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Thread: Prisms vs cyclindrical optics?

  1. #1
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    Default Prisms vs cyclindrical optics?

    Ok, so I'm thinking about my next blue and red modules (and maybe even green diodes come to think of it)
    I don't follow the in depth discussions on optics as much as I should but I've always gleaned that people tend to use anamorphic prisms for 445, and cyclinder pairs for red.

    But why?

    Would appropriately coated prisms also work for red, and do it in a shorter space too? What are the down sides?

    I read a bit of post by Planters who talked about available space being a deciding factor, but assuming you're using PBS cubed pairs of each colour, nice and tight arrangement, what is best to stick directly after the cube in each case, and why?

    I like the ease of use of prisms, and the compact dimensions, so why do most people use the cylinders?

    I'm happy with ~4mm*4mm diameters and <1.5mrad
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    You are using Bonetti's defense against me, ah?

    I thought it fitting, considering the rocky terrain.

  2. #2
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    I check for blue and mitsu prism and cylinder from Dave, in my opinion cylider give better result.

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  3. #3
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    Quote Originally Posted by norty303 View Post
    people tend to use anamorphic prisms for 445, and cyclinder pairs for red.

    But why?
    Probably as the G71/P73 's need a bit more magnification than the 445nm diodes to get things into check, and prisms suffer with increased reflection and higher power losses as the angles are increased to get that magnification, so cylinders were a better choice for those diodes. Either work well on the blues, each with merits and downsides.
    A little bit werrrr, a little bit weyyyyyy, a little bit arrrrgggghhh

  4. #4
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    If you are limited to PBSed pairs of diodes then stick with cylinder pairs. They have lower loss even at magnifications as low as 2.5 to 1. The big advantage with prisms is they can be used with the off axis diode input that occurs with quads and greater while here the lens pair introduces off axis distortions.

  5. #5
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    If you are limited to PBSed pairs of diodes then stick with cylinder pairs.
    LOL, I'm not sure 'limited' is the right term, it is more choice. ~3W 445 and ~1.3W 638 is about exactly what I need. I have no need for something bigger.
    To my mind, a PBS pair is a better solution than a knife edge pair - would you agree?

    Thanks for the replies, thats summed it up nicely for me.

    Can anyone remember the thread (link please?) which talks about the setup of the cylinder pairs, optimum distances, etc?
    I want to do some catching up.
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    You are using Bonetti's defense against me, ah?

    I thought it fitting, considering the rocky terrain.

  6. #6
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    I have found with the 71 (haven't tried with 73) the polarization is oriented the wrong way to use a prism for expanding the FA. You end up with huge losses over 50%. They do work for compressing the SA with minimal losses.

    Can anyone remember the thread (link please?) which talks about the setup of the cylinder pairs, optimum distances, etc?
    I want to do some catching up.
    I don't remember which thread but it can be summed up pretty easily. Some of the threads are needlessly long. Hopefully this can save you some reading.

    For a telescope with a negative and positive lens the distance between the elements is the positive - the negative. Example 100mm fl + and a -20mm negative should be spaced 80mm apart.

    The the magnification is the ratio of the two. Example 100mm/20mm = 5X.

    Because the FA is expanding so fast after the collimator lens the distance to the first element affects the magnification amount of the system. The further away the greater the telescope will magnify. Basically, the wider the beam when it hits the first telescope element the smaller the final spot size and divergence. This also effects the size of the beam at the scanner mirror inversely.

    I have most collimator and diode divergence info written down. So, it possible to come up with a close "guess" as to what mag telescope to use with the collimator you select for a given divergence you require.

    hope this helps

    -woody

  7. #7
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    TBH, I might just end up sending you another email Woody, and be done with it all
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    http://www.frikkinlazors.co.uk

    You are using Bonetti's defense against me, ah?

    I thought it fitting, considering the rocky terrain.

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