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Equipment Discussions >> Refractors

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John Huntley
scholastic sledgehammer
*****

Reged: 07/16/06

Loc: South West England
Re: Information on 120mm ED refractors? new [Re: jrbarnett]
      #6147274 - 10/19/13 07:01 PM

As Jim says, they have always used Schott and Ohara elements. Synta did sign an agreement with Schott in recent years to promote the use of their glass more overtly so the brand name now appears in promotional material and a Schott sticker has been added to the tubes.

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tonyt
professor emeritus


Reged: 09/02/09

Loc: Australia
Re: Information on 120mm ED refractors? new [Re: jrbarnett]
      #6147277 - 10/19/13 07:02 PM

Quote:

There has never been a prescription change on these. They all use the same glass irrespective of vintage.
- Jim




Tests over at astro-foren suggest otherwise. Repost of an earlier post I made:
I thought I should check that my memory wasn't failing so I've just been to astro-foren and noted the secondary spectrum figures for the Skywatcher Pro 120ED (old gold tubes) and Equinox.
Two samples of the Pro version had about 190 microns spread between the focus points of different colors, while two samples of the Equinox 120 had about 75 microns spread. One Equinox scope had a spread of only 22 microns.


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hfjacinto
I think he's got it!
*****

Reged: 01/12/09

Loc: Land of clouds and LP
Re: Information on 120mm ED refractors? new [Re: tonyt]
      #6147514 - 10/19/13 10:14 PM Attachment (26 downloads)

I have a 120MM EON, in my opinion one it is one of the best 5" scopes out there. Visually and for imaging.

Edited by hfjacinto (10/19/13 10:15 PM)


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skyward_eyes
Vendor - Sky-Watcher USA
*****

Reged: 12/12/06

Loc: California
Re: Information on 120mm ED refractors? new [Re: John Huntley]
      #6147638 - 10/19/13 11:28 PM

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

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Kunama
professor emeritus


Reged: 10/22/12

Re: Information on 120mm ED refractors? new [Re: skyward_eyes]
      #6147661 - 10/19/13 11:45 PM

Realistically there seems to be very little difference in visual use. I've looked through the Megrez, SW 120 ED Pro and a few others, I suggest you pick one you like the look and feel of and check that there are no manufacturing faults like dust or marks on elements.
The 120mm F7.5 seems me to be a great allrounder scope. I would not worry too much about the glass that is in it, they will all show some extraneous colour unless you step up to the likes of the TOA/TSA/AP triplets.


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tonyt
professor emeritus


Reged: 09/02/09

Loc: Australia
Re: Information on 120mm ED refractors? new [Re: skyward_eyes]
      #6147719 - 10/20/13 12:42 AM

Quote:

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?


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Ava
sage


Reged: 11/30/11

Loc: Sweden
Re: Information on 120mm ED refractors? [Re: tonyt]
      #6148522 - 10/20/13 02:37 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-service.de/Artikel_Teleskop-Express/Skywatcher/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.


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evhap
member
*****

Reged: 11/09/10

Loc: California
Re: Information on 120mm ED refractors? new [Re: Ava]
      #6148619 - 10/20/13 03:49 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


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tonyt
professor emeritus


Reged: 09/02/09

Loc: Australia
Re: Information on 120mm ED refractors? new [Re: evhap]
      #6148869 - 10/20/13 06:34 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.

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Ava
sage


Reged: 11/30/11

Loc: Sweden
Re: Information on 120mm ED refractors? new [Re: tonyt]
      #6149557 - 10/21/13 10:11 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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