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  1. #1
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    Default Super cheap eBay mirror

    I was looking through the eBay store of the guy I bought my t-thread to 2" adapter from, and I saw these super cheap mirror sets. I was thinking of picking one up to experiment on, as I think I'm really close to figuring out how to pack 30 hours into a day.

    My idea was to get my hands dirty and try grinding a mirror. Once the existing damaged coating was stripped, I'd grind it down to f/4.5 or f/4, and make a rich field telescope. The only worry I have is with the included 1.1" (28mm) diagonal. According to the diagonal calculator on Mel Bartle's website Mel Bartels' Diagonal Calculator, with a 114mm primary, 448 focal length, and 28mm secondary even if I can get a low enough profile focuser to put the focal plane 70mm from the diagonal, I will still have a decent amount of light loss at the edges. At the focal plane, a 68 degree EP at the minimum magnification (that gives 7mm exit pupil, about 16x magnification) would have an image radius of 16.32mm. The calculator gave me the following values.

    Off-Axis Illum. Light Loss
    0.00 mm 100.0% 0.00 mag
    2.00 mm 100.0% 0.00 mag
    4.00 mm 100.0% 0.00 mag
    6.00 mm 100.0% 0.00 mag
    8.00 mm 93.05% 0.07 mag
    10.0 mm 82.28% 0.21 mag
    12.0 mm 70.45% 0.38 mag
    14.0 mm 58.41% 0.58 mag
    16.0 mm 46.59% 0.82 mag
    18.0 mm 35.29% 1.13 mag
    20.0 mm 24.80% 1.51 mag

    At the 16mm that would be the edge of field, illumination is down to 46.6%. They say 70% loss is fine for visual use, but I'm worried about losing almost a full magnitude at the edge of the FOV. How noticeable will this effect be, do you think? Granted if I just use a little more magnification (ie, a Hyperion 24mm to give 18.67 magnification and a 3.64 degree FOV), the edge of the maximum field is only 14.25, so I'd be closer to 0.6 mag loss at the edge of field.

    Anyway, sorry about the rambling post, I like to talk as I think. I'll place a bid, worst case is I never use it and I'm out 30 bucks or less with shipping. Seems worth it to get a great rich field scope for looking at the Milky Way from a dark sky site. what do you guys think, is there anything I'm missing here that would keep this from working?
    Zhumell Z10, Celestron AstroMaster 114EQ, 10x50 and 15x70 Binoculars
    Zhumell 2" UHC Filter
    Hyperion 8mm GSO Superview 30mm and a whole case full of Plossls

    Saskatoon, SK - 106W, 52N

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    Art Bianconi (01-13-2011)

  3. #2
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    Default Your "rambling": is fine with me!

    Dave I am not an optician, not even close. My strengths are in materials, structures and mechanical design. When optical gurus start talking about strehl, I think they are talking about Bavarian pastry!

    I've been asked to develop an all composite tube for a 11" f-4.5 mirror by Waite Research. The quest is to come up with an inside diameter that allows certain equipment to fit inside without vignetting the primary yet keep the secondary as small as possible.

    No need for you to apologise for thinking out loud. I learned something from it. Please keep it up.




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