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  1. #1
    Alec in France's Avatar
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    Default Chromatic aberration in low cost refractors and binoculars



    Frankly I'm a bit puzzled folks. When I bought my Skywatcher Startravel 102mm refractor, I was not expecting the earth. As a lower cost achromat I knew that it would exhibit some CA on bright objects such as the moon and Jupiter. This proved to be the case. It doesn't really trouble me especially since the scope wasn't purchased for that purpose. I use it as my "cluster buster" where it performs remarkably well, giving me lovely views of M45 and the like.

    I also recently added a pair of really cheap (40 euros) binoculars to my arsenal. Made by Bresser they are 10 x 50s. Nice enough looking I suppose and the box proclaimed that they use Bak4 prisms. Terrestrial daytime views are impressive, but last night I used them "grab and go" fashion for a quick look and the moon and Venus.

    Venus was just a small crescent but focused well and exhibited no noticable CA. Puzzled, I looked at the moon. The night sky was very clear and the moon very bright. Looking through the bins I did notice that the very edge of the moon's disk seemed brighter than the rest of it but again little to no noticable CA.

    OK, the refractor wasn't expensive, but it did at least cost about 5 times the price of the binoculars. Is the CA problem largely overcome because its much easier to make 50mm glass than 102mm? Or does the fact that you are using both eyes also contribute?

    I'm not complaining of course, but these cheap bins seem to perform above their price tag. Maybe I'd be amazed by premium quality bins?

    Alec.
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  2. #2
    j.gardavsky's Avatar
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    Default Re: Chromatic aberration in low cost refractors and binoculars

    Hello Alec,

    Bresser offers the 10x50 binoculars in a broad price range, between 20 EUR and 260 EUR. The BaK4 label has nothing to say on the China made binoculars, they often use a phosphate glass for the Porro prisms, to reduce the transverse CA, similarly to the very old Asahi Pentax (Japan) wide field binoculars. Maybe, you have got one of those high definition bins, like here Bresser 10x50 High Definition Universal-Fernglas: Fernglas Preisvergleich - Preise bei idealo.de

    Congrats on your acquisition, and anjoy the binoculars very much,

    JG
    Binoculars: Leica Ultravid 7x42, 8x42HD; Swarovski EL 8.5x42 Swarovision; Nikon 10x70 Astroluxe; Docter Nobilem 7x50 Porro; Jenoptem 7x50W, 10x50W; BA8: 10.5x70, 15x85; 25x100FB, AsahiPentax 8x40, Refractors: Sky-Watcher 150mm/750mm; Leica APO Televid 82mm (25x-50x WW ASPH); EPs:Baader Classic Orthos; Fujiyama ortho, Leica B WW, ultrawide zoom ASPH, Periplan GF, HC Plan S, L; DOCTER UWA; Wild UW mil; Tele Vue Delos, Nagler Zoom, Plössls; Swarovski SW; Pentax XW; ZEISS diascope B WW T*, Carl Zeiss E-Pl; Hensoldt mil; Filters: Astrodon, Astronomik, Baader (CCD), TS;
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  4. #3
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    Default Re: Chromatic aberration in low cost refractors and binoculars

    Quote Originally Posted by j.gardavsky View Post
    The BaK4 label has nothing to say on the China made binoculars, they often use a phosphate glass for the Porro prisms, to reduce the transverse CA, similarly to the very old Asahi Pentax (Japan) wide field binoculars.
    JG,

    Thank you very much for your answer. You are 100 per cent correct, these are exactly the binoculars I purchased!
    So, if the phosphate glass is a cheap way to reduce the transverse CA, I assume that there must be a downside? Maybe their performance will degrade sooner rather than later?

    Alec.
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  5. #4
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    Default Re: Chromatic aberration in low cost refractors and binoculars

    Quote Originally Posted by Alec in France View Post
    JG,

    Thank you very much for your answer. You are 100 per cent correct, these are exactly the binoculars I purchased!
    So, if the phosphate glass is a cheap way to reduce the transverse CA, I assume that there must be a downside? Maybe their performance will degrade sooner rather than later?

    Alec.
    Hello Alec,

    I would not be afraid.
    The glass used has a high refractive index and simultaneously lower dispersion, something between BaK4 and BK7. It may be the same or similar material as in the very good BA8 United Optics binoculars (Garrett's Signature Series, Astro-Physics Premium Binoculars). The only question arising is the quality grade, e.e. the density of the microinclusions, which may lower the contrast. You can easily check it, when aiming the binoculars during the night against some street lamps. A high density of microinclusions in a cheap glass material would reveal a bright halo around the street lamp.

    Sooner or later, a good astronomy equipment will be much more cheaper, as we have seen it with the Explore Scientific eyepieces, and with the United Optics binoculars.

    Congrats to your choice, which is looking very promissing,

    JG
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  7. #5
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    Default Re: Chromatic aberration in low cost refractors and binoculars

    Good info ^.

    I too note a brighter white edge to the Moon (not a halo) but no CA that I can see. (BaK4 labelled).

    However is there an easier check of the lenses, my nearest street light is about 20 miles way and even then it will probably switched off.

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    Default Re: Chromatic aberration in low cost refractors and binoculars

    Quote Originally Posted by RichM63 View Post
    Good info ^.

    I too note a brighter white edge to the Moon (not a halo) but no CA that I can see. (BaK4 labelled).

    However is there an easier check of the lenses, my nearest street light is about 20 miles way and even then it will probably switched off.
    *sigh* with jealously.....
    Jim
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  9. #7
    j.gardavsky's Avatar
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    Default Re: Chromatic aberration in low cost refractors and binoculars

    Quote Originally Posted by RichM63 View Post
    Good info ^.

    I too note a brighter white edge to the Moon (not a halo) but no CA that I can see. (BaK4 labelled).

    However is there an easier check of the lenses, my nearest street light is about 20 miles way and even then it will probably switched off.
    Hello Rich, hello all,

    a test with the full (or nearly full) Moon should be enough. Move the binoculars around with the Moon in and out, and watch if there is any halo. There will be some, I bet, but if you can fix with your binoculars the Dark Cigar dark nebula (on a moonless clear and dark night) down the M39 cluster in Cygnus, then, there is nothing to worry about.

    Best,

    JG
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    Default Re: Chromatic aberration in low cost refractors and binoculars

    Quote Originally Posted by RichM63 View Post
    I too note a brighter white edge to the Moon (not a halo) but no CA that I can see. (BaK4 labelled).
    Thanks for that comment Rich! Good to note we had the same viewing experience.

    Alec.
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