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Thread: Check Quality of Optics

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    Default Check Quality of Optics



    Hi all,

    I'm looking to buy my first telescope, but living in Thailand means I don't have a great many options.

    One scope I have my eye on is an unbranded 150x750 reflector for about $300 (I'd post a link to it but I'm too new to the forum. However, if you go to lazada.co.th and search "telescope 750x150" there are four results all for the same scope). If I buy it and it turns out to be poor quality I can return it for a refund, but the problem is I don't know how to test the optics to tell if they're any good. What should I be looking out for?

    Any words of wisdom would be greatly appreciated!

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    Default Re: Check Quality of Optics

    Oops, apologies for the double post! I thought my first one didn't get submitted properly (dodgy internet) - didn't realise they have to be approved by a moderator first.

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    Default Re: Check Quality of Optics

    There is more to it than just the quality of the optics. The focal length and aperture are small so the view of the planets will not be anywhere near like what they show in the pictures. The magnification will be limited to 150 and that only under very good and rare circumstances. The mount looks underwhelming so the views will be very shaky. It has no tracking motor so you can't use a camera to take images except perhaps the moon. The optics are probably fine just watch out that it is not a bird-jones type that has a spherical mirror instead of parabolic, and a lens under the focuser (it does not look like one to me but see if you can check if the mirror is parabolic). It is a starter scope so do not expext too much of it. I don't see any red flags though, other than what I described.
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    Default Re: Check Quality of Optics

    CamelHat, thanks a lot for that. I thought the aperture was quite wide at 150mm, and that it would give me a max practical magnification of around 300x... It's very possible I'm missing something though - am I on the wrong track?

    I did wonder whether it's a Bird-Jones - the listing says the barrel is 645mm, so a bit shorter than the 750mm focal length. Hard to tell though, so I'll ask the seller.

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    Default Re: Check Quality of Optics

    I doubt it is a Bird-Jones, but it might be.

    The 300x is a "theoretical" maximum magnification and is unrealistic. Your local seeing conditions likely will limit magnifications to less than that anyway on most occasions. Don't get hung up on achieving high magnifications, you'll soon find that the vast majority of your observing is done at lower magnifications anyway.

    If you don't have a pair, I HIGHLY SUGGEST getting a pair of binoculars. Give this thread a look. https://theskysearchers.com/viewtopic.php?f=4&t=331

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    Default Re: Check Quality of Optics

    Quote Originally Posted by RunnyYolk View Post
    CamelHat, thanks a lot for that. I thought the aperture was quite wide at 150mm, and that it would give me a max practical magnification of around 300x... It's very possible I'm missing something though - am I on the wrong track?
    The practical limit is based on aperture because of the Rayleigh criterion. This would be about 0.5 to 1 times the aperture in mm. Then there's the atmosphere, which further deteriorates things, most of the time. To see the images in the ad you would need a C11 on a solid mount like a CGX. Your scope is not good for planets, if you see a few blurry stripes on Jupiter you wil be lucky. It is more aimed at wide field. For that, 150 mm is fine, Newts give you the most bang for your money.
    ... Henk. Telescopes: 6" Mak-Newt (Comet Hunter), ES ED127CF, ES ED80, Zhumell Z12, Coulter Odyssey 10, AT6RC, Venture RX-7, Celestron Skymaster 20x80, Mounts and tripod: Losmandy G11S, AVX, LXD55, Tiltall, Cameras: Fuji X-a1, Canon SX40, Xt, XSi, T6, ELPH 100HS, DIY: Dob and camera barndoor trackers, afocal adapter, Dob with foldable base and Az/Alt setting circles, Accessories: SSAG, Plossls, Barlows, Telrad, laser collimators (Seben LK1, Z12, Howie Glatter), Cheshire, 2 Orion RACIs 8x50, Software: DSS, ImageMagick, PHD, Nebulosity, Photo Gallery, Gimp, CHDK

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