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  • Re: Apochromatic Objective Question

    From Martin Brown@21:1/5 to quadibloc on Sun Nov 10 14:20:16 2024
    On 01/11/2024 09:51, quadibloc wrote:
    The first Cooke photovisual objective was sold in 1899.
    It was a kind of apochromatic objective; however, it aimed at bringing ultraviolet light as well as visible light to a common focus.
    This was long before 1969, when Canon started selling fluorite lenses
    for cameras.
    One source claims that it was discovered that the new Schott glasses
    that
    made such lenses as the Cooke photovisual possible were unstable; they deteriorated after a few years, and so this type of lens was realized to
    be impractical, and Schott stopped making those glasses.

    I think that is basically true. The early dispersion corrective glass
    for the concave meniscus lens tended to recrystallise at the surface
    under the influence of dew and humidity on a timescale of a few decades
    and required periodic repolishing to restore clarity.

    However, it seems as though this can't be the explanation, as there
    are reports of Cooke photovisual objectives that have survived to the
    present which are still in usable condition.

    It may well vary from batch to batch. The ~12" objective for the
    Northumberland telescope at Cambridge made by Chaucoix in Paris for Airy
    in the 1830's suffered that fate. It was replaced by a modern lens in
    the 1990's when it was beyond further refiguring. It lasted about 150
    years before the thing was deemed beyond repolishing. Not sure how many
    time it was done in its lifetime possibly 4 or 5 times maybe.

    https://www.ast.cam.ac.uk/telescopes

    When built it was for a short while the largest refracting telescope in
    the world.

    The other possibility is that since the correction provided by this
    kind of lens extends to the ultraviolet, perhaps that means that the
    degree of correction for chromatic aberration it provides within the
    visible spectrum is reduced, maybe the amount by which chromatic
    aberration in the visible range is corrected isn't much improved over
    that of plain achromatic lenses.
    But that also doesn't seem to be the case; the lens is described as
    virtually making chromatic aberration g away completely.

    So I'm puzzled. Why didn't the Cooke photovisual lead to a continuous availability of apochromatic lenses prior to Canon's introduction of
    the fluorite lens in 1969?

    I think they became problematic after a couple of decades of use and
    required refiguring and repolishing often enough to retain their full performance. The surface tended to recrystallise and mist at least in
    some batches of the material. The ones which survive today are probably
    the handful of batches where the trace water content was very low.

    We have antique normal glass in our village hall from that era that
    shows internal mistiness depending on the ambient humidity. It might
    also be that the surviving instruments with this glass formulation are
    in places with naturally very low humidity (unlike the UK).

    --
    Martin Brown

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  • From Martin Brown@21:1/5 to quadibloc on Wed Dec 25 09:36:50 2024
    On 16/12/2024 19:34, quadibloc wrote:
    I've researched this question further.
    It turns out that it was found that they could protect elements from
    this new glass from deteriorating by sealing them off from the air.
    Also, I've learned that Lanthanum glass has anomalous partial
    dispersion.
    This solves the mystery of how the Kern Macro-Switar, which came out
    years before Canon found out how to grow fluorite crystals, could have
    been truthfully advertised as an apochromat.

    A Kodak patent, though, indicates that contrary to another source,
    fluorite glasses may have existed before Canon's achievement, instead of arising in reaction to it. However, that patent referred to fluoride
    glasses, and maybe they didn't have anomalous partial dispersion to a
    useful extent - I mean, fluoride and fluorite may sound similar, and
    they both have fluorine in them, but a fluoride of something else with
    no calcium isn't going to have anomalous partial dispersion necessarily
    just because calcium fluorite does.

    Merry Christmas everyone!

    Old fluoride glasses would contain calcium fluoride if there was any
    calcium at all in the in the original melt. CaF2 is one of the most
    insoluble and stable inorganic compounds known.

    Almost all of the fluorine on Earth is locked up as calcium fluorite or
    one of the rarer group II metal fluorides.

    Looking for CFC's in the atmosphere of a remote Earth like planet is one
    way that we hope to be able to detect alien civilisations. You pretty
    much have to invent electrolysis before you can make CFC's.

    Nature can't really make them although a handful of plants can make
    fluoracetic acid which is an incredibly potent rodent (and other
    mammals) poison.

    But the early formulations really didn't get on with water.

    https://patents.google.com/patent/US2511224A/en

    Contains:
    "References to fluoride glasses are found in V. M. Goldschmidt, Vid.
    Akad. Skv., Oslo, 1926, No. 8, p. 138; and G. I-Ieyne, Angew. Chem, 46,
    p. 473, 1933. However, these glasses were water-soluble and hygroscopic.
    It is another object of this invention to provide fluoride glasses which
    are, for practical purposes, stable against moisture attack."

    Their invention was a version that was a lot more stable. Hard to look
    back any further because modern fluoride glass fibre compositions are
    too numerous and it is hard to see the one ancient tree for the forest.

    --
    Martin Brown

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