Re: [APML] Exposure time with filter

["Jim Ives" <flyboy9990@earthlink.net>]

It does make sense Robert thanks. Sounds like a low transmission % though!
Maybe better suited for Schmidt cameras than my f/3.3 Takahashi.
Jim
----- Original Message -----
From: "Robert Reeves" <reeves10@swbell.net>
To: <astro-photo@seds.org>
Sent: Thursday, November 28, 2002 12:47 AM
Subject: Re: [APML] Exposure time with filter


>
> > 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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