroTech AT80EDT, AT Voyager / Celestron C6XLT, AT Voyager / Meade 8" LightBridge / Vixen VMC 110L (Baader Solar Film), Porta II / Celestron Nexstar 102GT, ES Twilight I / Oberwerk 20x80 DIII, Orion 10x50 Resolux
Quote:And stars are...too small to resolve in the images.
150mm MCT f/13, 31% CO
"People say I'm in denial. I disagree."
Quote:Just got the latest Sky and Tel - huge Lx600 is pictured In it with this humungous CO!! It's F/8 I'm assuming but at what cost? It's gotta be at least 50% by diameter and I wouldn't rule out 60%. I'm getting it that it's an advantage to have a faster scope for wider fields in the interest of deep sky and imaging. Still, the light in those diffraction rings has got to be bothersome on a number of fronts and lunar and planetary has to be aggrevated in terms of contrast loss .Pete
Uncle Rod Uncle Rod's Astroblog: http://uncle-rods.blogspot.com/
Quote:This is an imageing system. That does not mean that it can't be used visually, but I am not sure why anyone would want to do that really. For visual use, an obstruction this large will rob a huge amount of contrast.
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Quote:I've often thought that percentage of diameter was a bit misleading as a measure of CO. I realize that this is the conventional method but still... Area is another matter. As an example, my C6 has a diameter based CO of 36% but calculated by area of primary minus area of secondary it's only about 13%. Still, I'd agree that that Lx600 has a large CO by any standard.
Quote:probably twice as apparent as his sims let on.
Quote:I've often thought that percentage of diameter was a bit misleading as a measure of CO. I realize that this is the conventional method but still... Area is another matter. As an example, my C6 has a diameter based CO of 36% but calculated by area of primary minus area of secondary it's only about 13%.
Quote:Anyway, the "Clear Aperture" formula offers an approximation for contrast transfer, but is not at all accurate when the contrast is for detail that is very fine.
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Quote:The vanes cause spikes on stars- more out there on this than there is on central obstruction effects.
Quote: Quote:probably twice as apparent as his sims let on.
I am interested to see how you would quantify it. Apparently you have a better way of measuring it. Please explain.
Quote: So if CO has such a "profound" effect on image quality
Quote:Norme has covered it pretty well.The clear aperture method though (the diameter of the primary minus the diameter of the secondary obstruction = clear aperture) is only an approximation.If fails when the obstruction is very small or very large.And it ignores the fact that when an obstruction is present, at the very very highest spatial frequencies, the contrast transfer in an obstructed scope is almost always better than in an unobstruced scope. That's right. At the limits of angular resolving power, the obstruction improves contrast transfer vs an unobstructed scope.But the damage cased by the obstruction is not really much to do with shading of the mirror reducing light transmission. Reduced transmission has no effect on cotrast transfer at all. All of the damage is done because light that would normally go into the Airy Disk in an unobstruced system is now going in to the diffraction rings around the point.84% of the light in an unobstructed scope goes into the Airy Disk, 7% into the first diffraction ring, 3% into the second ring, and the rest in the outer rings.By comparison, a scope with a very large obstruction, rather than putting 91% of the light into the Air Disk and first only puts about 70% of the energy into the Airy Disk and first ring.Now remember, the Airy disk may be small, but the first ring goes out to twice the diameter of the Airy disk, and the second ring goes out to three times the diameter of the Airy Disk.And this is what kills the contrast. Low contrast details of a size that are two or three diameters of the Airy disk will be washed out but the light that is being thrown from the point on the image that created it.And if the obstruction is very large, it can extend to four and five rings, which is why we see such a huge droop in the MTF plots around .5 in MTF plots. Small details simply get drowned out.Anyway, the "Clear Aperture" forumla offers an approximation for contrast transfer, but is not at all accurate when the contrast is for detail that is very fine.For example if you have two equal magnitude doubles, an obstructed system might show them better seperated than a perfect unobstructed system because the Airy Disk will appear smaller, if the stars are seperated by the widty of the gap between the edge of the Airy Disk and the first ring, they will actually appear to be seperated by a wider space in the obstructed instrument. For this reason, we say that the contrast transfer is better. The space between the stars appears wider, and hence, blacker.The damage is very real, and the MTF plots do a very exact job of describing how contrast transfer will differ between two different scope.Anyone that chooses to ignore MTF plots or treat them as witch-doctor stuff is simply ignoring the physics of how telescopes work. It is 100% reliable. If you accuratly model a telescope, an MTF plot describes exactly how it will behave.That is why professional optical engineers talk in terms of MTF and encircled energy. These describe how an instrument performs with great clarity.
Quote: Quote: So if CO has such a "profound" effect on image quality
I didn't say that at all.
I said a very large CO has a very big effect. My response was meant in the context of the scope the OP is talking about.
The effects of a 20% obstruction are so small that most observers would struggle to see it.
Even the 33% SCT obstruction does not degrade the image to the point that it is glaringly obvious, but in a direct comparsion by a good observer, it will be enough to see.
Once the obstruction gets over about 40% though, the damage becomes more apparent, and this is why this has been considered the cut-off for the maximum size allowed for visual use.
Quote:The whole MTF thing seems to breakdown for progressively larger aperture as more and more detrimental effects of seeing are resolved.
Quote: The honest to god truth? For most folks, seeing will be the limitation, not a too-big central obstruction. And, again, my observation is that the RCX400s did a respectable job on the planets. If I were assured of quality otherwise, I would not hesitate to buy an LX800/LX600 for all-round use.
Quote: The effect of the CO is still present
Quote:The argument for the soundness of MTF, which is in essence the product of diffraction, is easily visible simply by spending a few hours browsing our own Solar System and Imaging forum.
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Quote:The effect of the CO is still present, it adds itself to the effect of aberration and seeing and whatever else.You're correct, bold is a nice touch.
Quote:I wouldn't just use MTF by itself as some absolute thing as Suiter describes.
Quote:You want to do it right? Get a 6" apo and add varying central obstruction discs. Image it and process it fairly raw as wavelets applied can skew the results. Line up all the raw pix and there's a fair representation of CO to no CO differences. Attenuating fixed photos is playing. Just be careful the seeing is stable enough to allow even handed sampling.
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