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  1. #1
    isherwoodc's Avatar
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    Default Totally new to Eyepieces



    Hey guys, I recently bought a sky-watcher 200P 8" Dobsonian, and the lenses it came with are a 10mm and a 25mm wide angle?

    I have tried them both out, and noticed that the 10mm has better zoom on jupiter for example.

    I am very new to telescopes and i'm trying to look around at which lenses I should be looking at, but it seems to differ for each telescope.

    I am probably going to get a barlow lense, but other than that I simply have no idea.

    I am embarassed to say but I don't even know what the difference would be between a 10mm and a 25mm lense. The only thing I noticed when I tried mine out was that the 25mm wasn't as strong (I could see the entire moon in 1 view), where as the 10mm was much stronger and jupiter looked great.

    To be honest, i'd love to make jupiter look even bigger, or even the moon bigger (also looking at a moon filter)

    Please can you shed some light on this rather grey area in my knowledge


  2. #2
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    Default

    Isherwood:

    Sounds like you have figured out the basics. The lower mm's the more the zoomed in the eyepiece will be. And vice versa. The 25mm will show you more objects visually while the 10mm will focus and give you more detail on a certain object.

    I would suggest a moon filter for looking at the moon. That was one of the first accessories I purchased and I have never regretted. I also got a 2x Barlow and it was well worth the money.

    A Barlow lense will essentially double the magnification of your eye piece. Example...If you place the barlow with the 10mm, your new magnification would be 5mm. Using the Barlow with the 25mm would give you a 12.5mm. An investment of a Barlow would essentially double your eye piece collection because every eye piece yould serve as 2 pieces.

    I would try the Barlow and a moon filter and see where that take you. Using a Barlow with your 10mm should make Jupiter look bigger but it also means that Jupiter will move quicker out of the field of view.

    Hope this helps
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  4. #3
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    Default

    Hi there

    The different eyepieces are what give you your different magnifications.

    edited to add:
    LOL - got in there before me Dutchman

    apologies for duplicated info....

    If you divide the focal lenth of the telescope (1200mm for the Skywatcher 8" Dob) by the size (in mm) of the eyepiece - you get your magnification.
    So, the 25mm is giving you 1200/25 = 48x magnification
    The 10mm gives you 1200/10 = 120x magnification

    An 8" scope is theoretically capable of around 400x, but unfortunately atmospheric steadiness (known as the 'seeing') in UK rarely allows us to get a steady, sharp image at much above 200x

    You could get a 2x barlow lens - this fits between the eyepiece and the telescope focuser, and doubles the magnification, which in turn, effectively doubles the amount of magnifications you can get from your eyepiece.
    The 25 with a 2x barlow would give you 96x - the 10x plus barlow would give 240x


    Most beginners - start off by going for a high-magnification eyepiece as their first extra purchase - something like a 4mm or 6mm (I did it myself)
    Personally I'd advise against this - the atmosphere rarely allows us to use very high magnifications - so we get disappointed with the view through the new eyepiece, and it sits in the eyepiece case, hardly ever getting used.

    If I was starting afresh - my first purchase (after the scope package) would be a good barlow.
    (always buy good eyepieces/barlows - Scopes will change over the years, but you'll take extra eyepieces from scope-to-scope)
    Next would be a good-quality mid-range eyepiece (somewhere around 12mm to 15mm) as a 'workhorse'
    Then would come a wide-angle low power 'sky-sweeper' eyepiece of 30mm or more.

    THEN I'd go for a nice sharp high power 'planetary' eyepiece of 7mm or less.

    Of my personal eyepiece collection (about 30 of 'em) - the ones I use MOST are....
    40mm wide-angle 'Erfle'
    22mm Wide-angle (17mm Baader Hyperion with the nosepiece removed)
    13mm Baader Hyperion
    7mm 'Orthoscopic' planetary eyepiece
    And a 2x Orion 'shorty' barlow
    Rarely use anything else
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  6. #4
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    Default

    isherwoodc-

    First what is your "f" rating of your scope. Reason I ask, some EP's work well in like "f-10" scopes but not well in faster "f" scopes. The lower the "f" value the faster the scope.
    That being said your 10mm is a high power EP as compared to the 25mm EP. Usually a lower power with give better clarity because it will gather more light in the EP whereas the opposite for lower power EP's.

    I would suggest asking someone locally if you have a local astronomy club with a scope similar to yours.

    But remember there is a calculation table you can use to determine how much high power of an EP you can use in your scope. Most scopes are rated to their highest useful magnification by the manufacturer. But keep in mind it is usually a generous rating that in theory would work on the most ideal of nights and seeing.

    The formula once you have the highest useful mag. rating is simply:

    Magnification= Focal length of telescope in (mm) / focal length of EP in (mm)

    Then you can calculate how high you can go safely. This way you don't for instance buy a 2.5mm EP thinking you will get awesome close views and then find out your scope can't handle that magnification power of the EP.

    Hope this helps.
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  8. #5
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    Default

    Thanks a lot guys, I think my next purchase will be a 2 x Barlow and a moon filter and then I will progress from there if I need to
    Chris

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  9. #6
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    Quote Originally Posted by isherwoodc View Post
    Thanks a lot guys, I think my next purchase will be a 2 x Barlow and a moon filter and then I will progress from there if I need to
    That is a good decision, imnsho. As far as eyepieces go, you might consider a zoom. There is a pretty thorough discussion in the stickies at the top of the Eyepiece forum.
    Last edited by WWPierre; 09-13-2009 at 05:45 PM.
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  10. #7
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    Quote Originally Posted by WWPierre View Post
    That is a good decision, imnsho. As far as eyepieces go, you might consider a zoom. There is a pretty thorough discussion in the stickies at the top of the Eyepiece forum.
    if that in your not so honest opinion, does that mean your saying its not a good decision?
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    Quote Originally Posted by isherwoodc View Post
    if that in your not so honest opinion, does that mean your saying its not a good decision?
    Lol, it is my not so HUMBLE opinion.
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    Quote Originally Posted by isherwoodc View Post
    Thanks a lot guys, I think my next purchase will be a 2 x Barlow and a moon filter and then I will progress from there if I need to
    This is a great decision Isherwoods ...Just make sure you buy the best quality barlow you can afferd ...Possibly an ED glass barlow ...And fully multicoated ...The ED glass is great quality glass and will allow more light to pass through the barlow ...The multicoatings also allow more light to pass ...When ever you add more glass to the optical train (where you see)It makes the image dimmer because the light from the image has to pass through so much glass ...By using good quality glass it helps the light to get through ...Every little bit helps ...

    On the moon filter thing , Look at the variable polarizer filters ...They darken the image and show more contrast between the light and dark areas .. Like shadows in the craters ...It is pretty cool indeed ...Plus the brightness is adjustable from 1% to 40% .....
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  14. #10
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    roverich is right on the variable polarizing filter suggestion.
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