Re: [APML] Scanning and enhancing Halpa filtered shots

[Jerry Lodriguss <jml@astropix.com>]

>I am looking for advice on scanning and enhancing hydrogen filtered film
>shots on E200.  The slides look dense and red<g>  I used the Lumicon
>halpha filter on my Pentax 105mm lens at f4 for one hour exposure of the
>Milky Way.

I shot 52 minutes at f/2.8 with the Lumicon h-alpha filter, the slide 
looked to be the right exposure for my dark-sky site, so your's probably 
looks dense because it is about 1 stop underexposed.

Otherwise, it looks like a color picture taken through a red filter, that 
is, it's red. <G>

You can scan it in color, but only the red channel will have data in it, 
the green and blue channels will be empty, totally black.



  Will the red
>channel have the characteristic bell shape curve or will it be flat?

The red channel, if properly scanned, will have a normal bell-shaped 
histogram. The other two channels won't have anything in them.



>Just looking for tips on the best way to scan.  Should we scan as black
>and white possibly?

You'll probably get better results to scan in color and then just take the 
red channel out of the color image.


>  When I begin to enhance, should I just convert to
>grayscale?


Again, you will get better results to copy just the red channel and paste 
that into a new document and use that as your black and white image, if 
black and white is what you want to work in.



>  Are there any online resources for Halpha film shots?

Yes, there are some for h-alpha on techpan, but none that I know of for 
h-alpha on color film.

The benefit to shooting h-alpha is higher signal to noise in the h-alpha 
wavelengths that come from emission nebula. It only works with emission nebula.

Naturally, shooting an h-alpha filter will work much better on Technical 
Pan film because it has increased sensitivity at the h-alpha wavelength, 
and because it is so fine-grained and has such great contrast when 
developed in D-19 for around 10 minutes.  These factors combine to make 
h-alpha filtered shots on hypered techpan excellent.

My experiments with the h-alpha filter were done with the intention of 
finding out whether it was worthwhile to shoot the filter on Ektachrome 200 
because E200 probably is one of the best color films in terms of contrast 
and fine grain and red sensitivity, but it does not need to be 
hypered.  Sort of a lazy-man's techpan for h-alpha.

My plan was to take the red channel out and substitute it into the red 
channel of a normal color shot, and also see how well it worked as a 
luminance channel in an LRGB composite.

I have compared the red channel taken from an h-alpha E200 shot to the red 
channel of an IDAS LPR filtered shot. The h-alpha shot was 52 minutes and 
the IDAS LPR was 45 minutes. I would have gone 60 min on the h-alpha but 
the state police decided to drive up on me at 4am that morning with their 
spotlight on. I don't think there would have been much difference between 
52 min and 60 minutes.

Anyway, there is some difference in contrast between the h-alpha red 
channel and the IDAS LPR red channel, although not as much as I would have 
thought, and not enough to justify the extra time. You can acquire an 
entire color image in 45 minutes with the IDAS LPR whereas only a red 
channel in 60 minutes with the h-alpha. You are better off shooting two 
color images and stacking.

I'll have a section in my new book about all of this. I'm working on the 
last chapters now. Hopefully it will be out by Christmas.

Jerry







Astronomical photography: http://www.astropix.com




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