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  1. #1
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    Default Moon filter versus hole/port in lens cap



    I recently had a chance to look at the moon both thru a variable moon filter and a telescope with a round hole cut in the lens cap.

    I am not experienced enough to discern the pros and cons of each method of cutting down the brightness. Obviously the moon port method is the cheapest.

    Can anyone enlighten me?

    (In keeping with most of the other posts in this forum I guess I should ask which method will let me see the flying flag that we didn't leave there.)

  2. #2
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    Default

    Hi TM
    The main difference will be the resolution available - and hence, ability to see fine detail.

    Resolution (in arcseconds) and the ability to get a sharp image at high powers - is a direct function of the aperture of the scope.
    Larger scopes can discern finer detail, and will allow you to get a sharp image at higher powers (assuming the atmosphere is steady enough to actually use those higher powers)

    Using the 'hole' in the protective cap (known as an 'aperture mask') is an effective way to dim the image - but will also limit your usable magnification. You are basically cutting down your scope's aperture, to the size of the hole in the mask.
    Using the old 2x per mm rule - a 40mm 'hole' will only allow properly sharp images up to 80x.

    This may well be enough - but if you decide to do the 'Lunar100' observing list, some features do need significantly higher magnification - and will benefit greatly from using the full aperture of the scope.

    Personally - I prefer a 'Variable Polarising Filter' over a Moon Filter or aperture mask.
    With a Moon filter you are limited to only one degree of dimming - With a VP filter you can vary it infinitely, to suit the conditions/brightness.
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  4. #3
    TambourineMan's Avatar
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    Default

    The seeing was not good last night. And between the street light and various neighbor's flood lights, and the bright moon shining on the snow cover, the other viewing conditions were lousy also. But what was great was the temperature. I think it was above freezing.

    Anyway I got my 2x barlow last week and wanted to see what it would do with my 13mm so I checked out the moon - without a filter and without a mask. I am glad I remembered reading that I could not permanently hurt my eyes because for a while after I got done there was a big black splotch in my vision. fortunately it went away, but it had me worried for awhile.

    But I was reasonably satisfied with the magnification. My 9mm came today, so hopefully it will still be clear when I get home tonight and I can try that one.

  5. #4
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    Default

    If the moon's brightness is bothersome then using a moon/neutral density/polarizing filter is the best way to view the moon when it is near the full phase. Reducing the aperture to dim the moon reduces the telescopes resolution. If you don't want to spend money on a moon filter put on a pair of your darkest sunglasses. Just remember to take the sunglasses off before observing anything else or walking around.
    The moon's brightness will not harm your eyes but will definitely leave you night blind for a while. The larger the telescope used to view the moon the greater the area of the retina that will be fully illuminated.

  6. #5
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    Default

    Here's a different approach.
    If you are only going to be observing the moon, do it from a well lit area like in the garage with the lights on or with the neighbor's floodlights.
    That way your pupils are already the right size for looking at bright things, rather than being wide open for the dark and then having bright light (moon) shone into them. You gain nothing by being over dark adapted.
    It makes it easy to go from the eyepiece to a lunar map and you're less likely to trip over things.

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  8. #6
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    Default

    ... you know Steve ... that is madly brilliant ... seriously ...

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  9. #7
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    Default Holy Filter

    Hi TM.

    I think you could preserve the resolving power of the scope, while reducing brightness, by using 2 holes; each at the extent of the aperature of the scope. One might also think about lots of smaller holes randomly spaced over the aperature. So while the surface area of open space will control the brightness, the full diameter of the light sampled would preserve the resolution.

    methinks.

  10. #8
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    Default

    Yea , i think they refer to that as a hartman mask . For focusing , i believe .
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  11. #9
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    Default

    Quote Originally Posted by roverich View Post
    Yea , i think they refer to that as a hartman mask . For focusing , i believe .
    Good point. That would make capnron's idea not so hot unless you had perfect focus. The slightest amount out of focus will give you as many images as there are holes, all overlapping to some extent. Believe me, that's a quick way to a headache.

    Steve.

    PS. You've just reminded me of something - Where's that "dumb things" thread ?
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    Default

    Try fly wire over the objective might reduce glare and improve contrast

 

 

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