Re: [APML]: Pelican in Tri-Color

[Robert Hoyle <rhoyle@earthlink.net>]

The Astro-Photography Mailing List
------------------------------------

Great tri-color photo of the Pelican, Chuck.  Shades of David Malin.  The TP/PPF
comparison sure shows the beautiful contrast and grain of TP.  But I agree, PPF
seems to have held its own wrt "tri-color emulation" -- sort of the point Michael
Stecker (and others?) made on the APML back around Dec. '97.

For grain evaluation it would be interesting to know what kind of "magnification"
factor the comparison images are at -- that is, how many times larger are the
images posted, than the (35mm?) negative size? (does that make sense?).  Another
try: what size "print" would the posted images be equivalent to?  11x14, etc?

Rx

Chuck Vaughn wrote:

> The Astro-Photography Mailing List
> ------------------------------------
>
> Hi All,
>
> I just completed the first tri-color image I have ever done. This was a
> tremendous amount of work. This image is done according to the philosophy
> I wrote about in my article "Color Photography of Emission Line Sources".
>
> See: http://www.aa6g.org/Astronomy/articles.html
>
> The filter set used is the one shown in the article and was designed to
> meet as much as possible the ideal filter set suggested by Brad Wallis
> and Robert Provin in their book. It's not perfect but it's better than
> any commercial tri-color filter set I know of. The blue and green filters
> both have real response at 500nm.<g>
>
> You can go directly to my astrophoto page at:
>
> http://www.aa6g.org/Astronomy/astrophotos.html
>
> Be sure to click on the large Pelican image to see a comparison between
> tri-color and PPF. This turned out to be most interesting.
>
> But first the image...
>
> The tri-color image was constructed as follows:
>
> Calculations of Tech Pan response with filter responses showed that the blue
> and red exposures should be about equal times. The green exposure needed to be
> 2 1/4 times as long. This was impractical since I needed to run 3 hours with
> the red and blue filters. Instead I ran 3 1/2 hours with the green filter
> and stretched the green image twice as much in the Polaroid pre-scan software.
> I carefully choose the scan setups so that one image was not biased compared
> to another. Finally I scanned the negatives to preserve the grayscale and
> not block up the color in the nebula.
>
> The negatives are not perfect. The green has some glitches in the upper left
> corner and the red negative was taken last night and the temperature fell
> a lot during the exposure causing the refractor to shift focus which seemed
> to impact the corners more than anywhere else.
>
> Pelican comparisons....
>
> I compared the tri-color image to the PPF negative that is making the
> rounds in the scanner test. This is a single 90 minute exposure and
> shows the only part of the two images that overlap.
>
> Although some of the stars do show blue halos on the tri-color, overall the
> stars are smaller than on the PPF negative. This was a surprise since I've
> heard so many complaints about bigs stars on AP refractors due to out-of-focus
> blue light. My blue filter has response well into the near UV while PPF does
> not. I suspect the blue halo stars are just very blue stars and record better
> in that exposure. There are big orange stars as well.
>
> Note on monitor settings...
>
> This is related to viewing the images. PS 5.0 instructions for monitor
> calibration say to set the Contrast to maximum and then adjust the Brightness
> so blacks and whites are balanced. This turns out to be a good thing to do
> as it maximizes the RGB color space of your monitor and images have the most
> range possible.
>
> Chuck  <aa6g@aa6g.org>