Cats’ Eyes at Night: Seeing Next to Nothing with Your Smart Scope

COLOR VISIONS

Cats can see much better than we do in low light levels, in fact they can see about two magnitudes fainter if we used terms astronomers understand. This is because they have more rod-like photo receptors in their retinas. But, alas, their actual visual acuity and color vision are a poorer than ours. So, they don’t stand a chance of beating us as sky watchers, much less our smart scopes. This blog entry will, however, look at a famous “cat” in space to help us understand how to understand the color vision of our scopes for at least one category of object and to maybe better understand why dual band filters are designed to bring out the colors of interstellar space.

We have to rely upon our “rods” when the light level is low too. That’s why even when we look with our unaided eye through a large amateur scope, we’re still often only activating our more sensitive black and white system. It also preferentially operates in the periphery of our field of view, so we might need to avert our vision. I just see black and white in this case.

One of the biggest advantages of our smart scopes is that they have color vision. I know this is obvious, but I venture to say it’s under appreciated because so many sources we look at are broad band white light sources like stars. To see the colors, however, we need to look at colorful sources. In the months of the northern summer, luckily there are many good color saturated targets. We are also fortunate to have filters to help with the signal to noise. These interference filters that would have delighted William and Margaret Huggins exactly 159 years ago this August.

CAT’S EYE NEBULA AND A GLIMPSE OF CREATION

William Huggins is probably the most important amateur astronomer we never heard of. And I’m not exactly sure why he is considered an amateur astronomer, except for the fact that there were very few astronomers with paid academic or government positions in the early 19th Century. And, I suppose he was considered to be an amateur because he did “astrophysical” research, not the typical positional astronomy of his day.

Astrophysics, in the 19th Century, meant using a spectroscope to analyze starlight and to compare the results with the use of a spectroscope in the laboratory or in his case, his observatory dome. Today this approach is so fundamental, I think it’s safe to say there are very few of the old fashioned astronomers yet the majority of researchers use spectroscopic tools in their research.

Huggins built his observatory and laboratory in a suburb of London. His work on stellar, solar and nebular spectroscopy formed the foundation of astrophysics. His wife Margaret should be given credit where credit is due. At a time when few women were even allowed in an observatory, Margaret was his indefatigable collaborator. She too was an enthusiast for spectroscopy and helped with their elaborate spectroscopic and photographic techniques.

On the night of 29 August 1864, William Huggins was occupied with trying to solve the puzzle of the nebulae. What were these fuzzy patches amongst the stars? For over 100 years they could only be analyzed by studying their shapes when observed through large telescopes. Some even looked so much like round but stationary planets that they were dubbed, planetary nebulae. This roadblock to understanding came down abruptly when William peered at one planetary nebulae, NGC6543, AKA “The Cat’s Eye Nebula.” What he saw was like a revelation.

I looked into the spectroscope. No spectrum such as I expected! A single bright line only! At first I suspected some displacement of the prism, and that I was looking at a reflection of the illuminated slit from one of the faces. This thought was scarcely more than momentary; then the true interpretation flashed upon me. The light of the nebula was monochromatic, and so, unlike any other light I had as yet subjected to prismatic examination, could not be extended out to form a complete spectrum…*

He later was so moved about this and other observations that when he put his eye to a spectroscope he felt he was looking into a secret place of creation. In this case, a pure, relatively monochromatic emission was very different than the usual stellar spectra that showed dark lines against broad spectrum. He was looking at flame like one he might create in the laboratory out of different gases or elements. It was actually two bright blue-green lines very close together and another one nearby. But what was this blue/green element?

The Hugginses already knew about the Fraunhofer lines that were in absorption in the solar spectrum and identified them as Calcium, Sodium, Iron, etc. The wavelength of this bright line was close to ones of Magnesium. But William and Margaret were not ones to speculate, they had a unique observatory where they could produce laboratory spectra near the telescope and compare in real time.

Huggins Spectrum of the Cat’s Eye Nebula. Note that it shows three emission lines, the two in the middle are from an unknown element that would be attributed to a new element, christened nebulium, for a few decades. The line to the left is Hydrogen Beta, which is faint, but also present. Mg marks where they expected, but didn’t see, magnesium.

NEBULIUM

No one knew what nebulium could be. William and Margaret compared it to the spectrum lines from other known elements and the wavelength did not match. The primary lines he saw were at 495.9 and 500.7 nanometers (nm). The closest known candidate was magnesium at around 517 and 518 nm. Since in those days new gasses like helium and argon were being discovered in the solar spectrum, the name nebulium was suggested. But, unlike helium, for example, it was not seen in absorption in the Sun. Astronomers were not quick to assign a new element to these lines. They only seemed to be native to interstellar nebulae.

Nebulium continued to be a mystery until 1927, when the physicist Ira Sprague Bowen showed that these lines were actually from doubly ionized oxygen, [OIII]. The reason they didn’t appear in a laboratory was because any collisions between the atoms would cause them to decay before emitting the blue-green photons. Such emissions are called “forbidden” because in any earthly laboratory the pressure of the oxygen plasma will prevent the levels from radiating light before being knocked down. Nebulium was really oxygen behaving in a “forbidden” way. The brackets around the OIII indicate a forbidden transition from this ionized element.

(Whew! I’ll bet you didn’t think you’d get such an atomic physics lesson. Well, it’s good for you and I’ll be done in a minute. Just one more link in the chain….. )

What is amazing is that the OIII ions are energized because of a close resonance with a fundamental spectral line of helium. It’s in the ultraviolet, so Huggins wouldn’t have seen it. But, by a cosmic coincidence Helium resonates with OIII the same way a tuning fork will vibrate when the same note is played on a piano. The whole process is called the Bowen Resonance Fluorescence mechanism. Without it we wouldn’t see pretty green nebulae.

Here are the energy levels involved in the [OIII] emission. The forbidden lines are in green, the resonant ultraviolet lines are indicated in magenta.

BUT MY CAT’S EYE ISN’T VERY GREEN!!

Okay, so I was out the other night to image the Cat’s Eye and I got the following image. I had to tweak the saturation a bit to get any hint of green. (BTW, this was taken with my Vespera through my dual band filter.) The reason one can’t see anything that looks like a cat eyeball is that the resolution of this instrument is not good enough to pick out details in an object that is about 3 arc minutes in size. It also does look a lot like Neptune or Uranus, however.

But why is it so whitish? Well, I though a bit more about it and realized that although Huggins was seeing bright green, that's because with his spectroscope he wasn’t tuned out to 650 nm where the hydrogen in the nebula is emitting. Here’s a full spectrum of the nebula where I’ve inset a diagram showing how green and red light are going to give us a color somewhere in between — closer to yellow.

DUAL BAND FILTERS

So, this blog entry was supposed to be about the details of dual band filters, but I thought the physics and process of discovering the sources of colors in astrophysics had more interesting stories to tell. If you look back to the spectrum of the Dumbbell Nebula above you’ll see that the green [OIII] lines are off to the right and there’s a strong line around 656 nm. Well, the latter is a fundamental line of hydrogen — hydrogen alpha it’s called. It’s in the red end of the visible spectrum. Dual band filters basically just pass the light of these two regions of the visible spectrum. They show us doubly ionized oxygen and hydrogen plus maybe a bit of “forbidden” nitrogen near the hydrogen line.

Both the oxygen and the hydrogen lines are quite narrow. Hence they are essentially monochromatic. A filter that can just allow these lines to pass will block nearly everything else. This is fine for ionized nebula made mostly of hydrogen and helium and a seasoning of oxygen.

The overall upshot is that 99% of the light of nebulae are passed by these filters. And, they block much of the other light. So, at this summer time of the year it’s great pickings with all the HII regions and planetary nebulae about.

But in our smart scopes the Cat’s Eye is the size of a tiny BB. It’s only a third of an arc minute in size! If you want to see a fantastic planetary nebula, the Dumbbell (M27) is probably the best bet. It’s about 20 times bigger than the Cat’s Eye partly because it’s had more time to expand and partly because it’s half the distance away from us.

So, I did an experiment with the much larger nebula. Below is M27 (the Dumbbell Nebula) in two 8 minute exposures with my Vespera. The one on the left is with the dual band filter on and the one on the right just shows the results of the light pollution filter. Aside from the performance difference (more [OIII] on the left and more faint stars on the right), one can see that there is more hydrogen emission on the outer edge of the nebula.

BEST VIEW OF NEARLY NOTHING

When I look at emission nebulae like the Cat’s Eye or the Dumbbell I certainly reflect upon the fact that one is either seeing a stellar nursery or a star in its death throes. All of us who know basic astronomy do. But, I also have my own revelation.

I feel I’m looking at the closest thing to nothing we can see with our “eyes.” For, when we see the green light and pink light of these objects we are perceiving light generated from regions so rarified that they are able to emit “forbidden” lines. These monochromatic lines are the product of realms that typically have only 10 atoms per cubic centimeter. Even the best vacuums produced in labs on Earth have at least 10,000 atoms per cubic centimeter.

Thus, looking at interstellar clouds is our way of seeing “nothing.” So, whatever you do, if you post process your images, keep them diaphanous… don’t make them look like solid objects… ;-)

Form is Emptiness. Emptiness Form.... Buddha

(Okay, I admit I’m not a Buddhist. My take on nothingness is more akin to the concept in Stanislaw Lem’s story, “How the World was Saved.” Check it out.)

———————

* William Huggins and W. A. Miller, Philosophical Transactions of the Royal Society of London, Vol. 154 (1864), pp. 437-444.

Cat’s Eye Nebula spectrum is courtesy of Johannes D. Clausen, https://www.astrobin.com/s79n6g/0/