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Thread: Back to the zenit prisms on the refractors, and aberrations reduction

  1. #11
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    Default Re: Back to the zenit prisms on the refractors, and aberrations reduction



    Quote Originally Posted by Don Quixote View Post
    Thank you for all of this.

    I have in the mail to me a mirror prism from Baader that I want to use on my my f6, f6.5, and my 7.5 refractors.
    I hope I have not made a mistake.
    Hello Mark,

    which Baader diagonal prism it is? They offer quite a few, and all of them are pretty good.
    Best,

    JG
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    Default Re: Back to the zenit prisms on the refractors, and aberrations reduction

    Quote Originally Posted by j.gardavsky View Post
    Hello Mark,

    which Baader diagonal prism it is? They offer quite a few, and all of them are pretty good.
    Best,

    JG
    This is what I have coming. It is not a prism. It is a mirror I believe. I misspoke in my earlier post calling it a prism.

    Baader 2 inch BBHS Sitall Mirror Diagonal - MAX-2S
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  3. #13
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    Default Re: Back to the zenit prisms on the refractors, and aberrations reduction

    Quote Originally Posted by Don Quixote View Post
    This is what I have coming. It is not a prism. It is a mirror I believe. I misspoke in my earlier post calling it a prism.

    Baader 2 inch BBHS Sitall Mirror Diagonal - MAX-2S
    Congratulations Mark!

    This is the best mirror diagonal on the market, see for details:
    https://www.baader-planetarium.com/e...ock-clamp.html
    If I would not be keeping cash for some more other optics, I would already have it.

    Herewith you can capture Moon in NIR/IR to dig out finer details, or the turbulent swirls in Jupiter bands in the UV/violet.

    The BBHS enhances also the color contrast for visual use.

    Best,

    JG
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    Default Re: Back to the zenit prisms on the refractors, and aberrations reduction

    Quote Originally Posted by John Baars View Post
    Can't wait!
    Have to wait at least for a week or two, though.
    The first owner of my "new" 150mm SW Achro is still on holiday. But the purchase has already been concluded and paid for.
    Hello John,

    you will better see the difference due to the prism when you mount your Pentax XO eyepieces, or anything shorter than 10mm.
    Best,

    JG
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    Default Re: Back to the zenit prisms on the refractors, and aberrations reduction

    Hello JG,

    Last evening I did some observing and startesting (just out-and inside focus) with my 120mm F/7.5 Evostar. According to W. Rohr this instrument has a Rest Chromasie_Index of 0.7244 without Glasweg. Which means with a mirror it is an APO. The very same instrument has a RC_Index of 2.1588 with Glasweg, which means it is an Achromat with a prism. Apparently this is one of those instruments which are designed for use with a mirror. astro-foren.com - A116 SkyWatcher Equinox ED 120-900 ED APO Nr 3

    1- I could easily observe the difference in an Evostar ED doublet while startesting with an XO-5, the brightening in the red fringes with the prism is much more easily visible than the brightening in blue.

    Some time ago I did the same test with my much older 102mm F/9 Vixen ED. Although that was a while ago I could easily see differences too. In this case the prism did exactly what it was predicted to do: CA with the prism was significantly less than with a mirror. With the prism in the Vixen I did not notice differences in intensity between the red and blue fringes ( like in the prism and the Evostar). I don't recall exactly the eyepiece I used, but I suppose it was the XO-5 too, for I am used to using this eyepiece for testing.

    2- The Zeiss specified prism does its' work on the Vixen, which apparently was designed for working with a prism.

    Nr.1 and Nr.2 inevitable lead me to the conclusion that the prism-trick will work on the 150mm achromat too, being an older Fraunhofer design.


    Off topic mode on:
    [In general I am not so impressed by the cries of spoiled 21st century amateurs about chromatic aberration. (Dutch: roeptoeteren) As if short Apo's are the only saving dish in refractor land. I bet the majority of them have never looked at a nice NGC through a good achromat. Much knowledge about this is based on hearing knowledge from the internet, not based on experiential knowledge.

    That is why I will happily observe the DSOs smiling. And kindly give a look to the biggest CA-enemy, when NGC6995 passes by in my field of view.
    ]
    Off topic mode off.
    Last edited by John Baars; 08-20-2019 at 06:29 AM.
    Telescopes in Schiedam : SW 150 F/5 Achromat, SW Evostar 120ED F/7.5, Vixen 102ED F/9, OMC140 maksutov F/14.3, SW 102MAK F/13 on Vixen GPDX.
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    Default Re: Back to the zenit prisms on the refractors, and aberrations reduction

    Hello JG and John. I have always been happy with my 2" Orion dielectric 99% transmission diagonal. I only have bothersome chromatic aberration with very bright objects. This reminds me of the man who went to his doctor, raised his right arm over his shoulder and lowered it down his back and said; "It hurts when I do this." The doctor then replied; "Well, don't do that." So, I just observe the faint fuzzies. I will have to look more into this zenit prism for use with higher powers on my old Fraunhofer doublet refractor telescope. Thanks for the excellent read and information on this thread guys, and the very best of regards.
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    Default Re: Back to the zenit prisms on the refractors, and aberrations reduction

    Hello John,

    without knowing the materials and the lens thicknesses and radii in the doublet, it is difficult to predict what the glassweg through the prism will be doing.
    The Sky-Watcher (Synta) 6" F/5 reveals according to my measurements the secondary spectrum close to that of a canonical Fraunhofer with the C-e and F-e deviataions about Δƒ=0.35mm
    An approximate formula is given in the 7th paragraph in https://www.telescope-optics.net/des...t_achromat.htm :

    A doublet made from BK7 crown and F3 flint will have secondary spectrum close to the minimum possible with common glasses (~1/2000Æ’)
    The formula precise form is in the figure caption 150,

    Δƒ=(H/B)ƒ, with ƒ being the focal length. Numerically, H equals the relative partial dispersion differential, and B the Abbe number differential between the two glasses.
    This is the one point.
    The other point is the effect of the glassweg on the spherical aberration.

    Best,

    JG
    Last edited by j.gardavsky; 08-20-2019 at 07:21 PM. Reason: the editor of formulas
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    Default Re: Back to the zenit prisms on the refractors, and aberrations reduction

    Hello JG,

    Thanks for your thoughts.

    Well, yes, maybe I am a bit too enthusiastic about the outcome of the test.:-))

    The Synta 6" F/5 at least answers the standard canonical Fraunhofer characteristics, according to your measurements. Meaning in this case a secondary spectrum at 0.35mm and an colored blur of 11 times the diameter of the Airy disc. ( Telescope optics, by Rutten and Venrooij, second edition, chapter 6, the Refractor, page 55 ) Which is almost four times the usual tolerable limit for amateurs. Visible only on very bright pointsources ( until Magn. 6 ?) and objects like the planets and the Moon.

    Those are not the objects I had in mind for this instrument. I thought nice open clusters, small globulars and planetaries and maybe even emission nebulae were more of his kind.


    About the spherical aberration.
    I have learned that spherical aberration can be controlled more or less by slightly changing the distance between the two lenses. Probably with the introduction of other aberrations. Is this an option for the Synta, or has that optimization already been done? (probably)


    [Off Topic on] Here in my LP location I have found out that with my refractors on those faint objects ( exept the emission nebulae) an exitpupil of 1.0 - 0.8 mm is the best. Which means in this case a magnification of 150X - 190 X. On a dark location much lower. Will the Synta be able to cope with the 150X? )[Off Topic off]
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    Default Re: Back to the zenit prisms on the refractors, and aberrations reduction

    Quote Originally Posted by John Baars View Post
    Hello JG,

    Thanks for your thoughts.

    Well, yes, maybe I am a bit too enthusiastic about the outcome of the test.:-))

    The Synta 6" F/5 at least answers the standard canonical Fraunhofer characteristics, according to your measurements. Meaning in this case a secondary spectrum at 0.35mm and an colored blur of 11 times the diameter of the Airy disc. ( Telescope optics, by Rutten and Venrooij, second edition, chapter 6, the Refractor, page 55 ) Which is almost four times the usual tolerable limit for amateurs. Visible only on very bright pointsources ( until Magn. 6 ?) and objects like the planets and the Moon.

    Those are not the objects I had in mind for this instrument. I thought nice open clusters, small globulars and planetaries and maybe even emission nebulae were more of his kind.


    About the spherical aberration.
    I have learned that spherical aberration can be controlled more or less by slightly changing the distance between the two lenses. Probably with the introduction of other aberrations. Is this an option for the Synta, or has that optimization already been done? (probably)


    [Off Topic on] Here in my LP location I have found out that with my refractors on those faint objects ( exept the emission nebulae) an exitpupil of 1.0 - 0.8 mm is the best. Which means in this case a magnification of 150X - 190 X. On a dark location much lower. Will the Synta be able to cope with the 150X? )[Off Topic off]
    Hello John,

    and thank you for your discussing the topic.

    1. The color blur 11 times the Airy disc is noticeable on the densly packed globular clusters when I take the short eyepieces around 3mm. The color blur is then a grey blur, and it has as good as disappeared with the 2" glass prism.

    2. The spherical aberration control of the fast objectives is always difficult, that's why a triplet or a wide spaced quadruplet are a better choice. Controlling the CA and SpAb with a glass prism is the poor boys' choice.
    In the eyepieces, their own spherical aberration is controlled with the meniscus lens or 2 meniscus lenses, and with the thickness of the lenses.

    3. 150x magnifications? Well, the shortest EPs I have been using to pull out the spiral arms in some popular galaxies have been the Pentax XW 10mm and 7mm, and otherwise the KK Fujiyama Ortho 5mm on the M51 and M81. No problem under my backyard skies.

    Once again back to the prism Glassweg.
    The Glassweg in the prism acts both on the chromatic aberration in the sense of flattening and distorting the secondary spectrum curve, and on the 'on axis' spherochromatism in the sense of decreasing the light blur close around the Airy disc. The only question is, given an achromatic telescope, how much Glassweg is right.

    I am sure, you will be happy with your rich field 6" Synta, as happy as I am,

    JG
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    Default Re: Back to the zenit prisms on the refractors, and aberrations reduction

    Hello JG.,
    I have been musing / brooding about that spherical aberration. I have read on a Dutch forum ( and in your own post #19) that using a Barlow or Smyth-lens can reduce spherical aberration.(only visible while startesting though) I am not sure if lateral spherical aberration or axial spherical aberration was meant here. Astigmatism was not meant here, although it has it's effect on edge-astigmatism . Since you have mentioned the meniscus in eyepieces; it is an extra (negative) Glassweg. The two Pentax XW's you mentioned have a Smyth-lens too.
    What is your opinion / expertise and experience about this issue?
    Last edited by John Baars; 08-25-2019 at 09:28 AM.
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