Unpeeling Observations

Spiral galaxy NGC 2997

Objects detected at resolution scales 1 and 2

Difference between orignal and these objects

Observation and analysis requires separating out the different aspects of what we see. More often than not, this involves parts of what we see which are on different scales. Wavelet analysis offers a very good way to do this.

The wavelet transform is shown in the following sequence of images.

Commands used

To detect structures at levels 1 and 2, i.e. essentially stars, we use
mr_detect -n 7 -L 2 ngc2997.fits im_stars.fits
Options -n 7 -L 2 indicate, respectively, 7 scales for the analysis, and scales 1 and 2 for object detection.
To detect larger-scale structures, we use
mr_detect -n 7 -F 3 -L 6 -K ngc2997.fits im_gal.fits
The option -K keeps objects close to the image boundaries which would otherwise (for safety) be ignored, and the -F 3 option implies scale 3 is the starting scale used for object detection.
Extended structures, as shown in the animation, are obtained by
ngc2997 - im_stars (for example, in IDL, followed by writing of the images as GIFs).

To probe further

Image and Data Analysis: The Multiscale Approach, J-L Starck, F Murtagh and A Bijaoui, Cambridge University Press, 1998.