Imaging the disk of Beta Pictoris: a motivated amateur captures the beginnings of a nearby planetary system.
With these rapid advances in imaging technology, what is out there to shoot that can truly test an amateur's capabilities? Although another great photo of M42 or the Magellanic Clouds is nice, my imaging goals focus on taking pictures that are unusual in some way, either being of a rarely imaged object or a familiar object presented in a new way. For the last couple of years I've been wondering if it's possible for amateurs to resolve the circumstellar disk of debris and dust around the 4th-magnitude star Beta Pictoris, first detected by the Infrared Astronomical Satellite (IRAS) in 1983.
Beta Pic is a young star about 63 light-years away in the southern constellation of Pictor. Roughly 12 million years old, Beta Pic is thought to be similar to how our own solar system must have appeared some 4.5 billion years ago. Its circumstellar disk is seen edge-on from our perspective and appears in professional images as thin wedges or lines protruding radially from the central star in opposite directions.
The first optical image of the disk was taken in 1984 by Bradford A. Smith (University of Arizona) and Richard J. Terrile (Jet Propulsion Laboratory) using a 2.5-meter telescope and CCD camera at the Las Campanas Observatory in Chile. The difficulty in imaging the disk is the overwhelming glare from Beta Pic itself, which completely drowns out any features very close to the star. Images of the disk taken by the Hubble Space Telescope and other ground-based professional observatories are usually made by physically blocking out the glare of Beta Pic itself using an occulting disk within the optical path.
In pondering this challenge, I came across the paper "Observation of the central part of the Beta Pictoris disk with an anti-blooming CCD," by Alain Lecavelier des Etangs (Astrophysics Institute of Paris) et al., published in Astronomy & Astrophysics in 1993. Their idea consisted of imaging Beta Pic and then taking another image of a similar reference star under the same conditions. The reference-star image is subtracted from the Beta Pic image to eliminate the stellar glare, and the dust disk should then reveal itself. I realized that with this technique it might be possible to also record the debris disk with relatively modest equipment. As in the Astronomy & Astrophysics paper, I chose Alpha Pictoris as the reference star because it's near Beta Pic in the sky and also of similar spectral type and brightness. However, since the two stars have slightly different magnitudes, I needed to calculate how long to expose Alpha Pic in order to get a proper reference image that I could then subtract from the Beta Pic image.
The magnitudes of the stars are 3.86 for Beta Pic and 3.30 for Alpha Pic. The magnitude difference between the stars then becomes 3.86 - 3.30 = 0.56.
Due to the logarithmic nature of the magnitude scale, a difference of 1 magnitude equals a brightness ratio of 2.512. Therefore 2.512 to the power of the numerical magnitude difference gives the difference in brightness: [2.512.sup.0.56] = 1.67. This means Alpha Pic is 1.67 times brighter than Beta Pic. In order to obtain a reference image of Alpha Pic with equal brightness to the Beta Pic image, the exposure times for Alpha Pic should be 1/1.67th, or 0.597 times, that of Beta Pic.
By subtracting the Alpha Pic image from that of Beta Pic, I captured the first amateur picture of the debris disk on November 16, 2011. This image received a lot of attention from amateur and professional astronomers and was subsequently reported in the media all over the world. I was also interviewed live on Radio New Zealand and came home to a television crew and journalists in my driveway wanting to interview me for TV3 here in New Zealand.
After the initial success of capturing the disk, I was also contacted by several people in the amateur astronomy community, including Grant Christie of Auckland's Stardome Observatory. We discussed various techniques and possible improvements that could be made, and following his suggestion, I tried imaging it again using shorter exposures. I did this to minimize the area saturated by Beta Pic itself, which could potentially reveal more of the debris disk closer to the star.
The Sony ICX098BQ chip in my Philips ToUCam is an 8-bit detector that saturates fairly quickly when imaging bright stars, even when using very short exposures. Long exposures with the camera can only be controlled in 1/2-second increments, so I decided to use 7- and 4-second exposures for Beta Pic and Alpha Pic respectively, which translates to a factor of 0.571. This was very close to the calculated brightness factor of 0.597 and still significantly shorter than the 55 thirty-second exposures I used for the first image on November 16th.
For this second image, I recorded 344 images of Beta Pic at 7 seconds each, and 299 four-second exposures of Alpha Pic. I next calibrated both sets of images using darks and then stacked them separately in RegiStax (www.astronomie.be/registax). I then subtracted the Alpha Pic image from the Beta Pic image using PixInsight (http://pixinsight.com), and also created an "absolute difference" image between the two.
I found this absolute difference image easier to work with, but the subtraction image was important as a reference to examine which of the two images had contributed the various parts of the difference.
I created the "natural" appearance of the final image by taking the original stacked Beta Pic image and then blending in the central parts from a stretched version of the absolute difference image that showed the dust disk. I decided to also keep the black-spot result out of the difference image because the contrast with the protruding disk was more visually appealing--there was no occulting disk involved when recording the images. My new image was much better than my first attempt. The higher number of subframes (344 versus 55), coupled with the shorter exposure times, contributed greatly to its improvement.
Once I calibrated and combined all my data, I used the popular CCD imaging program MaxImDL (www.cyanogen.com) to analyze the final results. I first plotted the area intensity immediately around Beta Pic, shown at the top of the previous page. The circular plateau in the center corresponds to the saturated area caused by Beta Pic itself; the narrow trough immediately surrounding it is an artifact of the image processing. The debris disk is visible as the elevated red areas on each side of the star.
I then analyzed the image in MaxIm DL using the program's line profile tool. This feature works by displaying the intensity value of pixels between any two points. The two graphs on the facing page show the pixel intensity measured both through the debris disk plane (upper-middle panel) and perpendicular to it (lower middle). Because we know Beta Pic's distance, we can convert the angular scale on the sky into astronomical units (a.u.), which is plotted on the horizontal scale. The area saturated by Beta Pic itself is highlighted on the plots. Analyzing the second plot, the debris disk appears to extend roughly 250 to 300 a.u. before it becomes lost in the background signal.
Measuring the sky quality at my observatory, I have determined that the limiting magnitude with my ToUCam is about +20. So how far out should the debris disk theoretically be visible in my image? Based on the measurements of the signal intensity in the 1984 paper by Smith and Terrile and the limiting magnitude of approximately +20, the debris disk should be visible out to somewhere around 250 to 300 a.u. This corresponds well with my measured result shown below.
The disk surrounding Beta Pic is most prominent in near-infrared and longer wavelengths. The Sony CCD chip in my camera is very sensitive at near-infrared wavelengths, perhaps on par with CCD detectors available today. And while the detector uses a Bayer matrix of color filters over each pixel to record accurate color images, the process of modifying the camera for astronomical use removes the camera's built-in infrared blocking filter, requiring an additional one to be placed elsewhere in the optical path to take accurate color photos. Since my main goal was to image the disk rather than producing an accurate color image, I chose to forgo use of an infrared-blocking filter so that every pixel would still include a strong signal from near-infrared wavelengths.
After publishing this image, I was contacted by several of the scientists involved in studies of the Beta Pic disk and circumstellar disks in general, including congratulations from Alain Lecavelier des Etangs, one of the scientists behind the 1993 paper that inspired me to attempt this image. It has been very rewarding to see how professionals acknowledge and celebrate the achievements of amateurs who push the limit. I am very grateful for and impressed with all the comments and interest I have received from amateurs and professionals from all around the world.
I hope my success encourages other amateurs to go off the beaten path and try to photograph some of the more unusual things thought to be beyond our reach. Although Beta Pic is only visible from the Southern Hemisphere, many other exotic objects, such as gravitational lenses, distant quasars, and relativistic jets emanating from the centers of massive galaxies could be within reach of motivated astrophotographers. These obscure targets often have very interesting stories to tell.
Astrophotographers should consider targeting nearby stars with known debris disks. Small targets such as these have a fascinating story to tell.
For more images of exotic deep-sky objects such as galaxy clusters, gravitational lenses, and quasars, visit Rolf Wahl Olsen's website at www.rolfolsenastrophotography.com.
|Printer friendly Cite/link Email Feedback|
|Title Annotation:||Amateur Achievement|
|Author:||Olsen, Rolf Wahl|
|Publication:||Sky & Telescope|
|Date:||Aug 1, 2013|
|Previous Article:||The sky within your eyes: eye aberrations can have a big effect on visual observing. Here are some tips for seeing the universe more clearly by...|