Re: [APML]: M20 through DeepSky

[Jerry Lodriguss <jml@astropix.com>]

Philip Perkins wrote:
>Howard Anderson and Steve Bell expressed an interest in seeing comparison 
>photos of M20 with and without a Lumicon Deep Sky filter. I have posted a 
>second image of M20 on my web page, taken with the DS filter. Please click 
>on the M20 thumbnail ...
>
>http://www.astrocruise.com
>
>Conditions for the two exposures were so similar that I would consider this
>a valid direct comparison - full details on the web page. The differences
>are quite striking - I noticed:
>- amount of blue response
>- colour balance (esp the red of M20)
>- number of stars recorded


Philip, both of these pictures are very nice, but I don't believe you can
draw many conclusions from them for comparison purposes because you used
virtually the same exposure for both photos (80 min unfiltered and 85 min
with the DS filter). 

You have to add in the filter factor to the exposure.  I usually go 3x the
time for DS filtered compared to unfiltered. You mileage may vary.

In any event, what has happened is that your filtered picture is
underexposed compared to the unfiltered photo.  That is why the red is much
more saturated and deep. The unfiltered photo is comparatively overexposed.
Imagine a deep red on a slide that is overexposed. You can overexpose it to
the point where it goes white (completely transparent). So as you add more
exposure (white) to the deep red, you get pink in the unfiltered M20 shot.

Underexposure (compared to the unfiltered shot) also accounts for the fact
that you recorded less stars, and less blue nebulosity. 

Marling recommends 2x to 4x the normal exposure for the DS filter. You'd
have to build a sensitometer to really nail the filter factor.

Jerry