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SCT CO and Aperture question
"Stephen Paul" wrote in message ... Given an SCT has a 33% obstruction that will have some deleterious effects on contrast compared to a scope with a smaller CO, but of the same aperture, and, given that by comparison, a larger aperture provides the second benefit of a smaller Airy disk, and better (tighter) resolution, ... consider an 8" aperture of undetermined CO, and an 11" SCT with 33% obstruction. What would the obstruction of the 8" scope have to be, in order to perform similarly to the 11" 33% obstructed scope on planets? IOW, what is the relationship between CO by Area and aperture, as aperture increases, and CO by Area remains constant? Or, for those who wish to give a quick and less detailed response..., would the contrast ever/always be better by increasing aperture, given a constant ratio of CO by area? Get youself a copy of aberrator, and you can play with aperture, and CO, and see what the effect is on an image. The normal constraint on CO, is the focal ratio, and the fully illuminated field. If you want to have an 8" scope with a 20% CO, this can be built, but to have a reasonably useable field, the focal ratio has to rise to perhaps f/20. Hence normally the relative diameter of the CO, rises linearly with the scopes diameter, and the percentage CO, remains fairly constant. The traditional 'rule of thumb', puts an unobstructed 8" scope, and an 11" scope on pretty much the same terms. That being said though, the atmosphere will normally lower the difference to practically nothing... :-( Best Wishes |
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SCT CO and Aperture question
"Brian Tung" wrote in message
... Stephen Paul wrote: Given an SCT has a 33% obstruction that will have some deleterious effects Only for mid-frequency detail (about 0.75 to 3 arcseconds) is the dropoff in MTF both present and significant. I won't kid you, though--much of the good stuff, maybe even most, is in that range. But you can overcome the contrast dropoff if you are willing to shoulder the burden of extra aperture. Or, for those who wish to give a quick and less detailed response..., would the contrast ever/always be better by increasing aperture, given a constant ratio of CO by area? Always assuming equal optical quality and zero atmospheric turbulence, then yes. However, such considerations are often academic, since optical quality is not equal, and I haven't yet seen a night of literally zero atmospheric turbulence. Thanks. Answers = 1, Solutions = 0. g I live in an area that spends an aweful lot of time under the influence of the Jet Stream. Seeing here isn't usually too good, and planet observing, even in unobstructed scopes, is a real labor of love, with plenty of dissapointment. However, given that on nights of good seeing, the C8 does a fair job on planets, and given that under the same conditions (I assume) contrast improves with aperture, as %CO remains constant, I think I'll forego the 5" apochromat for the time being, and move up to the 11" or 14" SCT. -Stephen |
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SCT CO and Aperture question
Check out Thierry Legault's discussion of this:
http://perso.club-internet.fr/legault/obstruction.html Basically, it depends on the level of contrast. On high contrast images, ie. lunar features, the 11-inch can take full advantage of its aperture and cream any unobstructed 8" scope of similar quality. However, on low contrast planetary detail, the central obstruction becomes somewhat of a problem. With the values you gave, there are a few features that can be seen with the 8" unobstructed vs. the 11 inch. Generally, a 11 inch scope with 33% obstruction will perform like an unobstructed scope anywhere from about 8" to 11", depending on what type of detail you are looking for. Hope this helps. -George "Stephen Paul" wrote in message ... Given an SCT has a 33% obstruction that will have some deleterious effects on contrast compared to a scope with a smaller CO, but of the same aperture, and, given that by comparison, a larger aperture provides the second benefit of a smaller Airy disk, and better (tighter) resolution, ... consider an 8" aperture of undetermined CO, and an 11" SCT with 33% obstruction. What would the obstruction of the 8" scope have to be, in order to perform similarly to the 11" 33% obstructed scope on planets? IOW, what is the relationship between CO by Area and aperture, as aperture increases, and CO by Area remains constant? Or, for those who wish to give a quick and less detailed response..., would the contrast ever/always be better by increasing aperture, given a constant ratio of CO by area? |
#4
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SCT CO and Aperture question
George, I think your points are *very* well taken, precisely
because we often focus on otpics and what optics can or cannot do, or should or should not do on some theoretical level, with complete disregard of the object and its inherent traits as viewed under atmosphere. Your pointing out hi-contrast (lunar) features vs lo-contrast (planetary) features etc... as an inherent issue ... any optical set must deal with, is very well taken and needs to be kept at the forefront of any discussion of this kind... Thanks for the reminder! -Jerry George Wilkie wrote: Check out Thierry Legault's discussion of this: http://perso.club-internet.fr/legault/obstruction.html Basically, it depends on the level of contrast. On high contrast images, ie. lunar features, the 11-inch can take full advantage of its aperture and cream any unobstructed 8" scope of similar quality. However, on low contrast planetary detail, the central obstruction becomes somewhat of a problem. With the values you gave, there are a few features that can be seen with the 8" unobstructed vs. the 11 inch. Generally, a 11 inch scope with 33% obstruction will perform like an unobstructed scope anywhere from about 8" to 11", depending on what type of detail you are looking for. Hope this helps. -George "Stephen Paul" wrote in message ... Given an SCT has a 33% obstruction that will have some deleterious effects on contrast compared to a scope with a smaller CO, but of the same aperture, and, given that by comparison, a larger aperture provides the second benefit of a smaller Airy disk, and better (tighter) resolution, ... consider an 8" aperture of undetermined CO, and an 11" SCT with 33% obstruction. What would the obstruction of the 8" scope have to be, in order to perform similarly to the 11" 33% obstructed scope on planets? IOW, what is the relationship between CO by Area and aperture, as aperture increases, and CO by Area remains constant? Or, for those who wish to give a quick and less detailed response..., would the contrast ever/always be better by increasing aperture, given a constant ratio of CO by area? |
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