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Thread: First light through the 2inch zenith prism on the 6inch F/5 Synta refractor

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    Default First light through the 2inch zenith prism on the 6inch F/5 Synta refractor



    Hello all,

    the excellent seeing conditions on the 13th May have been a good reason to test the recently put on service 2” Wild Heerbrugg zenith prism.

    The eyepieces used in the test have been the Pentax SMC XO 2.5mm (magnification 291x, 44 AFOV),
    and the Pentax SMC XW 3.5mm (magnification 214x, 70 AFOV).

    Three targets for the tests have been chosen: Moon, Epsilon Lyrae double-double, and the globular cluster M13 in Hercules.

    The reason for the tests has been to check out if there is any significant improvement of the chromatic aberration (CA)
    and of the spherochromatism on the fast achromatic refractor when viewing the celestial objects.

    Moon

    The Moonwalk has started at north in he Birmingham area, with a stop at Plato to resolve the structures in its walls,
    and continued along the isolated montes at north in Mare Imbrium.

    Further south, Archimedes with Montes Archimedes, Erastothenes and Copernicus have been the dominating craters to spend some time with inspecting their structures into detail. Well resolved thin dorsae in Mare Imbrium and in Mare Insularum towards the terminator,
    and the very small flooded craters in the mare have been surprisingly very well visible.

    The Rupes Recta and Rima Hesiodus in Mare Nubium, and of course the crater Pitatus have been another highlights,
    both of them showing thin black lines with the highest contrast so far I remember.

    The end of the Moonwalk has been in Clavius, which is filled with smaller and smaller craters.

    The both eyepieces have offered through the prism very sharp views with high contrast, and with the resolution of the details comparable to the views through the zenith dielectric mirror when the green Baader Solar Continuum has been mounted to remove the CA and spherochromatism.
    Now through the prism, the high resolution and high contrast in the craters and around, has been achieved in white light without any green filter at all.

    Even if the Pentax SMC XW 3.5mm shows all what can be already resolved, the Pentax SMC XO 2.5mm blows up the finest details into the more comfortable view.
    None of these eyepieces has distracted the view with the floaters, even if I have enough floaters in my right eye reserved for Moon and planets.

    The only CA still visible has been a blue hue out of the rim on Moon in the bright sunshine. Trying the filters in my arsenal, this CA has been suppressed on the both eyepieces with the Baader UV/IR filter (420nm – 680nm passband) at best.

    Epsilon Lyrae doble-double

    The constellation of Lyra has been just rising through the atmospheric haze above the eastern horizon, and the Epsilon Lyrae is one of the classic test targets for the small scopes.
    Pentax SMC XO 2.5mm has easily resolved the both doubles, leaving enough black space between the components.

    Hercules Cluster M13

    I have been quite curious about the appearance of this glob through the zenith prism, have this globular cluster already seen more than 100 times since I am keeping a record.

    Having focused the Pentax SMC XO 2.5mm as precise as possible on one of the both foreground stars,
    I have slowly moved the scope into the glob. It took less time than usual to let the cluster condense into the individual stars. My impression has been that through the zenith prism the cluster stars become better separated, and that an enormous amount of cluster stars at the larger distances from the cluster center became visible.
    As the apparent field of view of 44 is at the magnification close to 300x not enough to oversee the whole M13 cluster,
    I have moved the scope above the cluster, and waited on the cluster to rise and to transit the eyepiece.
    What a fantastic view, I have been enjoying.

    Thank you for reading,

    JG
    Last edited by j.gardavsky; 05-15-2019 at 08:22 PM. Reason: typo
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    Default Re: First light through the 2inch zenith prism on the 6inch F/5 Synta refractor

    A fantastic report JG, glad that you were able to enjoy the views as well.
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    Default Re: First light through the 2inch zenith prism on the 6inch F/5 Synta refractor

    Great review and report, my friend!
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    Bryan
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    Default Re: First light through the 2inch zenith prism on the 6inch F/5 Synta refractor

    Sounds fascinating JG! I assume this is a prism from a theodolite?

    I will have to try a similar test with my Tak 1 1/4" prism vs a mirror diagonal. I'll see what the moon and Omega Centauri can show me...

    Keep up the good work!

    - Dean

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    Default Re: First light through the 2inch zenith prism on the 6inch F/5 Synta refractor

    Thank you all for the kind words!

    Hello Dean,

    the Wild prism has been evidently designed and manufactured for the Swiss Army tactical optics,
    maybe like the extra wide field XR Heliflex from Rodenstock, https://www.ebay.de/itm/Rodenstock-X...kAAOSw9wxbkM5S
    The face and the rear surfaces have antireflex multicoatings.
    The prism is isolated from a heat contact with the metal of the prism house, the prism house is a preowned part from TS,CNC machined they say, but who knows, you know.

    The old 2inch dielectric zenith quartz diagonal from TS, accessorized with the filter wheel, is further on service for the diffuse nebulae.

    Best,

    JG

    PS: It is the continuing bad weather offering me enough time to play with the gadgets.
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    Default Re: First light through the 2inch zenith prism on the 6inch F/5 Synta refractor

    Hi again JG. Although I primarily use my Celestron 6" f/5 refractor for wide angle DSO observing, it gives a good account of itself on the moon with medium high powers. The only chromatic aberration noted is the blue/violet at the limbs. Based on what you reported, the use of the prism may not only reduce this, but allow me to use even higher powers. Thanks for this analysis JG, and the best of regards.
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    Default Re: First light through the 2inch zenith prism on the 6inch F/5 Synta refractor

    Hello Marshall, hello all,

    different eyepieces will respond differently to the zenith prism on a fast achromatic refractor.
    And moreover, even the eyepieces with different focus lengths within one scaled design sequence may respond differently.

    So far tested: BCO 6mm, Pentax SMC O-6, O-18, and Fujiyama 5mm, on the terrestrial targets,
    and Tele Vue Delos 17.3mm, Leica B32x WW 14mm, Zeiss D 30x/40x B WW 12.8mm, on Moon on the 14th May - one day after the Moonwalk.

    In the first group the BCO 6mm seems to show the highest improvement,
    in the second group the winner has been unexpectedly the Zeiss D, followed by Leica B,
    and leaving the otherwise excellent Delos at some distance behind.

    Thank you for the kind comments,

    JG
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