I don't want to confuse the issue; I think Richard's advice is
very good for astronomy. But I do want to quibble with something:
> Hbeta is a good filter for visual because its 486nm wavelength is
> in the blue-green part of the spectrum and the eye is sensitive to
> that. The Halpha is a poor visual filter because the longer 656nm
> wavelength is in the deep red part of the spectrum and the eye just
> isn't very sensitive to that wavelength which is why we use red LED
> lights and red Lucite for covering out laptop displays.
These observations only apply to night vision. Night vision uses
a pigment called rhodopsin with a peak sensitivity near 500nm (http://
en.wikipedia.org/wiki/Rhodopsin). So an Hbeta filter makes good
sense, as do red LEDs and red laptop displays. BUT. Daytime vision
uses other pigments, and red sensitivity due to those pigments is
actually a good deal higher than it is for blue (the standard formula
is that perceived brightness is 0.30*red + 0.59*green + 0.11*blue),
which is why red traffic lights are red and green, while ruled paper
typically has blue lines on it.
I would also note that different spectral lines have different
strengths. The 486nm line for hydrogen is several times less intense
than the 656 line (speaking now of energy, and ignoring the
sensitivities of the human eye). A nice table showing this is at:
http://hyperphysics.phy-astr.gsu.edu/hbase/tables/hydspec.html#c1
I don't think this should changes Richard's advice at all, since
rhodopsin's preference for blue light probably outweighs it; I just
think it's interesting. :-> And it might be of some relevance to
CCD imagers, since CDDs don't have the same sensitivity curve as
rhodopsin; it's probably good to know that there's a lot more Halpha
than Hbeta emission out there. (I don't know whether that matters to
CDD folks, since I am not one myself, I'm just guessing...)
Yes, I'm absolutely nitpicking. :-> I just didn't want anybody
to walk away thinking that the eye was more sensitive to blue than to
red, period; the truth is, as usual, more complex!
-B.
Received on Tue Oct 11 22:20:59 2005