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Thread: cylindrical lens pair vs anamorphic prisms?

  1. #1
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    Default cylindrical lens pair vs anamorphic prisms?

    Can someone please list the advantages and disadvantages of each in comparison?
    I've found a seller that has bunch of stuff I need but doesn't have anamorphic prism pair, only cylindrical lens.

    All I know is prisms are easier to set up.

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    There is a great thread on this somewhere on this site. It's about 4-5 years old. It depends on what your source is and what you want to do. The lens has less loss but he prism lets you adjust easier but only in a single axis which is what you want for a diode. lens are tricky to adjust and you have to be exactly on axis. Prisms are a little more forgiving but you loose more light. Think I go that right.

  3. #3
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    Are you able to locate that thread? I can't find it.
    And thanks. But I think you mean spherical lenses, not cylindrical ones, right? My guess is cylindrical lenses also adjust in one axis (provided they are perfectly on the same axis as you explained).
    And my guess is cylindrical lenses would modify the beams hitting them on the exact middle differently than beams located closer to the edges.
    But that's as far as I can go by logic and I lack the scientific knowledge.

  4. #4
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    He definitely means cylindrical lenses. I have found that prisms do offer easier beam shaping adjustments but at the expense of output efficiency. Even with nm specific AR coatings , there is a serious amount of wasteful reflection off each prism surface. Conversely, I find that cylinder lenses give off a bit more beam trash and requires spatial filtering if you're picky. Each has their pros and cons but most people here use cylinders, especially for red diodes because every precious mW is needed to match the big blues and greens.

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    Here is a great thread showing cylindrical lenses combined with spatial filtering for a well groomed beam.
    https://www.photonlexicon.com/forums...l+filter+video

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    Just a thought;
    It seems everybody uses a galilean (positive + negative) cylindrical lens pair, to correct astigmatism.
    Is there a particular reason why no keplerian (positive + positive) cylindrical lens pair is used ?
    That way, a spatial filter could be integrated into this telescope (for the fast axis).

  7. #7
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    I've watched TechIngredients channel's video few months ago, all of them and I don't regret any minute spent.
    And thank you for the info.

    However I still seem lost on deciding between cylindricals and prisms because few things are still unknown to me. I think these are all.

    1) How much is a typical power loss between a typical prism pair and cylindrical pair? Just knowing one is greater than the other doesn't help much because if it is 3-5% more I might not care personally, but more might make me reconsider.

    2) Do the beams on the edge of a cylindrical lens get transformed differently than the ones on the middle? I feel like they would but not sure how. With prisms I don't feel I'll find out something undesirable *after* I buy and test them.

    3) I still don't understand how cylindrical lens pair can affect the vertical axis if the lenses are positioned planar.

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    1) the greater the magnification, the smaller the angle of incidence the greater the losses. Prisms do well between 1,2x and 2x. Properly paired cyl's can magnify virtual limitless.
    2) a light ray traveling right through the middle of a lens passes unaffected, the more to the edge it enters the lens, the smaller the angle of the air-glass interface(s) it encounters, the stronger it gets deflected.
    3) it does not ! The divergence of the 'vertical axis' should be purely determined by the focusing-lens. (at least when the correcting optics are perfectly aligned.)

  9. #9
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    Quote Originally Posted by -bart- View Post
    1) the greater the magnification, the smaller the angle of incidence the greater the losses. Prisms do well between 1,2x and 2x.
    Thanks but again, what does doing well mean? (around how much losses with 3x and above?)

    a light ray traveling right through the middle of a lens passes unaffected
    But does that mean that the middle beam in a knife edge won't be corrected as much/the same way as the leftpost and rightmost beams?
    If you just answered this, then sorry, please rephrase it for morons.

    3) it does not ! The divergence of the 'vertical axis' should be purely determined by the focusing-lens. (at least when the correcting optics are perfectly aligned.)
    kecked said prisms can only correct in one axis so I could only assume the case is different with cylindrical, glad this is cleared up now.

  10. #10
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    When Bart says that a "ray" traveling thru centre of lens is unaffected by it , he refers to an infinitely skinny "ray" not a real beam with a finite width.
    So answer is no middle beam is not affected differently....HOWEVER ... I deduce that you have made a big mistaken assumtion in how you imagine the knife edged beams to be arranged with respect to the cyl lenses.

    They should be stacked vertically (thin on top of thin) so all beams pass as nearly as possible thru the centre of the lens.
    Remember that the thin part of the beam after the collimator lens in near field is the fat(wide) stripe in far field.

    The cyl lenses simply re-correct this slow (narrow) axis, which has been inevitably "over-corrected (making it focus and then diverge again) by the collimator lens", in order to tame the "fast" (wide) axis as early as possible so as to keep the near field beam small.

    The thin bit early on is really the fat bit later on .....or to put it yet another way .... the cyl lenses (or prisms) make the thin bit of the beam fatter in the near field, so as to make sure that what becomes the fat stripe bit of the beam, is thinner (less wide) in the far field. It does sweet FA to the originally (hopefully) corrected (by the collimator lens) fast axis.

    If you stack the thin beams side by side as you seem to be suggesting, they will not be collectively thin anymore so when you make them fat, they will become VERY fat and so will the narrow spaces in between them ... they will not all fit on your scanners unless you use massive mirrors, which would make them a tad slow, and in short, it would be a ghastly mistake!

    Hope that makes some sense.

    Cheers

    PS. Cyl lenses against prisms controversy ....losses not different enough to be important, (prisms slightly worse), what is important is just that prisms are dead easy to set up but lenses do wierd things if you tilt them in ANY axis so they are a bit more difficult to get into the right place at the correct a angle in all directions. (just imagine the multi axis adjustor/translator I had to make, just to get a pratical idea of the error magnitudes involved). Most people rely on the physical edges of the lenses to be aligned with their various optical axis which works most of the time although many here choose to correct the "rotate" axis" and take care great that the lens is "facing" the beam as best they can.
    Last edited by catalanjo; 05-18-2016 at 17:55.

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