View Poll Results: Max useful magnification?

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24. You may not vote on this poll
  • 20x per inch of aperture

    0 0%
  • 25x per inch of aperture (~aperture in mm)

    5 20.83%
  • 30x per inch of aperture

    6 25.00%
  • 35x per inch of aperture

    3 12.50%
  • 40x per inch of aperture

    6 25.00%
  • 50x per inch of aperture

    3 12.50%
  • 60x per inch of aperture

    1 4.17%
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  1. #1
    Diagoras's Avatar
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    Default Opinions regarding maximum useful magnification



    Ok, so I've been reading various recommendations for the maximum "useful" magnification for a given scope aperture, and even here on this forum there seems to be some variance of opinion regarding this issue. By "useful" I mean the maximum magnification level beyond which any additional magnification does not add any more detail (for planetary viewing, for example).

    So, I decided to take a poll on the subject, just to see where the posters here stand. Given that most of us do not have perfect terrestrial seeing conditions, what do you think is the optimal maximum magnification as a function of aperture on nights of good seeing for planetary viewing?
    Celestron NexStar 6SE
    Orion Apex 127mm

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  2. #2
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    Default

    Most manufacturers (Meade and Celestron use 60x per inch and we all know that would require us to be observing on the surface of the moon..

    40x is a much more reliable "guess on my part "...on a normal night anyway...My CPC1100 is rated at 660x.. its a rare night I can push it up past 300x ....

    EXCEPT ON THE MOON..which is big and bright and very close to the earth it takes magnification very well..

    Bob G
    CPC1100 housed in a slotted domed observatory (Exploradome) 4 and 5 inch refractors for use from the lawn, a 8" Sct (NS 8i) for star parties...
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  3. #3
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    Default

    If you include fine collimation than 100X per inch of aperture is both appropriate and useful. Here you are actually trying to look at diffraction paterns (not the objects themselves).

    But realisticaly:: The atmosphere almost never allows more than 300X and basically never allows more than 600X (well, maybe twice a decade.)

    Some people can see all that there is to see with an exit pupil of 2mm while others require an exit pupil of 1mm to get it all. 1mm exit pupil is equivalent to a magnification of 25X/inch(aperture). Beyond 25X you are making the image bigger, but rarely are you making it better.

    So the best answer to your question is::

    MaxMag = collimation ? 100X/aperture : MIN( 300X, 25X/aperture) ;

  4. #4
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    Default

    The scope limit is due to diffraction. It begins at the magnification equal to your aperture. An 8 inch scope is 200 mm. The diffraction limit is 200x. That is also 25 times the aperture in inches because there are 25 mm per inch.

    The next limit is the eye resoluion. It can see one arc minute. So magnification larger than the diffraction limit helps the eye to pick up detail available. At some point there is no more detail available and it just gets larger.

    Countering these limits are the eye pupil size, the weather conditions, scope collimation and a few other minor effects.

  5. #5
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    Default

    With my 12 inch Dob I can usually use 300X and sometimes I get a night of exceptional seeing when I can use 428X, so I voted for 35X.
    Name: Sam
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  6. #6
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    Default

    Quote Originally Posted by Diagoras View Post
    By "useful" I mean the maximum magnification level beyond which any additional magnification does not add any more detail (for planetary viewing, for example).
    What you have defined is the diffraction limit. The magnification at the diffraction limit is equal to the aperture in millimetres.

    Manufacturers like to double that, because, in excellent conditions, it can be useful to go beyond the diffraction limit. You won't see more detail, because there isn't any more to see, but the detail that is there is easier to see if it is bigger.

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    pro; C-90 on wedge; 20x80 binos; Etc: Canon 350D; Various EPs, etc. Obs: 8' Exploradome;
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  8. #7
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    Default

    Whatever it is ----- ? 675 power is usually impossible in a 60mm refractor from a department store.
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  9. #8
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    Default

    Quote Originally Posted by KeithBC View Post
    What you have defined is the diffraction limit. The magnification at the diffraction limit is equal to the aperture in millimetres.

    Manufacturers like to double that, because, in excellent conditions, it can be useful to go beyond the diffraction limit. You won't see more detail, because there isn't any more to see, but the detail that is there is easier to see if it is bigger.
    Yeah, that's what I was trying to get at. Assuming a good scope, with a good eyepiece, good seeing conditions, good collimination, etc. (but not necessarily perfect in any of those areas)...

    In other words, how much (if any) beyond the diffraction limit do most people think is feasible under good conditions.....
    Celestron NexStar 6SE
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  10. #9
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    Default

    Quote Originally Posted by Diagoras View Post
    In other words, how much (if any) beyond the diffraction limit do most people think is feasible under good conditions.....
    Double the diffraction limit, or 400x, whichever comes first.

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  11. #10
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    Default

    Well, I haven't voted on the poll myself, since I am primarily interested in what others think on the subject...

    ...but on the nights I've had with the steadiest seeing, going over 200x (between 30x and 35x per inch with my 6SE) seems to really be pushing it. Having colliminated my scope to the best of my ability, using a 7mm EP (~214x magnification with my scope) seems like it is just too much. The 8mm (~187x) seems a better fit for me under the best seeing conditions that I've had, and even that might be just a tad too much for most nights. This seems to be in-line with most of the responses in the poll (between 25x and 30x per inch).

    I've been thinking of going ahead and getting a 10mm EP in order to get magnification right at the diffraction limit of my scope (150x), and thus hopefully get the best possible planetary views. Then, I would only use the 8mm when the skies are exceptionally steady.
    Celestron NexStar 6SE
    Orion Apex 127mm

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