Re: [APML] Exposure time with filter

[Robert Reeves <reeves10@swbell.net>]

> Robert do you have any idea off hand what the % of transmission is through
a
> Wratten #92 at the Ha line?
> Jim Ives

Ho Jim,

My Kodak Wratten filter book is on the other side of the house and I don't
want to disturb the sleeping significant other, so I'll use info from Table
5-2 of my book Wide-Field Atrophotography.  It was sourced from the Kodak
book.

The #92 filter passes 76.4% of the H-Alpha wavelength and has a 50% cutoff
at the 640 nm wavelength.  Anything further toward the blue end of the
spectrum gets cut off progresively more.

Interestingly, the #29 actually passes more H-Alpha (88.5%), but it has a
50% cutoff at 620 nm and thus passes more light pollution.  This more than
offsets the added gain from the greater amount of H-Alpha and you end up
with more sky fog which washes out the added gain in H-Alpha.

So bottom line is the #92 works better for H-Alpha because it blocks more
light pollution, alowing longer exposures to more than compensate for the
12% reduction in H-Alpha passing through the filter.

Does this make sense or am I talking in circles this late at night <G>?

Robert Reeves                      reeves10@swbell.net
520 Rittiman Rd.                   www.robertreeves.com
San Antonio, Texas 78209    210-828-9036
USA                                     29.484  98.440  200 meters


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