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Rolf Olsen | all galleries >> Astrophotography >> Stars > Annotated crop of the Beta Pictoris debris disc
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Annotated crop of the Beta Pictoris debris disc
04-DEC-2011

Annotated crop of the Beta Pictoris debris disc

See the main Beta Pictoris page for full details: http://www.pbase.com/rolfolsen/beta_pictoris

This is an annotated crop of the final image which shows the extent of the debris disc on both sides of Beta itself.
This image shows the famous circumstellar disc of debris and dust orbiting the star Beta Pictoris 63.4 light years away. This is a very young system thought to be only around 12 million years old and is essentially similar to how our own Solar System must have formed some 4.5 billion years ago. The disc 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.
For the last couple of years I have been wondering if it was possible for amateurs to capture this special target but have never come across any such images. The main difficulty is the overwhelming glare from Beta Pictoris itself which completely drowns out the dust disc that is circling very close to the star. Images of the disc taken by the Hubble Space Telescope, and from big observatories, are usually made by physically blocking out the glare of Beta Pictoris itself within the optical path.
But I then found this excellent 1993 paper 'Observation of the central part of the beta Pictoris disk with an anti-blooming CCD' (Lecavelier des etangs, A., Perrin, G., Ferlet, R., Vidal-Madjar, A., Colas, F., et al., 1993, A&A, 274, 877)
Full article available here: http://adsabs.harvard.edu/abs/1993A%26A...274..877L

I realised that with this technique it might not be entirely impossible to also record the debris disc with relatively modest equipment. I followed the technique described in the paper above, which basically consists of imaging Beta and then taking another image of a similar reference star under the same conditions. The two images are subtracted from each other to eliminate the stellar glare, and the dust disc should then hopefully reveal itself. However, since the two stars have different magnitudes I needed to calculate how long to expose Alpha for in order to get a similar image which I could subtract from the Beta image:

The magnitude difference between the stars is 3.86(Beta) - 3.30(Alpha) = 0.56
Due to the logarithmic nature of the magnitude scale we know that a difference of 1 magnitude equals a brightness ratio of 2.512. Therefore 2.512 to the power of the numerical magnitude difference then equals the variation in brightness.
2.512^0.56 = 1.67, so it appears Alpha is 1.67 times brighter than Beta. This means that exposure for Alpha should be 1/1.67 = 0.597x that of Beta.
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 required brightness factor of 0.597.

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. The absolute difference image is simply easier to work with, but the subtraction image was important as a reference to examine which of the stars had contributed the various parts of the difference. It was 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.


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