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Thread: 20mm or 6mm eyepiece?

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    Default 20mm or 6mm eyepiece?



    Which is better for viewing the moon?
    20mm or 6mm eyepiece?

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    Default Re: 20mm or 6mm eyepiece?

    Quote Originally Posted by galileoDSLR View Post
    Which is better for viewing the moon?
    20mm or 6mm eyepiece?
    What telescope is it being used in? Both work good on the moon but the 20mm will be a wider field of view and will encompass more of the moon than the very high power 6mm regardless of the scope. In all of my scopes if I had to use those two I would always start with the 20mm and if I wanted a tighter closer view of a certain feature on the moon the 6mm would get placed in the focuser. Hope this helps.
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    Default Re: 20mm or 6mm eyepiece?

    You have enough posts now you should make a signature of your gear so we can answer these questions better/easier.
    A 20mm eyepiece is a mid power eyepiece where a 6mm is a high powered eyepiece so it would depend on the view wanted and the telescope used.
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    Default Re: 20mm or 6mm eyepiece?

    Definitely need to know the scope in use. In my C11 20mm is a fair moon magnification...in my ST80 the moon is very a distant orb. Likewise the 6mm in my C11 is screaming magnification...in the ST80 if kinda fills the EP .
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    Default Re: 20mm or 6mm eyepiece?

    Thanks for the suggestion Gabby. I added my gear to my signature like you suggested. Good idea!
    Quote Originally Posted by dagadget View Post
    What telescope is it being used in? Both work good on the moon but the 20mm will be a wider field of view and will encompass more of the moon than the very high power 6mm regardless of the scope. In all of my scopes if I had to use those two I would always start with the 20mm and if I wanted a tighter closer view of a certain feature on the moon the 6mm would get placed in the focuser. Hope this helps.

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    Default Re: 20mm or 6mm eyepiece?

    Have you used this eyepiece calculator before? N.A.A. Telescope Math Calculator
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    Default Re: 20mm or 6mm eyepiece?

    I checked out that link.

    I didn't really know what it all meant though.

    Oh well.....that's cool.
    Thanks anyway...

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    Default Re: 20mm or 6mm eyepiece?

    On average night 20 mm will deliver better view of the moon in your telescope. If the seeing is very good (very steady atmospheric conditions) you may try 6 mm for high power close up views.

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    Default Re: 20mm or 6mm eyepiece?

    I would suggest if you can swing it... add a 10-12mm eyepiece. I think that is the sweet spot for moon viewing to get good detail in average conditions for you. An alternative of course would be a 2x adapter to your 20mm eyepiece.
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    Default Re: 20mm or 6mm eyepiece?

    A 6mm eyepiece is pushing it a bit with your scope. It will give you a magnification of 116x, and you may find that it is not super sharp at that power in a typical 76mm reflector telescope. A 10mm eyepiece is a good suggestion: this will give you 70x, which is nice for close-up views of the moon, and an exit pupil of around 1mm, which is about right for high-power viewing. (The exit pupil is the diameter of the light "cone" when it hits your eye.)

    I'm not sure what you don't understand about the link that Gabby gave you. I know there may be a lot of new terms there, so I will walk you through it:

    If you enter "76" into the second green rectangle, hit "Enter diameter", this will fill all the boxes (mainly with zeros!). Then just move down to the Focal Length box, put "700" in and hit "Change focal length". You then have all the stats for their default eyepiece, which is a 25mm one with a 55 degree "apparent field". For your eyepieces you just enter "20" into the Eyepiece Focal length field, and hit "Change fl", and "50" (I am assuming you have a typical "plossl"-type eyepiece, with a 50-degree apparent field) into the "Eyepiece Apparent Field (degrees)", and "Change AF".
    (Note: the "Apparent field" is the width of the "light cone" that hits your eye when you look through the eyepiece, or if you like the width of the image that you are looking at. The "True field" is the actual width of the image you are seeing, taking into account the magnification that you are using. Thus the bigger the "apparent field", the bigger the "true field"; and usually the more expensive the eyepiece!)

    This will give the following information:

    Resulting magnification: 35
    Power per inch of aperture: 12 (this just means that for your 76mm (or 3 inch) diameter telescope you are getting the equivalent of 12 magnifications per inch, which is confusing when you are talking mm! However, is is a measure of what the scope is capable of. It is generally accepted that an excellent telescope will give a maximum magnification of 50x per inch and still be sharp, on a very good viewing night. For your 6mm eyepiece, this figure will read "39"; and this is pretty high for an average 76mm telescope.)
    Exit pupil (mm): 2.17
    True Field (degrees): 1.429
    True Field (arcminutes): 85.7
    Field transit time at 0°declination (sec.): 343 (this is the time the object will stay in the field of view without moving the telescope)

    If you then do exactly the same thing, but enter "6" into the Eyepiece Focal length field, you get the specs for your 6mm eyepiece. You will notice at the Exit pupil box it will say: "<1mm, image may be dimmed". This is because it is such a small amount of light going into your eye.

    I hope that all makes sense.

    Good luck and all the best,

    - Dean
    Last edited by DeanD; 12-19-2017 at 04:30 AM.
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