Now that I’ve calmed down about the supernova in M101 and galaxy season will be giving way to summer nebulae soon, I keep thinking about how to get better galaxy images. One of the obvious questions is, Which of the filters we have available to us will give us the best images of galaxies? After reviewing the ones I know about and thinking about all this a bit, I realized that the best filter for galaxies is a DIY project and it’s very cheap. I’ve named it the GOT filter. Here’s what my thinking is based upon and then I’ll describe how to use the GOT filter on your smart scope.
The supernova in M101 is a great teaching tool. I managed to image it in both my smart scopes and it’s about magnitude 11. Plus, when just starting the stacking process the brilliant Type II explosion is visible even a bit before the nucleus of the galaxy. The spiral arms take at least a few minutes to become apparent in my scopes in my Chicago west side skies. But to get those arms to look great would take me easily a couple hours.
Of course, we know that by stacking more and more images we can beat down the light pollution quite well. But maybe there’s a filter that would help even more? Good deep sky imaging is all about signal to noise or S/N. In my case, the galaxy M101’s stars supply the signal and the noise is mostly supplied by the photons rattling around from the three street lights in my alley, the Chicago Loop, Wrigley Field on game nights, but mostly that goofy Volvo auto dealership tower a mile away. (Oh, and our friend the Moon can be a big problem too, but I'm assuming here that we're talking about observing on moonless nights.)
Before I start my reasoning here, we need to know what a nanometer is, because that’s how we will talk about light. A nanometer is a billionth of a meter and it’s one of the units used to characterize visible light. The entire visible band extends from about 400 to 700 nm. The longer the wavelength, the redder the light. 400 nm is deeply violet, 550 nm is green and 700 nm deep red. With this vocabulary we can look carefully at what wavelengths our astrophysical sources emit.
Optical filters work by transmitting the wavelengths of photons we want and rejecting those we don’t want.* One of my favorite filters is my dual band filter (Optolong L-eNhance Light Pollution Dual-Bandpass, which I use on my eVscope) which just passes photons from two very narrow wavelength bands. I use it for HII regions like the Orion Nebula or the Eagle Nebula or even the Ring Nebular. These objects emit nearly all their light in two very narrow optical bands: 1. the doubly ionized oxygen [OIII]; and 2. the bright red fundamental line of hydrogen — H-alpha. Each of the filter band passes for these two bands in my Vespera filter are about 12 nm (=nanometers) wide. [OIII] lines are centered at 496 and 501 nm and appear cyan. H-alpha is at 656 nm and appears red. Basically, emission nebulae just shine in these two narrow wavelength band. So, if we want to emphasize them against the background, all the rest of the light is wasted light — even light from other astrophysical objects.
Let me now introduce you to an analogy that might help. Try to imagine the optical spectrum is mapped out onto a full 88 key piano keyboard. If the lowest key were 400 nm and the highest key were 700 nm, then the light from an HII region, if we translated it to piano audio would be: 1. The D below middle C and 2. the C three octaves above middle C. If you know how to play a piano, and have one, sometime sit down and just play those two notes. That’s what the M42 optical nebula would "sound" like if it were translated into audio. (I’m not a piano player, but I live in a family in which half the people do.) It’s an unresolved, dissonant sound, but nevertheless based upon rather pure tones.
But what about galaxies? Of course they have nebulae in them, but unless you’re using the Hubble Space Telescope or a relatively higher resolution telescope on something close like the Magellanic Clouds, then you’re basically looking at star light. And it’s integrated star light since our little scopes really can’t pick out individual stars unless they are supernovae. Many galaxies, like M31, emit most of the light we see from stars called K2 giants. K2 is a spectral classification and for our purposes says the star is a little cooler than our Sun, which is a G2 star. What if we mapped a K2 star’s emitted light onto the piano keyboard? Well, stars emit thermal spectra that are much wider than even the optical spectrum we see. If we could hear such a spectrum played on a piano, it would be like slamming down all the keys at once! (I actually know what this sounds like because it happens when unsupervised grandchildren come over and visit our house with its Steinway.) Even if you haven’t hear this you can surely imagine it.
To see the difference in these two emitters I also created a plot. The gentle curve is the spectrum of a typical giants that make up the bulk of the light from the galaxies we would observe. The green and red lines are where the emission lines (or “notes) would be coming from a nebula like the Orion Nebula, M42.
Finally, we need to understand what the spectrum of our light pollution looks like. I won’t go into details here, but the basics are the following. At one time much outdoor lighting was from Mercury or Sodium vapor lamps. These emitted spectrum lines. So, blocking them was relatively easy. The so-called light pollution filters basically try to block these lines and leave the rest. It would be as if you might deaden a few keys in the middle of the piano. A facile performer could still play recognizable pieces if they could scoot over the bad keys.
The problem is that our outdoor lighting is rapidly becoming dominated by LED fixtures. These are broad band. So, they slam down on the keyboard too. Filtering them without removing too much signal from stars, in particular, is nearly impossible. This is whey we can hardly see any stars with our eyes in a light polluted environment.
Emission line nebulae are not really a problem if we have a dual narrow band filter. But how can we filter out light pollution without filtering out the light of the stars in our galaxy? The answer is, we really can’t. But the thing to notice is that they are based upon starlight and relatively average stars at that. When using a nebular filter we are suppressing the sweet spot of where galaxy light is emitted. So, that’s a bad thing to do too. This is why in nebular pictures stars seem to be suppressed. The image stacking and infrared capabilities of our smart scopes really can help beat down the background noise, but we can do better.
But how? Well, it’s always about getting rid of background. The only way to do that for galaxies or even star clusters is to “Get Out of Town.” I call this strategy the GOT Filter. It really works and it’s really the only way to improve the signal to noise on galaxy images if you are impatient like me.
I can give you an example of how going from my Chicago home to Yerkes Observatory in Wisconsin last year resulted in a factor of two improvement in signal to noise. I live in a Bortle 9 environment. Yerkes Observatory’s back lawn is about Bortle 5. I took images of the Whirlpool Galaxy, M51, at both locations. What took me 22 minutes in Chicago I was able to achieve in just 5 minutes at Yerkes. Since 22 is about 5 squared, I would say this is a factor of two in S/N for a given amount of time. I was in a Messier Marathon, so I couldn’t spend more time on the galaxy at Yerkes, but it was a verifiable reminder to me of how well the GOT Filter works.
(Here are my unprocessed images. You can tell where they were by the coordinates, if you know them, but more importantly the time spent in enhance mode. First Chicago, then Yerkes.)
I have many pleasant other memories of working and studying at Yerkes Observatory. One was getting to know the famous astronomer W. W. Morgan. He may not be the sung hero Edwin Hubble became, but Morgan’s work in stellar physics was extremely important in the 20th Century. In working on this blog I was reminded of how Morgan and his colleagues worked to classify the integrated light from galaxies too in the 1950's. This went a step beyond Hubble’s classification based upon mere shape. In Morgan’s paper with Mayall of Lick Observatory I learned that M31 basically emits the light of K2 giant stars.
So, don’t even think of trying your narrow band filter on a galaxy. It will just suppress the light you’re trying to capture.
Just put your scope in the back seat of your car and, Get Out of Town!
—
* This is not strictly true in the same way as for a white light solar filter. This type of filter throws away just about everything and preserves only enough photons to not fry one’s telescope. In other words, a solar filter with a 0.0003 percent transmission. (eVscope 100,000 times reduction). These come to 3x10^-6 and 1x10^-5. What are these in magnitude reduction? 10^-5 reduction takes the Sun down to about the magnitude of the full moon. No, you can’t just put an H-alpha filter you might use on a nebula on your smart scope. You’ll fry it and void your warranty -- it'll cost ya. I’ll talk about H-alpha solar scopes in another blog.
Comments