| David Knisely |
| (Postmaster) |
| 04/16/08 09:40 PM |
|
Re: Using a Nightime H-Alpha filter for Solar view
|
Byoesle wrote:
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If we look closely at the bright continuous spectrum of the sun, we observe many "spectral lines" corresponding to the various elements of the gases in the photosphere -- the "absorption spectrum." Relatively cool (6000 degrees C) hydrogen gas in the photosphere absorbs light coming up from below, and we see a very dark line – the Hydrogen alpha absorption line -- in the continuous spectrum. It is the darkest line, and relatively wide, because the sun is mostly hydrogen, and it is this gas that predominates the layers of the photosphere.
Visually in a simple spectroscope, the H-alpha line is not necessarily the "darkest". Indeed, the H and K lines of Calcium are quite broad and somewhat easier to see with a spectroscope than is H-alpha. Even the G-band, the H-Beta line, and the Sodium doublet tend to be somewhat easier to notice than H-alpha (mostly because of the eye's lower red sensitivity), although the H-alpha line is still fairly dark and deep.
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The sun's chromosphere lies above the photosphere, and is much hotter -- 20,000 degrees C. At these higher temperatures we see what is know as an "emission spectrum". Here light is being given off instead of absorbed, corresponding to the energy given off when the electron shifts back down to a lower energy state, and at the same frequency or "color" of light which is absorbed in the photosphere.
Actually, light is still being absorbed in the chromosphere (otherwise, the hydrogen atoms in the chromosphere would remain in the ground state and not have excited electrons to decay back to the ground state and release an H-alpha photon). Indeed, many of the features seen against the solar disk in sub-angstrom solar H-alpha filters do appear dark as the gas they contain absorbs light from the sun. What happens is that the chromospheric feature absorbs the intense light from directly beneath the feature (over a somewhat limited solid angle) but then re-radiates it in all directions, with only a small amount of the re-radiated energy heading directly on a line to the observer. This is why filaments and fibrils appear dark on the solar disk. On the limb, we finally see the weaker emission component of the filament against the black background of space, so the filament becomes a brighter "prominence". Unlike the photosphere's surface, the chromosphere does not really have a well-defined temperature, as it reaches a minimum at its base (somewhat lower than the photospheric temperature), and then rises both as altitude increases and as you get over areas of the magnetic network where it can also be higher. What can be said is that the temperature is probably somewhere between 3500 K and the 1,000,000 K of the Corona, although I have seen figures of 8000 K to 10,000 K for some portions of its upper edge.
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This is why you can't use a white light solar filter with a nebula filter and see chromospheric features like flares and prominences: The white light filter reduces the H alpha emission from the chromosphere by the same amount it reduces all the other wavelengths from the photosphere. The wide bandwidth of a nebula filter only lets you see the photospheric features from light coming from either side of the photospheric absorbtion line, and the chromosphere emission itself is reduced by the white light filter to invisibility.
It might be a little more general to say that the reason broader "nebula" H-alpha filters won't show the chromospheric detail is that they are letting in a little too much off-band light from the photosphere. That light is basically "drowning out" the weaker chromosphere. If you use one of those really narrow night H-alpha filters (say, under 70 Angstroms FWHM) and an occulting disk to cover the photosphere, you might still occasionally photograph some of the brighter prominences on the limb, but something narrower would still be better (4 angstroms or less). With a regular white-light solar filter, you would, of course, dim the chromospheric emission, but the H-alpha filter would still be letting in too much light from wavelengths well away from the H-alpha centerline, and that is the crux of the problem. Clear skies to you.