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Thread: I read that the shorter collapsible setting is for binocular-viewer lenses...?

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    Default I read that the shorter collapsible setting is for binocular-viewer lenses...?



    So - I read somewhere on the SkyWatcher website that the mysterious short setting on the collapsible truss is for binocular viewers! Now we know!

    Okay - you probably already knew that. It was a surprise to me.

    Can anyone explain why bino-viewers need a shorter focal length?

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    Default Re: I read that the shorter collapsible setting is for binocular-viewer lenses...?

    To use binoviewers with a scope you normally either have to shorten the optical path a bit or lengthen it. That is why most people use a 2x barlow with them unless the scope has been built as binoviewer ready.
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    Default Re: I read that the shorter collapsible setting is for binocular-viewer lenses...?

    Okay - I'm thinking about why this might be...
    Is this right: bino-viewers add length by mirrors and such, so you have to compensate elsewhere?

    I've been thinking about adding these to my dream-list, but not if they are a major pain. The mid-point on the collapsible truss seems like a neat solution.

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    Default Re: I read that the shorter collapsible setting is for binocular-viewer lenses...?

    Quote Originally Posted by moffjarjar View Post
    Is this right: bino-viewers add length by mirrors and such, so you have to compensate elsewhere?
    Yes, exactly. The focal length is what it is and you can't change it. And the eyepiece needs to be at the focal point. So, if you add tuvbe somewhere, you have to remove it somewhere else.
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    Default Re: I read that the shorter collapsible setting is for binocular-viewer lenses...?

    Surely the best option though is to use a Barlow or Focal extender as you will still use the entire light cone. Surely if you use the SW collapsible at the lower setting you are going to be cutting a reasonably large portion of light which will fall outside the secondary. Seems a bit of a daft idea to me... still it's cheaper I guess but if I bought a 12" scope I'd rather use every one of those inches...
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    Default Re: I read that the shorter collapsible setting is for binocular-viewer lenses...?

    If you compensate for the the required length of the bino viewer by moving the secondary assembly closer to the primary, you will effectively reduce the aperture of the scope. This is because the secondary is designed to reflect the whole primary light cone at a given distance from the primary. By moving the secondary toward the primary much of the light cone will miss the secondary. With bino viewers, which split the light cone to begin with, a dimmer image will be perceived due to the reduced amount of light reflected by the secondary.

    A better way to use bino viewers is with a good OCA (Optical Corrector Adapter) that doesn't add much extra magnification. The Siebert Optics 1.25X OCA for Dobsonians would be a good choice. This would keep the scope at it's native focal length with no reduction in the light cone...important for bino viewers.

    Siebert Optics OCA's

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    Default Re: I read that the shorter collapsible setting is for binocular-viewer lenses...?

    Quote Originally Posted by Davesellars View Post
    Surely if you use the SW collapsible at the lower setting you are going to be cutting a reasonably large portion of light which will fall outside the secondary.
    That assumes that the secondary is sized for the longer position. It could be sized for the shorter position. (In which case, it would be reducing the light in the longer position, I know.) Or maybe it is sized for the average of the two.
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    Default Re: I read that the shorter collapsible setting is for binocular-viewer lenses...?

    Another reason that Sky Watcher may have added the mid position for Bino Viewers is balance. Bino viewers and the extra eye piece + barlow can add some serious weight to the upper truss assembly.

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    Default Re: I read that the shorter collapsible setting is for binocular-viewer lenses...?

    The light cone just fits in the secondary in my SW dob in the long position... there's not that much wiggle room so I assume light would definitely be cut in the short position.

    IMG_0403.JPG

    This is with a collimation cap and the focuser racked out to pretty much at the focal plane.
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    Default Re: I read that the shorter collapsible setting is for binocular-viewer lenses...?

    Quote Originally Posted by Davesellars View Post
    The light cone just fits in the secondary in my SW dob in the long position... there's not that much wiggle room so I assume light would definitely be cut in the short position.

    IMG_0403.JPG

    This is with a collimation cap and the focuser racked out to pretty much at the focal plane.
    It is sized perfectly for it's native focal length.....Like you said "Not much (any) wiggle room"!
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