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Information on 120mm ED refractors?

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#26 tonyt

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Posted 19 October 2013 - 11:42 PM

Our 120ED scopes use Schott BK-7 and Ohara FPL-53ED glass.


What accounts for the improved performance of the later scopes - different glass, different spacing or something else?

#27 Ava

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Posted 20 October 2013 - 01:37 PM

There is a comparison of the older golden Skywatcher 120ed and the equinox 120 (and a TS 127 triplet, that may use the same or similar optics as the ES 127, there seem to be many scopes with an objective using a similar prescription) on the TS optics site:

http://www.teleskop-...r/ED-Refrakt...

In german only but you should be able to run it through google translate. I'm unsure if TS did the comparison themselves or if it was someone else, there is no source stated. It seems very thorough, and if you scroll down towards the end you can see some in and out of focus images, where the equinox does show obviously less CA than the other two; this is also the conclusion in the text of the comparison.

Like I said, no certain source but it's a data point at least.

#28 evhap

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Posted 20 October 2013 - 02:49 PM

That's a fascinating review, Anders. It supports a comparison I saw elsewhere between the 127 triplet and an Orion ED120 that concluded there was precious little to choose between them. It boggles my mind that a doublet can perform essentially as well as a triplet. There does seem to be a discrepancy with specs on the Skywatcher web site, however. The web site gives a weight of 5.2 kg for the Equinox, whereas TS reports 6.2. The lower number is essentially as the same as for the Pro (5.13 kg). Perhaps the 5.2 is a misprint?

To reiterate Tony's question, if, as Skywatcher insists, the Pro and Equinox use the same optics, what accounts for the difference in performance?

Evan

#29 tonyt

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Posted 20 October 2013 - 05:34 PM

That TS comparison is in agreement with the interferometric testing Wolfgang Rohr has done on the three scopes. Thanks for posting the link Anders.

#30 Ava

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Posted 21 October 2013 - 09:11 AM

Regarding the difference, maybe they DO use the same optics, but have a spread in production quality for the lens sets. They put the ones with somewhat better performance into the equinox series, and ones with somewhat less (but still very good) pergormance in the 120ed series (maybe using a measurement of color spread as mentioned earlier in this thread). This is pure speculation on my part but similar approaches are used in many businesses to increase yield, the semiconductor industry for example users this approach frequently (though very openly). Just an idea that might account for the differences but could still be said to have "the same optics".






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