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    Re: EBBCO/filters
    From: Michael Daly
    Date: 2007 Nov 21, 18:49 -0500

    Isonomia wrote:
    
    > My concern with using two polarising filters is that the UV may behave
    > differently and not be polarised, so although the optical light is
    > diminished, the UV comes through just as before!
    
    This is a legitimate concern.  Depending on the source of filters, the
    degree of UV filtration can vary over quite a range.  If the filtration
    is appropriate for sunglasses, that's not necessarily enough for direct
    observation.
    
    > One solution I'm considering is using solar filters for astronomy and
    [...]
    > Only problem is that it doesn't say how much the light is attenuated
    > and although it ought to be suitable, I've no way of checking out
    > before hand.
    
    If they are designed for solar and sold by a reputable maker, they
    should be safe.  Amateur astronomy products like this are tested by a
    lot of people and anyone pushing inferior products will be run out of
    town.  Amateur astronomers take their eyesight seriously.
    
    > The other alternative I'm considering is buying some sheets of optical
    > filters - unfortunately with no guarantee on UV transmission so only
    > for lunar work (is there any significant UV in moonlight?).
    
    The aluminized mylar can only be used effectively if one side is dark -
    i.e. mounted in a cell in tight contact with the telescope at the
    objective lens.  If the cell is not mounted square to the optical axis
    of the scope, you can get very poor contrast and/or multiple sun images
    due to internal reflection on the inside of the filter and the internal
    components of the scope.
    
    As Nicolas has pointed out, this means you won't see the horizon.
    
    Your best bet is a welder's filter.  The welding arc can generate more
    UV (with a broader spectrum) and low energy x-ray than what gets through
    the atmosphere from the sun.
    
    Mike
    
    
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