From: Pertti Tapola (pertti.tapola_at_antispamdomain.not)
Date: Tue Jun 24 2003 - 07:36:36 EDT
--- In digital_astro_.at._yahoogroups.com, "Samir Kharusi" <samirkharusi_.at._h...>
wrote:
> With all the recent discussion on a lack of red in 10D astro images
One thing that makes astrophotography really difficult is that
instead of the continous spectrum of light that all other photographs
consist of, some of our subjects contain most of their light in
a few discreet wavelengths.
As an example, the Oxygen III spectral line(s), at 496 and 501 nm
fall between blue and green. The optimal case would be that both
the blue and the green sensors were equally sensitive at that
wavelength. However, this is usually not the case.
In the worst case, there could even be a hole at this wavelength, resulting
in a blind spot and no registration in either blue or green sensor.
This would *not*, in fact, show much in terrestrial photography.
In the normal case, however, the sensitivities of the blue and green
sensors are not equal at Oxygen III. Hence, the resulting hue is
tilted either towards green or blue.
I got first-hand experience on this after looking at images taken through
the same Oxygen III filter with Nikon 990 and Canon 10D. The Nikon
image was definitely greener - while in ordinary daylight photography
there wasn't any distinct difference. It is simply a case of what the
blue and green sensitivity curves look at their edges.
As for Hydrogen alpha line (656 nm), the problem is that it appears
on the borderline area of the sensitivity of the eye. Hence, it is not
an important wavelength for terrestrial photography and need not
be specially catered for by digicam manufacturers. In fact, the
camera should *not* be any more sensitive to it than the eye is (to
get natural-looking daylight photographs)!
Emission nebulae emit most of their visible light through these two
wavelengths (Oxygen III and Hydrogen alpha), though there are others
as well. Anyway, it is no wonder that different cameras perform very
differently, even though they look similar in terrestrial photography.
Most CCD sensors are *very* sensitive in near-infrared (a lot more than
human eye). Hence, most commercial digicams have an internal IR filter
to make the result look more like what the eye sees. I suppose this
might also be the case with 10D (even though it does use a CMOS sensor).
In any case, Canon has tried to make the response curve of the 10D
correspond as much as possible to that of the human eye.
Anyway, the Hydrogen alpha line is a victim of the IR filter (being too
near it). Hence astrophotography may make it seem that the camera
is missing reds, while in terrestrial photography it is not.
I've seen a web page that shows how to remove the IR filter from a Nikon
Coolpix camera, and hence multiply its infrared sensitivity (as well as the
high reds). But this is very risky, and most probably cannot be applied
to Canon 10D. Sony has a model where you can control whether the
filter is there or not, in fact being able to photograph in "complete darkness"
(for the human eye, not for the IR-sensitive camera). Such a camera
might be very sensitive to Hydrogen alpha, too.
I think it should be possible to calibrate your camera, by finding out the
sensitivity for different wavelengths. However, I am not very certain as
how to perform the measurement. Does anyone have a spare Hydrogen
alpha or Oxygen III source with known intensity? ;-)
But then again, what is our goal? If we try to show our subjects like
our eyes saw them if there just were brighter, then 10D probably does
a good work as it is. But if we want to catch all "light" photons that are
there, then a CCD camera probably works better for many astrotargets.
Anyway, the bottom line is that it is very difficult to get "real" colors.
And also difficult to define what is real. So one solution is to be
happy with what we get, as long as it looks pretty. ;-)
Pertti
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