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Thread: Star image without and with a barlow in a F/7.5

  1. #1
    John Baars's Avatar
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    Default Star image without and with a barlow in a F/7.5



    On several occasions I noticed a slight but remarkable improvement of the star image while using my 120mm F7.5 Evostar and a Zeiss barlow. The changes are visible too with the 2.5X Televue Powermate. There is no difference visible between the barlow or the Powermate.

    In the first image I used no barlow, just the Evostar (doublet ED APO) together with the Televue 6-3mm. zoom, which was set on 3 mm.
    In the second I used a Zeiss barlow with the same eyepiece set at 6mm.

    [IMG

    ]Star image In Evostar 300X without and with Zeiss barlow by John Baars, on Flickr[/IMG]

    The difference is obvious (and not imaginative) in the sketch.
    I have heard from one observer on this forum of noticing it too. Asking on a Dutch forum gave no results.
    I am a bit puzzled by the cause of this phenomenon. It seems the first diffraction ring is more narrow but above all the instrument seems to have less troubles with the seeing.

    As causes I could think of:
    - Longer focal distances seem to have a less restless star image. ? Although the distance the light travels in the tube is in this case nearly the same, shorter even.
    - F15 systems give a bigger Airy disc than F7.5 systems. Which means less light in the surrounding arcs?
    - Frank Theys, a known telescope maker in Belgium gives some reasons, some of them do not count here, but others do: Why achromats? - FrTelescopes He mentions "complicated optical reasons", but doesn't give them.

    Did anyone notice this behavior too? And if yes, what are your experiences with it?

    Thanks!
    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: Star image without and with a barlow in a F/7.5

    Just some thoughts, the longer focal length will help reduce chromatic blur of the objective to a degree, longer focal lengths are more tolerant of any slight/ minor collimation issues with the optics train.
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    Default Re: Star image without and with a barlow in a F/7.5

    Quote Originally Posted by Gabby76 View Post
    Just some thoughts, the longer focal length will help reduce chromatic blur of the objective to a degree, longer focal lengths are more tolerant of any slight/ minor collimation issues with the optics train.
    I think both thoughts are true and will contribute to the phenomena. Thanks for your thoughts!
    Gabby76 likes this.
    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.
    Binoculars: AusJena 10X50 Jenoptem, Swarovski Habicht 7X42, Celestron Skymaster 15X70, Swift Observation 20X80.
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    Default Re: Star image without and with a barlow in a F/7.5

    I also discovered by chance that a good barlow (TV 2x and 3x in my case) clean up the views. Now I used this trick to split tough doubles and get steadier views of planet details. I think it has to do with barlow increasing effective focal lengths of scope.

    Unfortunately, it seems to work only if you start with good optics. In my case using 8" F10 SCT or 127mm F7.5 APO. I did not get any noticeable improvement on planets with barlow in 150mm F5 achro. The view was bad barlow or no barlow.

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    Default Re: Star image without and with a barlow in a F/7.5

    Hello John,

    and thank you for the documentation!

    If I understand the pictures correctly, then the Barlow :
    1. widens the central Airy disc,
    2. and fades away the second (and higher order) diffraction rings.

    Ad 1., This is expectable, as the Barlow can't increase the Strehl of the telescope.
    Ad 2., This is less understandable without the Fourier transforms calculation of the optics, against the classical diffraction + ray tracing calculus.

    And more ad 2., the Barlow may increase the modulation transfer at the medium to high spatial frequencies, the observers may perceive as higher contrast performance.
    Such tricks are used in some short buildt photo telelenses, where the fast front optics is followed at the end of the telelens by a 'teleconvertor' integrated into the telelens body.

    If it works, then why not to use it,

    JG
    Last edited by j.gardavsky; 03-29-2019 at 08:27 PM. Reason: typo
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