Hi All,
As you may have noticed I managed to capture the first amateur image of the debris disc around Beta Pictoris on 16th November 2011 (see this thread:
http://www.iceinspace.com.au/forum/s...ad.php?t=83185).
This image generated a lot of attention from both professional and amateur astronomers and was reported in the media all over the world. I had some fairly hectic days last week and I managed to get IceInSpace.com mentioned a couple of places too, for example in the
Space.com article
I have now completed a new and much improved image based on new data. The full details and a couple of different versions of the image are available here:
http://www.pbase.com/rolfolsen/beta_pictoris
After the initial success of capturing the disc I was contacted by several people in the astronomy community, among which were Dr Grant Christie of Auckland's Stardome Observatory. Following his suggestions I had a go at imaging it again using shorter exposures. This was to minimise the area saturated by Beta Pictoris itself and potentially reveal more of the debris disc closer to the star. The ICX098BQ chip in the ToUCam saturates fairly quickly for bright stars, even at very short exposures. Since long exposures with my ToUCam can only be controlled in 0.5s increments I decided to use 7.0s and 4.0s for Beta and Alpha 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 30.0s I used for the first image on 16/11/2011.
So for this second image I collected 344 images of Beta Pictoris at 7.0 seconds each and 299 images of Alpha Pictoris at 4.0s each. Both sets of images were dark subtracted and stacked separately in Registax. I then subtracted the Alpha image from the Beta image in PixInsight LE, and also created the absolute difference between the two. It is clear that there is a fairly strong signal corresponding to the exact location of the debris disc and that it is coming from the Beta image.
I created a more natural looking final image by taking the original stacked Beta image and then blending in the central parts from a stretched version of the absolute difference image that showed the dust disc. I decided to also keep the black spot of the central glare from the difference image since the contrast with the protruding disc just seems better this way. So there is no occulting disc involved, it is simply for the sake of presentation. I created a couple of versions which can be seen in the gallery below. This is a vastly better image than the first one taken on 16/11/2011. I believe the higher number of subframes (344 versus 55) coupled with the shorter exposure times are responsible for the improvement.
I used MaximDL to produce some more in-depth illustrations of what is going on in the difference image. First a area plot of the intensity immediately around Beta Pictoris. The circular plateau in the centre corresponds to the saturated area caused by Beta Pictoris itself (The narrow trough immediately surrounding it is an artifact of the image processing). The debris disc is visible as the elevated red areas on each side of the star.
Also attached are profile plots taken both through the debris disc plane and perpendicular to it. The horizontal scale is Astronomical Units and the area saturated by Beta itself is highlighted on the plots.
The visible part of the debris disc seems to extend to roughly 250-300AU before it falls below the background noise levels.
I have found that the limiting magnitude with the ToUCam from my location is around 20.0. So how far out should the debris disc theoretically be visible in my image? Attached is a plot of the magnitude pr. square arcsecond for the debris disc (
Smith, B. A. & Terrile, R. J. 1984, Science, 226, 1421 A Circumstellar Disk around Beta Pictoris)
It seems that if a limiting magnitude of approximately 20.0 is assumed, then the debris disc should be visible out to somewhere around 250-300AU. This corresponds with what is seen in the profile plots.
Finally, an annotated crop of the final image shows the extent of the debris disc on both sides of Beta itself.
Some notes on ToUCam and IR sensitivity
The disc is most prominent in IR and fortunately the ICX098BQ CCD chip is very good at picking up signals in IR, probably on par with or even better than some modern CCD's in this aspect.
Another property of the ICX098BQ that helps here is actually the Bayer colour matrix. This is because each pixel either receives only red, green or blue light due to the overlaid Bayer matrix. But since the matrix dyes are practically transparent to IR light, every pixel therefore receives signal from the IR band. So the IR S/N ratio in the final image can be assumed to be correspondingly higher than the RGB S/N. This could easily explain why the ToUCam seems to pick up IR light so well; the Bayer matrix effectively acts like a IR pass filter and allows a proportionally higher number of pixels to be IR illuminated than RGB illuminated.
While I have not tested this in practice it could mean that the limiting magnitude of the ToUCam is higher in IR than in RGB.
Hope you enjoy, I have certainly had a lot of fun with this lately.
Regards,
Rolf