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			<title>wolfgang schreibt:</title>
			<link>http://r2.astro-foren.com#comment-96</link>
			<description><![CDATA[Quote: "Regarding lens no. 25. the problem is NOT SECONDARY spectrum, but "TERTIARY" spectrum. In the case of secondary spectrum, both blue and red colors are de-focused in the same direction. So, both red and blue lines should "bend" in the same direction. Just like achromats do this." Normally the term "Secondary Spectrum" belongs to an achromatic lens http://rohr.aiax.de/ref-rutten03D.jpg and the term "Tertiary Spectrum" belongs to apochromat: http://rohr.aiax.de/Apochromatic_focus_shift.jpg Serial #025 shows at http://rohr.aiax.de/CFF_tel_10.jpg a tertiary spectrum and GREEN is Null. Then BLUE is shorter with -71.2 mircron and RED is longer with + 92.2 micron. But this spectrum of #025 is much longer than the spectrum of #013, see here: http://rohr.aiax.de/CFF_tel_12.jpg I would calculate both this with the formula: http://rohr.aiax.de/RC_Index.png. many regards and thank you for answering]]></description>
			<dc:creator>wolfgang</dc:creator>
			<pubDate>Tue, 05 Apr 2016 00:32:05 +0200</pubDate>
			<guid>http://r2.astro-foren.com#comment-96</guid>
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			<title>Pal Gyulai schreibt:</title>
			<link>http://r2.astro-foren.com#comment-95</link>
			<description><![CDATA[Dear Mr. Rohr, thanks for your comments! Regarding lens no. 25. the problem is NOT SECONDARY spectrum, but "TERTIARY" spectrum. In the case of secondary spectrum, both blue and red colors are de-focused in the same direction. So, both red and blue lines should "bend" in the same direction. Just like achromats do this. In the image you kindly linked, the blue lines of lens no. 25 are bending "upwards", while the red lines are bending "downwards". Adding e.g. 30mm glass path would decrease the de-focusing of BOTH colors and 50mm glass would make an even stronger change in color correction. It only depends on taste of the observer. The difference between secondary and tertiary spectrum is that tertiary spectrum can be easily decreased by adding some glass in the light path, so, with simple a prism you can decrease it significantly, or "null it out" totally. This is why we can happily leave tertiary spectrum in the lens INTENTIONALLY, in order to make the color correction range wider (400nm - 1000nm) for photography. On the other hand, you can NOT decrease the amount of secondary spectrum with a prism, if you add glass and improve the lens in e.g. blue, then red color will become even worse and vice versa. So, secondary spectrum CAN NOT be left in the lens intentionally as it CAN NOT BE CORRECTED by simply adding a prism. With lens no. 25 the difference in measured spherical correction was much smaller than it was for no. 13, because the second lens had much more time to warm up. 12 hours is perfect, so, there the difference was minimal (and could be caused by different measurement wavelength, I measure green at 532nm). But for the first lens you measured spherical null in orange color, while that lens was also perfect at 532nm. That lens obviously had less time to warm up (and that was in the colder winter). Kind Regards, Pal Gyulai]]></description>
			<dc:creator>Pal Gyulai</dc:creator>
			<pubDate>Mon, 04 Apr 2016 16:54:11 +0200</pubDate>
			<guid>http://r2.astro-foren.com#comment-95</guid>
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			<title>W.Rohr schreibt:</title>
			<link>http://r2.astro-foren.com#comment-94</link>
			<description><![CDATA[Dear Pal, nice to hear from you. My main interest for serial #013 ist the Secondary Spectrum. For serial #025 it's longer than for serial #013. And I hope to test this a second time, to controll my own testings. Serial #025 I tested it twice: first the lens was to cold about 10°C and after 12 hours later it was a perfect lens, as my report shows. But I don't know, how many hours this type of lens has to temper. So I think, it was enough time for that. Quote"The no.13 lens has "visual" color correction," No, perhaps photografic correction. Compare this image: http://rohr.aiax.de/CFF_tel_12.jpg. Serial #025 could be visual corrected, but with a long Secondar Spectrum. Nowhere I've found any information, that you designed these lenses with a zenith prism. Normally I test this lens without any 50 mm prism or glas path. So the lens serial #013 I hope to get it for a second test, and then I could confirm you or not. Many regards from Wolfgang]]></description>
			<dc:creator>W.Rohr</dc:creator>
			<pubDate>Mon, 04 Apr 2016 14:57:00 +0200</pubDate>
			<guid>http://r2.astro-foren.com#comment-94</guid>
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			<title>Pal Gyulai schreibt:</title>
			<link>http://r2.astro-foren.com#comment-93</link>
			<description><![CDATA[Dear Mr. Rohr, I am Pal Gyulai and all the APO lenses in CFF telescopes are designed and built by me. I aspherize, measure and adjust them by my own hands. No part of production is done in China. ALL the lenses made by me have better than lambda/25 spherical correction in green color, at 532nm wavelength. When the no.13 lens arrived back, I measured it after 24 hours of waiting. Result was lambda/29,78. You measured a different value because the lens was colder than the room, when measured. This is natural. A few hours of thermailizing is not enough for these large lenses if you want to measure Strehl with high precision. A minimum wait time of 4-6 hours is recommended. The no.13 lens has "visual" color correction, so, it performs optimally without glass in the light path. In the meantime we changed the default color correction to "photographic" because: - with a, 1.25" prism diagonal these lenses perform absolutely optimally for planetary observation (but with a mirror they are still excellent, the difference in the eyepiece is small) - without glass in the light path they are corrected for very wide wavelength-range, so, they can be used with CCD sensors (from 400nm to 1000nm) without false color in the photographs - adding an 1.25" prism diagonal causes lambda/50 spherical overcorrection (at this focal ratio) which is mostly negligible, but on the other hand, it moves the spherical null towards red color, which is good for planets, as they have much more surface features in the red part of the spectrum (surface features of Mars, Jupiter, etc.) We believe this lens design is optimal. Pal Gyulai]]></description>
			<dc:creator>Pal Gyulai</dc:creator>
			<pubDate>Sun, 03 Apr 2016 22:26:33 +0200</pubDate>
			<guid>http://r2.astro-foren.com#comment-93</guid>
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