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Thread: Prism diagonal comparison

  1. #11
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    Default Re: Prism diagonal comparison



    I use both the full size 2" and the T2 version of the BBHS and they have outperformed anything I have used previously. The one note that Bill made concerning color rendition was the one aspect that I noticed clearly, and fairly quickly. Jupiter indeed shows more 'depth', and star fields and clusters are simply amazing. The BBHS materials seem to be more sensitive to the red wavelengths as they are much more vivid amongst the myriad of colors in the star field. I have a BBHS T2 "Zeiss spec" prism version on its way, and hope to be able to do a head to head with the mirror version to evaluate which can go deeper and crisper. https://www.baader-planetarium.com/e...part-01b).html
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  3. #12
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    Default Re: Prism diagonal comparison

    Hello John,

    and thank you for reminding the Baader BBHS, which might be the best choice when the costs don't play a role.

    The planarity (waves distortion) and the scatter (surface r.m.s) of the prisms and on the mirrors can be easily tested with a laser interferometer, and I keep wondering why nobody, and even not Bill Paolini, have not made this test before putting a read on the CN.

    An easy visual test is to send a ray of a green laser on the mirror, or let it pass the prism.
    The diffusion of the laser beam on the air-to-mirror interfaces, and on the air-to-glass, glass to the internal reflecting surface, and finally glass-to-air at the prism exit, yield the first impression of quality grade of the mirror or of the prism.
    The comparison to be made is the size of the diffusion spot against the cross section of the laser beam.
    Large diffusion spots indicate high r.m.s. and high local peak-to-valley surface deviations on the surfaces.

    The second visual test is about the scattering loss in the prisms.
    The brightness of the green laser beam trace is an indicator of the scattering loss in the prism. Ideally the trace is very faint, or even better it is hardly visible. So, a bright laser beam trace indicates an inferior glass quality.
    Similarly to the Porro prisms in the binoculars, the light loss due to the scattering on the microinclusions in the prism glass increases for the shorter wavelengths in green/blue and blue (Rayleigh scattering).

    Best,

    JG
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  5. #13
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    Default Re: Prism diagonal comparison

    Small prisms for planetary. Then most any mirror for galactic targets. If you use 2 inch eyepieces then 2 inch diagonal should be used. I tried the 2 inch screw on end on the BBHS T2 mirror had previously but the edges of the field were dimmer than the middle. That was my introduction to why you need big diagonal for big eyepiece.

    As far as dielectric surface roughness do not think it would be noticeably at low to medium powers like is used for galactic objects as I rarely go over 200x except for planetary nebula or double stars.
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    Default Re: Prism diagonal comparison

    Quote Originally Posted by tomjones View Post
    Small prisms for planetary. Then most any mirror for galactic targets. If you use 2 inch eyepieces then 2 inch diagonal should be used. I tried the 2 inch screw on end on the BBHS T2 mirror had previously but the edges of the field were dimmer than the middle. That was my introduction to why you need big diagonal for big eyepiece.

    As far as dielectric surface roughness do not think it would be noticeably at low to medium powers like is used for galactic objects as I rarely go over 200x except for planetary nebula or double stars.
    Agreed. I keep the T2's set up for 1.25 use only. I only have three 2" EPs in my lineup, and depending on the target of the night, I'll typically switch to a scope that better suits the target FOV and magnification needs, or swap out the entire diagonal for the full size version.
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  7. #15
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    Default Re: Prism diagonal comparison

    Look at the colors man. The BBHS mirror diagonal really shows the reds. All those red giant stars in the double cluster look nice. Saturn's moon Titan is a reddish brown star in my 5 inch apo. But.....for details on planets this high priced mirror is still inferior to the standard Baader prism.
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    Default Re: Prism diagonal comparison

    Quote Originally Posted by tomjones View Post
    (...)The BBHS mirror diagonal really shows the reds. All those red giant stars in the double cluster look nice. (...) But.....for details on planets this high priced mirror is still inferior to the standard Baader prism.
    Agreed on the first part.
    There is the tendency to give apo's a good correction in the blue part of the spectrum. All amateurs are looking for blue fringes around stars, and manufacturers know this. A slight red spherochromatism as a result. The BBHS shows it. A prism can be used to to shift it slightly back, depending on the sort of correction the manufacturer gave to the apo.

    Not completely agreed on the second part.
    A mirror that shows the spherochromatic deviation of the main telescope is not inferior, on the contrary. Apparently the telescope needs a prism for planets. As a given fact. No need to disqualify the mirror as inferior

    Like you I switch to a Baader ( Zeiss) prism when planetary details are involved (with less "glare" as a good first reason) , and I use a BBHS mirror for DSO's.
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  9. #17
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    Default Re: Prism diagonal comparison

    Helloo Frankie,

    and back to your post:

    Quote Originally Posted by SpyderwerX View Post
    I use both the full size 2" and the T2 version of the BBHS and they have outperformed anything I have used previously. The one note that Bill made concerning color rendition was the one aspect that I noticed clearly, and fairly quickly. Jupiter indeed shows more 'depth', and star fields and clusters are simply amazing. The BBHS materials seem to be more sensitive to the red wavelengths as they are much more vivid amongst the myriad of colors in the star field. I have a BBHS T2 "Zeiss spec" prism version on its way, and hope to be able to do a head to head with the mirror version to evaluate which can go deeper and crisper. https://www.baader-planetarium.com/e...part-01b).html
    The Baader/ZEISS proprietary BBHS Broad (spectral) Band Hard Silver technology is due to the reflected very wide spectral band of 390nm up to near infrared the best choice, as you'll get the Miras and the Carbon stars in vivid red, and on the planets and Moon the NIR increases the contrast.

    This goes lost on the dielectric mirrors with the UV/NIR cut, and to some extent on the prisms which absorb UV and drop the transmissivity in NIR-IR.

    Best,

    JG
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  11. #18
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    Default Re: Prism diagonal comparison

    Best mirror diagonal ever had for the planet's was the William Optics 2 inch enhanced aluminum. It was only slightly inferior to the standard Baader prism. All other mirrors have been significantly less good.
    Newtman but some refractors allowed.

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    Default Re: Prism diagonal comparison

    great report thanks for letting us know

 

 
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