Date: January 27, 2011
Title: Astroimaging Under Light-Polluted Skies
Podcaster: Robert J. Vanderbei
Description: Robet Vanderbei, Author of “Sizing Up the Universe” and Princeton University professor talks about Astro Imaging Under Light Polluted Skies.
Bio: Robert Vanderbei is professor and chair of the Department of Operations Research and Financial Engineering at Princeton University. He received his doctorate in applied mathematics from Cornell University and contributed to many of the leading design concepts for NASA’s Terrestrial Planet Finder space telescope. As an amateur astronomer, Vanderbei has taken, from his own backyard, high-quality images of astronomical objects that rival the best images from our greatest observatories. His photographs can be found in National Geographic’s new book “Sizing Up the Universe,” co-written by Vanderbei and best-selling author J. Richard Gott.
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Hello and welcome to this episode of 365 Days of Astronomy. I’m Bob Vanderbei, a faculty member at Princeton University and, for this podcast, I’d like to talk about astrophotography under light polluted skies.
I live in New Jersey. New York City is about 50 miles to the northeast and Philadelphia is about 50 miles to the southwest. The so-called Northeast Corridor, where major highways wind their way from Boston to Washington pass only about 8 miles to the southeast of where I live. That said, the particular town I live in has very good lighting laws. For example, in residential areas, we have street lights only at intersections. The Milky Way is barely visible late on clear summer nights.
My childhood interest in astronomy was rekindled about 12 years ago. I subscribed to Sky and Telescope magazine and was awestruck by the beautiful pictures seen on almost every page. I bought myself a small telescope and soon became one of those fanatics who goes out observing on every clear night. The bright Solar System objects were spectacular, especially Saturn and Jupiter.
But, living in New Jersey, I was disappointed by much of what I saw. Along with Jupiter and Saturn, open clusters were nice. But, globular clusters looked just like very faint blobs and most nebulae and galaxies were completely undetectable when viewing through my admittedly modest telescope.
Eventually, it occurred to me that one big difference, perhaps the biggest difference, between my visual impressions and the pictures I saw in Sky and Telescope was that the photographs were always very long exposure images. So, I investigated the various camera options available. Back then, about 10 years ago, digital CCD cameras were just coming on the market. There were two types: consumer market point-and-shoot cameras and special purpose black-and-white cameras specially designed for astrophotography. These special purpose cameras are designed for long taking long exposures. Single exposures often range from tens of seconds for star clusters to tens of minutes for nebulae and galaxies.
My first camera was from a company called Starlight Express. I bought what seemed to be the best camera they sold at that time. I also bought the adapters needed to attach it to my telescope and computer software that would operate the camera and do the subsequent image processing.
The camera arrived in August and so naturally my first target was the famous Great Globular Cluster in the Constellation Hercules, also known as M13. I took 26 six-second exposures. Later that evening I used the image processing software I had bought to stack these 26 exposures into a single final image. The result, although black-and-white, was simply stunning compared to what I had seen visually through the telescope. Instead of a faint fuzzy blob, the cluster came alive with a myriad of stars of varying brightness clearly looking like of glob of stars in space.
All of a sudden it became first-hand clear why a globular cluster is called a globular cluster. That’s exactly what it is and now I had “seen” it for myself.
Soon I invested in red, green, and blue filters so that I could assemble color pictures. But, the biggest advance for imaging under light polluted skies occurred when I bought so-called narrow band filters. While narrow band filters don’t help with galaxies, they work wonders with most nebulas in our Milky Way galaxy from the Crab Nebula, to the Lagoon, the Dumbbell, the Ring and many many others. These nebulas are called emission nebulas. The light from these nebulas comes in very specific wavelengths. In the visible part of the spectrum, there are only a few wavelengths. The most abundant of these wavelengths is called Hydrogen-alpha and at 656.3 nanometers is deep red. These photons are produced by hydrogen atoms in the nebula as an electron transitions from the third to the second lowest energy level.
The second most common wavelength is called Oxygen-III, which at 495.9 nanometers is a blue-green colored photon emitted by doubly ionized oxygen atoms.
Narrowband filters let almost 100 percent of these wavelengths through and block almost 100 percent of the rest of the light. Since most of the light pollution is outside these narrow wavelength bands, these filters cut out the vast majority of the light pollution. These filters have made imaging possible under light polluted skies. It is even possible to take nice pictures of emission nebulas on a full moon night.
To illustrate the amazing effectiveness of narrowband filters I once took an H-alpha picture of the Orion Nebula one half hour after sunrise. Light pollution doesn’t get much worse than that! While my telescope was taking the picture, I used a point-and-shoot camera to take a snapshot of the Sun shining off from my neighbor’s window as proof that the Sun was up and shining brightly when I took this picture. Now, granted, this is perhaps the worst astrophotograph I have ever taken. But, it proves that one can take narrowband images even under the worst imaginable conditions.
You can check out this picture of the Orion Nebula as well as my other, better, astrophotographs at website for the book I wrote recently with Richard Gott published by National Geographic and entitled “Sizing Up The Universe”. The website is www.sizinguptheuniverse.com.
End of podcast:
365 Days of Astronomy
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