#1
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11" SCT
Hi,
How is the quality of the latest generation of celestron 11" SCTs? What is the most lemon one you've received. Cie |
#2
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"Cie" wrote in message oups.com... Hi, How is the quality of the latest generation of celestron 11" SCTs? What is the most lemon one you've received. Cie There was a page, showing a set of tests carried out on a C11 in Germany, published here a few weeks ago. I'd say that this was a pretty 'fair' example (it showed about 1/7th wave PV), with some slight surface roughness from the machine figuring. Generally, I have not seen a 'bad' C11, in any recent scope. My own one, is pretty comparable with the German example, showing fairly smooth optics, with a 'best corrected' point for SA, about 4.5" behind the rear port (slightly further than is often quoted), and at this point, nice parallel Ronchi lines. I'd say the quality was pretty good. Best Wishes |
#3
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C-11 was all ways more expensive high end telescope for more series
observers, even back in the older days, unlike the 8"-er. So, quality was all ways closely watched the company, they did more hand figuring on them too. Very rare to find lemon amongst them, never heard of any. I own an Ultima 11 OTA, 22lb, sharp scope. Julius "Cie" wrote in message oups.com... Hi, How is the quality of the latest generation of celestron 11" SCTs? What is the most lemon one you've received. Cie |
#4
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quoted), and at this point, nice parallel Ronchi lines.
I'd say the quality was pretty good. What do you think is the accuracy of the Ronchi test in this configuration? Thanks. Dennis |
#5
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On Wed, 15 Dec 2004 08:42:59 -0500, "Dennis Woos"
wrote: quoted), and at this point, nice parallel Ronchi lines. I'd say the quality was pretty good. What do you think is the accuracy of the Ronchi test in this configuration? Thanks. Dennis Provided spherical aberration is controlled, the thing to look for in the Ronchi test is rough optics. They should be checked for with any large SCT. -Rich |
#6
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"Dennis Woos" wrote in message ... quoted), and at this point, nice parallel Ronchi lines. I'd say the quality was pretty good. What do you think is the accuracy of the Ronchi test in this configuration? Thanks. Dennis The potential accuracy will depend on the grid spacing, and the number of lines being shown. My LX200 in the same configuration, shows lines that bend quite visibly at the ends, (normally a sign of TDE). I was using a 120LPI grating, and on a normal mirror in the same configuration, would expect to easily reveal defects around 1/10wave, and with care, perhaps push to 1/20th wave. However the light source was red, which downgrades the accuracy compared to the green light normally used. The Ronchi test is rather 'qualitative', rather than 'quantitative', but by comparison with simulated results, and mirrors with known defects, measured by Foucault testing, I feel the scope is in the same 'order' as the example in the German tests, with defects in the order of 1/7th wave PV in the green. At the time, I was using the test for one thing only. To find the point where the scope was best corrected for SA, and was pleased to find that at the best point, the lines were much straighter than on the LX200, and were noticeably smoother... Best Wishes |
#7
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Roger Hamlett wrote in message ... "Cie" wrote in message roups.com... Hi, How is the quality of the latest generation of celestron 11" SCTs? What is the most lemon one you've received. Cie There was a page, showing a set of tests carried out on a C11 in Germany, published here a few weeks ago. I'd say that this was a pretty 'fair' example (it showed about 1/7th wave PV), with some slight surface roughness from the machine figuring. Generally, I have not seen a 'bad' C11, in any recent scope. My own one, is pretty comparable with the German example, showing fairly smooth optics, with a 'best corrected' point for SA, about 4.5" behind the rear port (slightly further than is often quoted), and at this point, nice parallel Ronchi lines. I'd say the quality was pretty good. Best Wishes that was on the Baader-Planetarium page . Talking with some friends from Germany apparently that interferometric test was conducted by a fellow named Wolfgang Rohr, who in some people's opinion is less than credible due to a past test of a Chinese refractor, in which he arrived at almost the same Strehl of 0.98 . Later he was contradicted by other measurements and admitted (allegedly) that he had tested the refractor only over the center 20mm of aperture , where it had indeed that high Strehl , then simply claimed the number for the whole scope. Please note that I do not have any first hand knowledge if this is the case or not, and would be interested to learn more . Based on what I understood from that test report, it just seems incomplete in the sense that the test setup and method was not described, pictured or in any way specified accurately . Any test should be taken with a grain of salt if the test setup is not published , especially the ones floating over the internet , which has no peer review and anybody can mae any claims . I would really be interested in any feedback from the German guys who read this list which is really the case with that Celestron test. My own experience with Celestron was along the general lines of that test , the same surface roughness level and that weird radial pattern but really nowhere as good a Strehl . best regards, matt tudor |
#8
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http://www.pliniocamaiti.it/test_main.htm#C11
How 'bout them radial spokes. They are almost certainly caused by the radial slots cut into the glass master-block that deforms the corrector plate during polishing, presumably as the block approaches or exceeds its usable life (the block wears as it is cleaned. This link is obviously a severe case, whereas they are just hinted at in the interferogram from baader. The two C11s I have owned certainly did not look this bad, more in line with the quite decent .029 rms of the German result. |
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