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Thread: Question about telescopes / optics

  1. #1 Question about telescopes / optics 
    New Member
    Join Date
    Apr 2010
    I'm new to astronomy, just got an 8 inch dobsonian a few weeks ago and am looking to buy eyepieces for it. It's focal length is 1200, and the eyepiece is 25mm so I'm only at 48x right now.

    I was under the impression that the higher the magnification used, the less light one gets and this was apparently an inherent rule of optics, no way around it.

    But on the sky and telescope website, they have a calculator for scopes.

    What I noticed was the smaller eyepiece I chose, the higher the magnification got, as expected, but it also told me with a long eyepiece I would see around 12 magnitude and with short around 14mag. (It gave precise #s of course but I'm approximating for brevity).
    Assuming I'm not confused and 14 mag is indeed fainter than 12 mag, why would I be able to see fainter stars with higher magnification?

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  3. #2  
    Join Date
    May 2008
    As I see it, with more magnification, you are going to see smaller, so fainter stars. At 48x you are only going to see fairly large stars. At 120x you'll obviously see smaller stars. It is only when you look for very faint stars (below 14 magnitude) that you will find a balance between magnification and light.

    Be aware that a 25 mm eyepiece is not a set standard. I have a cheap one where the bit you look through is about half an inch and an expensive one where it is over an inch across, so lots less eye strain and a far wider field of view. This is especially noticeable with the smaller focal length eye pieces. A Barlow lens severely reduces light.

    A good pair of binoculars and sky chart (maybe one on your computer if outside your house) will help you find your way around. A torch should be red, even red cellophane over it, as the eye quickly recovers from red light and you don't lose your night sight. A telescope should be out for a little while before use so it is around the same temperature as outside and you don't get any heat distortion. You will also get heat from nearby buildings.

    Find out where the galaxies and clusters are. Also check out the Orion Nebula. Andromeda galaxy (which can be seen with the naked eye) is disappointing but you have to remember that professional photos have large telescopes, CCD's and long exposures so you will not see the same in your telescope, as even professionals do not see the same image live.

    Warning. Obviously never look at the Sun through your telescope but you'll also find a full Moon too bright too (as well as lacking in detail since it is the shadows that make things show up.) Careful when observing Mercury. Being closer to the Sun than us, it goes through phases like the Moon and Venus, but it is never far from the rising or setting Sun, which you do not want to accidentally look at.

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