Tuesday, October 20, 2009

Didymium filter fol-de-rol!

I've been thinking about taking a "portrait" of McCormick Observatory, with star trails behind the dome, but good ol' Charlottesville has been growing over the years and now fills the sky with a diffuse orange glow. So rather than slip over to Bear Mountain and get a REAL dark-sky starfield, and then PhotoShop the image behind the dome of the Observatory, I decided to try to knock down the skyglow in-camera as it were.

Perusing the internet, I came upon web pages detailing the use of didymium glass filters to do this very trick. Glassblowers have been using the glass in safety glasses for some time so that they can see the melting glass they're working on despite the sodium light of the flame they're working with. Photographers (as contrasted with astrophotographers) use didymium filters to boost the color saturation of the red end of the spectrum (as compared to the more yellowish middle) in landscape photography.

I found several photographers who claimed the Hoya filter to be the best, so I ordered one for my Nikon:

The filter is supposed to knock down the yellow of sodium outdoor lights in particular. After some additional internet skulduggery I found out more about skyglow & filter specifications Here is a compilation of a spectrum from sodium lights & the didymium filter's transmission performance:


The blue line is the light pollution relative brightness, while the red line is the transmission curve for the filter. The higher the blue line, the brighter the sodium light is for that wavelength. The lower the red line, the less of that color light gets through the filter. So what I want is the "valleys" in the red line to cover up as much of the "peaks" in the blue line as is possible. I carefully aligned the wavelengths (color) for the 2 graphs so that they're as accurate as humanly possible.

You can see that the dip in the red line just below 600nm takes out a bunch of the peaks of the blue line there. It's not perfect, but every little bit helps. The sodium lines that I particularly was interested in is a pair at 589.0 & 589.6nm. This pair of lines is very familiar to chemists & astronomers alike, they're very distinctive.

The blue spectrum line, by the way, is courtesy testone22 and can be found here:
testone22's page in a new window.
And the red line is courtesy of Starna Cells, Inc. a Califormia manufacturer of spectrophotometer cells:
Starna's graphic in a new window.
Starna's home page in a new window.

He informs me that the actual peak has been inverted because of self-absorption (the discovery of which netted Kirchoff the 1862 Rumford medal). Thus the worst part of the light pollution has cancelled itself out! How convenient. Nonetheless, there is ample yellow light on either side of there from the pressure broadening in the high pressure lamps that is coloring the sky and a goodly part of this light is taken out by the filter.

By the way, here's a source for more didymium information:
Wikipedia's didymium page in a new window.

Where I came on this fascinating tidbit:
"During World War I, didymium glass was reputedly used to send Morse Code across the battlefields. Didymium did not absorb enough light to make the variation in lamp intensity obvious, but anyone with binoculars attached to a prism could see the absorption bands flash on or off."
Wow! Using the unique spectral signature of the glass to hide the fact that Morse Code was being sent! Triksy hobbitses!

After seemingly innumerable days of clouds & rain (the "new astronomy gear curse"), it was clear enough last night to do an experiment. I quickly drew a little grid on a post-it note with my plan for exposures: 2 min, 4 min & 10 minute, both with & without the filter.

I headed outside with the camera on my fluid head tripod ready for use. I plunked down the camera and carefully framed the shot so that a neighbor's mercury-vapor "security" (aka "the burglar's friend") light was behind a small tree trunk and started a 2 minute exposure. I used the stopwatch feature of my iPod Touch to time the shot. I would have put red plastic over the bright screen but that prevents the touch control feature from working. I vowed to buy a simple stop watch like I used to have back when I worked in TV news.

So far so good, one exposure in the bag, now for a 4 minute shot...

Halfway through this shot, a motion control burglar assistance light came on in another location, off the frame to the right. I moved around to the front of the camera so my shadow was being cast on the lens and thought about what to do. I knew that this new source of light pollution was lighting up any close-by dust in the sky as well as the foreground with a different lighting regimen than the 2 minute shot had, so I decided that this image wouldn't work for me. You may have noticed a certain negative attitude toward outdoor lighting at night. I'll have to work up a blog post about the issues surrounding light pollution & light trespass. Stay tuned!

As the 4 minutes was up then anyway, I ended the shot as I normally would have and thought about my new options. Should I wait out the light and resume the exposure series I had planned on? I decided to press on and get a next shot, the 10 minute one, with me stuck standing in front (but not in the shot) to shade the lens.

I started the exposure and after a couple minutes I had a rude awakening.

Yet another motion controlled light came on! Aagh! This time it's a light that is supposed to scare away deer from a different neighbor's wine grapes. The deer are completely used to the light now and ignore it, but now I had a second source of light pollution that wasn't in either the 2 minute or 4 minute exposures! Worse of all, it was on-camera! I terminated the 10 minute exposure at 3 minutes or so.

Now I'm in a pickle. I have no good options about camera placement and no way to block all 3 lights that were shining in my face. I briefly considered getting out my stepladder and unscrewing the 2 offending lights when one after the other they both went out. Yay! I quickly decided to just get two 4 minute long exposures, one with and one without the filter. I held my breath while the camera's shutter was open and hoped for the best.

Okay, I didn't really hold my breath, but I was nervous, I'll tell you!
Success! Have a look:


You can easily see the difference between the non-filtered image and the filtered one. The annoying orange skyglow is not so orange any more and the contrast in significantly improved. It doesn't look like a dark sky site all of a sudden, but it is a definite improvement!

My next step in this saga is to make the short trek to McCormick and see what trouble I can get into up there. I'll have to take along my trusty Mac laptop so I can check up on my progress and respond to changes. Maybe Charlottesville will have an unexpected blackout and I can get the real thing!

Yeah, like THAT's ever gonna happen!

Monday, October 12, 2009

LCROSS found ice after all! At least I think so...

All weekend I've been telling people that the mainstream media (MSM) have been missing the best part of the LCROSS lunar impact story. The MSM have reported that it was a dud, a failure. They were SO disappointed that they didn't see any of the expected debris plume. There was a brief flash of sodium light on impact (seen in mid-IR wavelengths by the trailing spacecraft) but no ejecta.

But maybe this was not so much a mission failure as a reality check from the Moon itself. The NASA press office was simply following the lead of the scientists, who all expected a pretty ejecta plume that people with telescopes (or a TV set) could see & enjoy. They didn't really overhype the story in their press releases, they were all caught of guard by the unknown conditions on the crater floor. That there was no plume is a heck of a story in itself and the MSM just didn't get it.

Fly that Centaur upper stage into the Moon anywhere else and you will get a regolith explosion and a very pretty conical cloud. But whatever LCROSS hit didn't do that.

Very interesting!

I've been telling anybody who'd listen that the booster must've hit ices that were internally shaped like a sponge. The energy of the impact was dissipated into the interstices of the ice, where there are gaps & voids left by sublimation. Rather like the aerogel that captured cometary dust particles a couple years ago. Take the green foam material that florists use (called Oasis) and jab a pencil into it and you get an idea of what happened.

However, Spaceweather on Sunday the 11th. got it right:

"The absence of debris plumes does not mean LCROSS was a failure. On the contrary, by offering up the unexpected, LCROSS is teaching us something new about the lunar surface and the products of lunar impacts. That makes it, by definition, a successful experiment. All that remains is to figure out what the new information is. Researchers will be announcing their findings in the days and weeks ahead. Stay tuned."

Now if only MSM paid attention to science bloggers they could've gotten it right too. They've fired all their science reporters and now the reporters they're left with haven't a clue what they're writing about. The NASA press office needs to get busy getting things straightened out and the real story out there.

It's probably a good thing long term that we didn't make a debris plume. The ices would've largely gotten sublimated to gas and blown away by the solar wind. What a waste. Now the ice is still there, but we have to send a rover down into the crater to discover it.

Friday, October 9, 2009

What is spectroscopy after all?

One of the most amazing things about astronomy is our ability to see inside the atoms that make up a star. We can't resolve the star into a disc, but we can peer into its inner workings like it was nothing!

It's like I'm looking through my telescope and see that there's a speck on a hillside. I know the speck is a person, but I can't tell if it's a man or woman, or if it has both legs, nothing. But I can read this person's mind!

Completely counter to what we'd expect, huh?

But how the heck is this even remotely possible? By spectroscopy, that's how.
Way back in 1814 Joseph von Fraunhofer, a German glassmaker discovered dark lines in the Sun's spectrum of colors. (He also discovered the diffraction grating and was the first to examine the spectra of stars, but that story is for another time.) It took till 1859 for Kirchhoff (and Bunsen of Bunsen burner fame) to figure out that these Fraunhofer lines (as they had come to be called) were a result of and depended on the energy levels of the electrons in the atoms of the material.

Fraunhofer lines.

Kirchhoff discovered these 3 laws:
1) That a solid body (like a lump of iron) when very hot will produce a continuous wash of color in its spectrum.
2) That a hot gas will produce discrete lines of color at specific points in the spectrum, and
3) That a hot solid body surrounded by a cooler gas will show darker lines like the Sun does.

Item #2's lines are called an emission spectrum, and #3's lines are called an absorption spectrum because the one is emitted and the other is absorbed. Not only that, but the color position of the dark & light lines are the same when the gaseous element is the same:

Interesting, but in order to explain how it works we need to take a little trip. A trip into the world of the atom.

We like to think of the atom like a miniature solar system, with the nucleus of the atom being like the Sun and surrounding it the electrons take the place of the planets. In the solar system the planets approximately orbit in a plane we call the ecliptic. (Note the spelling there, it's not elliptic, it's ecliptic, with 2 "c"s.) In atoms the electrons don't have to keep to a flat plane but can orbit however they please, but you can continue to think of them being in a plane - it'll help.

Now in the solar system, if you were to fly a spaceship from Earth to Venus you would set out and presently you'd find yourself a quarter of the way there, then halfway, then 2/3 of the way and so on. The world of the atom, however, has a different set of rules called quantum mechanics that basically says that you can be at Earth or Venus and not in between. Yeah, it's weird, but that's how the universe is built.

These different orbits represent different levels of energy. So if an electron gains energy it can jump instantaneously from Venus to Earth. Likewise, if it looses energy it can jump from Earth to Venus. This is what is meant by a "Quantum Jump."

So how does an atom gain or loose energy? What does this have to do with spectra? Where you goin' with all this?

Here's how! Light! Yup, it's all done with light. A photon (a particle of light) has energy that is proportional to its color. A redder color equals a low energy photon and bluer color is a higher energy photon. In the case of the Sun (like #3 above) the Sun's continuous spectrum is partially absorbed by the cooler gas above the surface. There are photons of all energies in the continuous spectrum coming up from below. Those few photons that have the exact energy (the exact color) required to make an electron jump from one level to another are absorbed, making a dark Fraunhofer line.

It's that simple.
The atoms of cool gas strip out all the photons that have that particular color/energy from the continuum of colors, leaving a gap in the spectrum. That particular color is filtered out from the light coming up from the Sun's surface.

The hot gas in #2 emits lines as its electrons loose energy and drop from a higher energy orbit to a lower energy orbit. They loose the energy by emitting a photon of light whose color (or energy) is equivalent to the difference in energy of the 2 orbits, the ammount of energy lost by the electron. So the atom sits there, cooling down with the electron still at the higher orbit level until it is cool enough for the electron to exist at the lower level, then PING! the electron drops down (it makes that quantum jump) & a photon is emitted.

So simple! Scientists use the word "Elegant" to describe phenomena like this. Elegant. It fits.

So looking at a star's spectrum, even though you can't see the disc of the star, gives us a look inside the workings of the atoms in the star's atmosphere. This works for planets, too, when our robotic spacecraft go zipping by taking pictures. This is how we know what the volcanoes of Jupiter's moon Io are spitting out.

When the Hubble Space Telescope takes a pretty picture of a gas cloud or galaxy (or whatever) the public wants to see the visible light picture and go "Oooh!" but scientists, ever hungry for deep knowledge, ask for the spectrum. They want to know what they're really looking at and only a spectrum will do the job.

Spectroscopy absolutely rocks! It's a cornerstone of science.
And now you know how it works. Now you rock!