Re: [APML] Exposure time with filter

[Robert Reeves <reeves10@swbell.net>]

> Dear All,
>
> I send this question again as you probably didn't receive my first one.
> I wanted to buy a dark red filter in order to prevent light pollution
> but filters like wratten #92 has a factor 8 as absorbtion. Is this true
> that I need to expose for a time 8 times longer than before? So an 10
> minutes exposure becomes an 80 minutes exposure? I need to understand
> this because if this is true I cannot do that way as I don't have
> autoguide....:-/
>
> thanks
> Nicola

Nicola,

I assume you are getting the #92 filter for H-Alpha work with red-sensitive
B&W film?  If so, then consider the filter nearly transparent to the H-Alpha
wavelength you want to record.  Here is a simplified explanation that ignors
some other important emission lines from emission nebulae.  The #92 filter
appears dark to the eye and needs a filter exposure correction in daylight
for terrestrial photography because it blocks all wavelengths except red.
With most of the visible spectrum removed, the remaining red wavelengths
will need longer exposure to form an image on film.  But in
astrophotography, it will pass nearly all the red H-Alpha wavelength you do
want while blocking the light pollution you don't want.  Since the H-Alpha
is the primary wavelength building up the image of an emission nebula, that
is the very wavelength you wanted in the first place.  The bluer wavelengths
with the light pollution were just fogging your film.  With the filter they
will be blocked, while the desired red wavelength passes.  Because the light
pollution is not fogging the film anymore, the filter allows longer
exposures which record deeper H-Alpha detail.  You can expose up to ten
times longer than without the filter.  But this is not to compensate for the
"filter exposure factor", it is because light pollution is radically reduced
and not rapidly fogging the film.

Remember that when using the red filter, the lens infinity focus will shift
slightly.  The lens will have to be focused with the filter in place.

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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