Re: [APML]: Color Film and Filters

[Jerry Lodriguss <jml@astropix.com>]

>The only color photos I've seen through a DS filter are the ones by Jack
Marling in 
>AMOACP and the ones recently presented by Jerry Lodriguss on his hoempage.
If you look 
>at Jack's, you'll notice that they are all of red-colored emission nebulae.
He also 
>presents a couple of galaxy photos, but he took these *without* filtration.
Since 
>galaxies are generally more blue than anything, I don't know if he'd agree,
but from 
>that I'd suppose Jack Marling views the DS filter as basically a red
filter, best used 
>on emission nebulae.
>
>Also, I think its reasonable to judge the predominate color transmission of
a filter 
>by holding it about a foot from your eye in a room well lit using your
standard GE 
>frosted light bulbs (which (I think) irradiate in all colors, ie, give off
white 
>light). When I do this with my DS, it looks purplish-red to me,
strengthening my 
>opinion that for photography (be it B&W or color), it's best used on emmission 
>nebulae, where you want to record essentially nothing but red. But then
again, there's 
>Jerry's photo of the Veil, and I'm amazed (and perplexed) at how he got
anything 
>yellow recorded through a DS, since in that region of the spectrum, its
transmission 
>is zipola.


>I forgot to mention this is my previous post, but if you look at Jack
Marling's color 
>photos (in AMOACP) through a DeepSky (DS) filter, the stars look white, not
red. This 
>is also true for Jerry Lodriguss' recent color photos through a DS on his
page. Now, I 
>distinctly remember an article in S&T or Astro. a few years back stating
that a DS 
>makes everything red on color film - stars and all. Has color film changed
that much 
>over the years, or am I loosing my mind (the most probable answer <G>)?


Yes, you are losing your mind, we all are around here. :-)

Please see the June 1991 issue of Sky and Tel, page 664. Jim Riffle has
shots through a Lumicon Deep-Sky filter of the Veil, the Horsehead, AND M31,
and the colors on all look pretty normal. So, it IS possible to shoot
galaxies through a deep-sky filter.  In fact, in his ads, Jack Marling says
" the Lumicon Deep-Sky filter improves your view of ALL deep-sky objects,
including galaxies and comets".

I dont' know why everyone is saying that the deep-sky filter is a red
filter.  Mine is distinctly blue-green when held up to the eye.  Now, mine
is about 12 years old, so maybe the new ones are red.  The OIII filter is a
red filter visually, maybe people are confusing that one.

As Chuck has already noted, most of the stars get overexposed and turn
white. I see a LOT of stars on the deep-sky fitler pix with red halos around
them. I have had this problem for a number of years with the Fuji films on
red stars and I always attributed it to light spreading in the red layer, or
halation off the film base since the red layer is next to the base.

Now that I've had the chance to think about the Veil picture and the yellow
and oranges and greens and reds it recorded, I am going to throw out a
couple of theories as to how these yellow colors were created on the film.

If you look at the transmission curve of the nebula filter on page 324 of
AMOACP, you will see that in the real world filter, there is quite a peak of
transmission at about 625nm right in the orange/red boundary. The filter
actually starts to transmit light here at 600nm.

If you look at the spectra chart on page 321, you will see that there is a
He-1 line at about 570nm, on the border of the yellow, orange region of the
spectrum.  Note that this is a spectrum for M42, and not the Veil.  I don't
have a spectrum of the veil, so I don't know what it's putting out in the
yellow-orange region.

Here are the two things I'll throw out for speculation.  One is that my
filter is 12 years old, and possibly the band pass has shifted, and yellow
light is getting through.

The other is that there is something emitting in the orange region of the
spectrum in the Veil, and it is simply overexposed on the negative and
appears more yellow than orange.

Jerry
email: jml@astropix.com 
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