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Re: [APML] Exposure time with filter
Hi Bert,
No I don't mind thanks for jumping in! I'm trying to figure out a way
tet around spending $500 on RGB filters for my Takahashi. I'm thinking the
Wratten filters might be the way to go if I can figure out how to use them
without epoxying a filter holder inside my optical tube in front of the
focuser!
Thanks Bert!
Jim
----- Original Message -----
From: "Bert Katzung" <katzung1@attbi.com>
To: <astro-photo@seds.org>
Sent: Wednesday, November 27, 2002 8:19 PM
Subject: 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/> ---
> > > Unsubscribe at <majordomo@seds.org>
> > >
> >
> >
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> >
>
>
>
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