From: Alan Friedman <alan@greatarrow.com>
Date: October 31, 2005 9:18:07 AM MST
To: Astro_IIDC@yahoogroups.com
Subject: Re: [Astro_IIDC] Re: Colors are making me mad!
Hi Duane -
Tim is right - the exposures for the different filters have to be different. Red and Green are about the same (green requires slightly less exposure) - Blue requires significantly more. You can adjust the histogram for each color image easily in Photoshop - go to the channels view and select one color channel so it is the only one highlighted - but be sure that RGB is checked (visible) too so that you can see the result obtained by adjusting one color component. Use the curves or levels adjustment and watch the change. Another way to do this is in the color balance tool. Either of these methods will let you tweak the colors of the image.
Here is an image of mine with the color channels broken out.
http://www.geocities.com/alanfgag/mars100305.jpg
The color channels have had minor adjustments for clarity, but they should give you some idea of the "proper" value for each separate color component of the RGB image.
Don't give up on the B&W method - it has major advantages over a color camera.
best,
Alan
On Oct 31, 2005, at 9:48 AM, Tim wrote:
duane:
Sorry you're having so much trouble! One possibility is that maybe the color filters were
intended for visual use, and so don't effectively block other colors from being transmitted
to the CCD even with a IR blocker in place.
But it sounds more likely that the ccd isn't sensitive to blue (most are more sensitive to red
light). It isn't unusual to have to take exposures much longer through blue than green or
red. Even with a good exposure, the blue image shouldn't show much in the way of detail,
except at the limb, polar caps and hoods. I remember years ago watching Don Parker
image Mars with his old LynxxPC camera. Red exposures were a fraction of a second in
length, green a second or two, and blue was 25 seconds long! (but what was most
amazing is that Mars held still that long!).
So, I'd try setting the exposure times individually for each filter, and then adjusting the
levels in Photoshop individually after the channels are combined. I set my exposures so
that the histogram peak is about halfway to 2/3 of the way toward the high end, so I avoid
saturating the bright areas on the planet.
Depending on the scope you're using, you might also want to adjust the focus for each
filter as well. Probably only necessary for an achromat, though. I have a Jaegers 6" f/10,
and even with a contrast booster and IR block filter in place, I get goofy colors at planetary
image scales. But since I don't have a monochrome camera (except the DSI Pro, but that's
a topic for another forum, and a much lower quality imager than the Flea I use most of the
time), I haven't taken the time to experiment with refocusing for different colors.
best,
-Tim.
--- In Astro_IIDC@yahoogroups.com, "Duane" <macastronomer@m...> wrote:
>
> I am trying to shoot with this B&W camera, and I must be doing something wrong.
>
> First off, I am having a lot of difficulty trying to get an exposure that will work with all
three
> colors. If I expose the Red properly, the blue is non-existant. If I expose so the blue
shows
> ANYTHING, the Red is blown way over-exposed.
>
> OK, so I shot a few Mars shots and I stack them up. These are straight from Astro IIDC
and all
> three shots have the same camera settings. Put the red in the Red channel, Blue in the
blue
> and green in the green. What do I get? A very mucky red color that doesn't show any real
> white or come close to the colors I get with color camera.
>
> This time I did shoot through an IR filter for all exposures.
>
> Am I missing something there?
>
> Please help me before I send this camera back for a color version :)
>
> Duane
>
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