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  1. #1
    Dan O's Avatar
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    Default Binoculars and apparent magnitude?



    I have recently bought binoculars for observing the night sky. It says 8x21, Field 7.3°, and 128m/1000m
    Can somebody tell me if it is a good one and up to what magnitude can it see. I know some binoculars can see up to a magnitude of 9, but i want to know exactly how much magnitude can mine see.
    Thanks for answering

  2. #2
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    Normally-all viewing conditions being ideal-8 power can bring in magnitude 10. However the size of the objective lens makes a big difference.
    8x35 or 8x50 will give you a better chance of seeing lower magnitude objects.

  3. #3
    Philip J's Avatar
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    According to Wikipedia, a normal human eye pupil has a diameter of 3 to 4 mm. Let's say your fully dilated pupil diameter is 3.5 mm. That's 1/6 of the 21 mm diamter of the binoculars. So the binoculars gather 36 times as much light as you naked eyes; that makes stars appear 36 times brighter. A long time ago, some idiot astronomer decided that 5 orders of magnitude should equal a brightness ratio of 100. So one order of magnitued is a factor of 100^(1/5) = 2.512. If 2.512^x = 36, then x = log(36) / log(2.512) = 3.89. Therefore, your binoculars will increase the brightness of stars by 3.89 orders of magnitude. With normal vision and a very clear dark sky, you can see magnitude 6; with your binoculars you might see magnitude 9.9. That's how you solve the problem; to get the right answer, measure you fully dilated pupil and redo the math.

    Little binoculars are great for daylight, but for astronomy, you should get bigger ones.

  4. #4
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    Quote Originally Posted by Philip J View Post
    With normal vision and a very clear dark sky, you can see magnitude 6; with your binoculars you might see magnitude 9.9. That's how you solve the problem; to get the right answer, measure you fully dilated pupil and redo the math.
    Yeah, that's a bit of overestimation there, as mag. 6 is probably only visible to the naked eye when it's fully dark-adapted, i.e. when it has an aperture of around 6mm (or more). So that would be 21/6 = 3.5; 3.5^2 = 12.25; 2.512*log(12.25) = 2.73, so those small binoculars would only get you to see stars up to a magnitude of 6 + 2.73 = 8.73, not 9.9.

    So yes, it turns out they're about as powerful as Dan suspected "some binoculars" are, i.e. mag. ~9.

  5. #5
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    Quote Originally Posted by Philip J View Post
    According to Wikipedia, a normal human eye pupil has a diameter of 3 to 4 mm. Let's say your fully dilated pupil diameter is 3.5 mm.
    That 3 to 4 mm is during the day though.
    At night, pupils will dilate to around 7mm (that's why binoculars very rarely have an exit-pupil diameter of more than 7mm)
    Binocular exit pupil is calculated by dividing aperture by magnification.
    So a 10x50 will have a 5mm exit pupil - 7x50 has a 7mm exit pupil - 15x70 has a 4.7mm exit pupil, and so on...............

    As we get older though - maximum pupil dilation does suffer - and people beyond 50 or so years old, commonly have a max pupil diameter of around 4.5 to 5 mm

    For astronomy, you really want binoculars with an exit pupil of 4mm or bigger.
    Binocs with a smaller exit-pupil = dimmer images, especially at night.


    I'm very lucky
    I have an eye condition (since birth) which includes thin, weak irises.
    My max dilated pupil is 8.5mm
    It's a big advantage at night, but the downside is that they don't close down like they should in bright light, and I need to use sunglasses before most other people.
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  6. #6
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    Quote Originally Posted by Carlos_dfc View Post
    For astronomy, you really want binoculars with an exit pupil of 4mm or bigger.
    Binocs with a smaller exit-pupil = dimmer images, especially at night.
    It depends on how dim and what you want to look at exactly. Within reasonable limits, a smaller exit pupil can also mean a smaller region of Space is being zoomed into and seen in better apparent contrast.

    See here: garyseronik.com/?q=node/13
    (Has comparative pictures.)

  7. #7
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    Hi there, I am afraid that your 8x21 binoculars won't be much good for the night sky. They may fit in your pocket nicely but you need bigger lenses. You will get some pleasing views but to really benefit night sky viewing you need something like 7x50's if they are to be handheld. This is about the sensible limit without using a tripod to steady the image. Night viewing is all about light gathering to view fainter objects. For instance a 50mm lens will gather 5.7 times as much light as a 21mm lens. (50 squared divided by 21 squared). This is nearly 2 whole star magnitudes brighter. I would suggest buying something like Oberwerk or Strathspey glasses for good value. Best regards bristars

 

 

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