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    Default Maximum usable magnification on a celestron 12



    A couple of members have suggested in order to get a larger view of the planets I should get a 5 6 or 7mm EP. My question is at a 480x maximum usablable magnification wouldn't anything below 12mm be useless?

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    Roughly 300mm objective means that you will seldom be able to use more than 300x. But under exceptional seeing you might use as much as 400x.

    Technically you could use up to 600x without the image grossly deteriorating but due to atmospheric issues it is really almost impossible to exceed 350-400x with fairly normal amateur equipment. Some good people with really big instruments and in really excellent seeing report being able to use up to 1,000x but some think they are delusional.

    Upshot, though, is that with a 12 inch Dob you will almost never be able to exceed 300x and it is exceedingly unlikely that you will ever benefit from more than 400x.

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    the F ratio of the scope is a good starting point, if the scope is a F5 it can use down to a 5mm eyepiece on most days and barlowed on those far and few perfect nights.

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    Quote Originally Posted by invision View Post
    the F ratio of the scope is a good starting point, if the scope is a F5 it can use down to a 5mm eyepiece on most days and barlowed on those far and few perfect nights.
    I may not be correct but most mass produced Dobs are 1200 mm scopes and a 5 mm eyepiece would give you 240x and I agree that you should be able to use a 5 mm eyepiece on a normal night BUT to use a 4 mm eyepiece would require a night with darn good above average seeing....

    scopes f ratio really is not a good indication of what eyepiece one can use in my opinion.

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    I believe most Synta and GSO 12 inch Dobs are running a 1500mm focal length. Pretty sure the OP has a Celestron Star Hopper which definitely has a 1500mm focal length.

    So the 5mm eyepiece yields 300x.

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    Quote Originally Posted by Bob327 View Post

    scopes f ratio really is not a good indication of what eyepiece one can use in my opinion.

    Bob G.
    It's actually a neat calculation shortcut - using an eyepiece with focal length in mm equal to the scopes f-ratio will give you a magnification equal to the scopes aperture in mm, which is one of the common recommendations for highest normally usable magnification.
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    But, Isn't that the "bottom line" and the "name of the game"? Everyday and every place is different, so the challange is to reach into your "EP bag of tricks" and see "how great you can make it"? Then if your really Lucky (or good), take a picture to share it.
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