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Re: [APML] Exposure time with filter
Hi Jim:
Sorry to jump in here but it gives me a chance to use my Kodak Filters book
(not often that I crack it!). It shows the W92 filter spectrum at what I
would guess to be 90 % at 656 nM. The log scale for transmittance is not
very well calibrated, but you can make a guess from the 10% transmittance at
625 nM and stabilizing at maximum (about 95%?) at 670 nM. If the book
interests you, it's called Kodak Photographic Filters Handbook, has a
copyright date of 1990 and ISBN # 0-87985-658-0. Probably available through
larger photo retailers, Amazon, or B&H.
Bert
Bert Katzung
katzung1@attbi.com
www.astronomy-images.com
----- Original Message -----
From: "Jim Ives" <flyboy9990@earthlink.net>
To: <astro-photo@seds.org>
Sent: Wednesday, November 27, 2002 4:37 PM
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
> ----- Original Message -----
> From: "Robert Reeves" <reeves10@swbell.net>
> To: <astro-photo@seds.org>
> Sent: Wednesday, November 27, 2002 6:58 PM
> Subject: Re: [APML] Exposure time with filter
>
>
> >
> >
> > > 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
> >
> >
> > -- APML Archives at <http://astro.umsystem.edu/apml/> ---
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> >
>
>
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