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  1. #1
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    Default Photographic lens as guide scope?



    Is anyone else using a camera lens as a guidescope?

    I have a Lodestar guider and I am about to purchase a Canon 200mm F2.8L lens for use with my Astrotrac. I use my orion mini guide scope for drift aligning the Astrotrac - but I came across an adaptor to use c-mount cameras with Canon EF lenses. So if I were to buy that I am thinking I should be able to use the lens as the guidescope and thus cut down the weight of the kit further - by removing the mini guidescope and its mount. I am also thinking that I should see much faster drifts on PHD graphs with the 200mm lens with a 2x extender than I will with the Orion mini guidescope.

    I am also thinking the real bonus will come when framing the shot as the Lodestar is so super sensitive. All I will have to do is to find and centre the object in the lodestar and carefully swap it for the EOS camera.

    I do not autoguide my Astrotrac - I just use the guider for drift alignment with the PHD graphing method. The tracking performance of the Astrotrac is such that it does not require guiding at my image scales. 200mm max focal length with modded 450D is the longest combination which works out to 5.36 arcsec/pixel. Astrotrac is rated to 5 arcsec peak to peak.

    Any comments?
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    Default

    I must be misunderstanding you because it sounds like you want to replace the Orion miniguider with a Canon 200mm f/2.8 lens to 'reduce weight'. That to me sounds like adding a ton of weight. What am I missing?

    Allan

  3. #3
    bfonseka's Avatar
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    Default Re: Photographic lens as guide scope?

    The current method is to
    1.Install the Orion Mini Guider + Lodestar on ball head with quick release plate
    2.Do a drift alignment with PHD
    3.Remove Miniguider+ Lodestar
    4.Attach DSLR + Lens
    5. Point Camera to target and start imaging

    The changed method being considered is:
    1. Attach Canon 200mm F2.8L via triopd collar to ball head with Quick Release plate
    2. Attach Lodestar via EOS to C-mount adaptor and do PHD drift align
    3. Point lens and centre target with Lodestar, Lock ball mount
    4. Remove Lodestar and attach DSLR
    5. Proceed with imaging

    Now the weight advantage is that I don't have to take the mini guider travelling with me at all. The 200mm lens takes it place. The real bonus is in being able to use the Lodestar to find the targets and centre them. The current method is quite painful as I need to do star-hopping, then try an contort myself to look through the viewfinder. The Lodestar is so sensitive that I think I will be able to see all of the Messier Objects in 5 seconds or less at F2.8.
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  4. #4
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    Default Re: Photographic lens as guide scope?

    So, it sounds like you want to do away with the "Guidescope" and use the 200mm as a "Finderscope".
    I'm not too familiar with the Astrotrac, whether it can actually take a "Guidescope's" Autoguiding signals, or whether the Mini Guider was actually used solely as an "Electronic Imaging Finderscope" (taking images to confirm that you have found the target DSO - rather than actively evaluating pixel-location of Guide Star on Lodestar sensor and sending position correction signals to Astrotrac RA Controller).
    Function as a Guidescope is ongoing throughout the Imaging Session, and can't be done if the Lens is removed to swap from the "Guidescope" to the "Imaging Camera".
    I think we have a misunderstanding in terminology.
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  5. #5
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    Default Re: Photographic lens as guide scope?

    I don't see why it can't be done, it's been done for terestial use.

    http://www.dl-kipon.com/en/articledetail.asp?id=40

    There are pleny of vendors that sell the adapters, but you'll only be picking up about 50mm of focal length from the Orion Mini 50? And, you could probably barlow the Mini 50 to accomplish the same end result? Just a thought.

    Cheers

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  6. #6
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    Default Re: Photographic lens as guide scope?

    I just ordered the lens and the adaptor today - should have them next week. The adaptor I ordered is just a physical mount adaptor and does not contain any optics. This is because the EF lenses have a flange back of 44mm and the c-mount is a flange back of 17.52mm This leaves a lot of room for the mount adaptor. Since it contains no optical elements, I will have the full 200mm focal length of the Canon lens. Indeed I was thinking of adding either a 1.4x or 2x extender (already own both) to the lens see the drift much more quickly or to fine tune the drift if needed. 200mm is more than the focal length of the mini guide scope (160mm) and F2.8 makes much it faster as well.
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