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If transmission at wavelength X = 95 % , the stacking two "identical" fllters will result in overall transmission of : 0.95x0.95=0.9050 = 90.5%
And so some desireable light will be absorbed and / reflected.
So longer exposures will be required, that will likely counter any benefits in cutting Hg and Na vapour lighting and W filament lighting pollution of stacking.
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