Advanced Processing Techniques

I have been trying to find a way to remove the characteristic banding visible in my images.  For example, see the bottom of the Coma Cluster page.  There are some solutions, such as wavelets, but most erode the fine detail in the image.  I am now developing a method based on subtracting a 'flat field' image, that is, an image made with uniform lighting from the main image.  I have now used this on my recent image of M51.  This is the M51 image before this subtraction.

The banding is visible in the brighter parts of the galaxy.  There was some wavelet noise reduction done, but it was kept light in order not to destroy details.  Another consequence of the banding is that it limits the extent to which unsharp masking can be applied.  Any attempt to emphasise detail results in the bands being emphasised as well.

This is the same image after subtraction of the 'banding flat'.

The banding is now almost unnoticeable, with no loss of detail.  In fact, unsharp masking (with the bright stars de-selected) done after the banding subtraction has improved detail.

The steps involved are as follows;

1.  Take the flat field image.  I directed the telescope at a random spot in the sky with some sky-glow.  The focus was deliberately left way off.  Long exposures (>100) were taken ensuring that the frames are not saturated and that the minimum levels were about 50, or more.

2.  Using the same settings, I covered the aperture of the telescope, and took a number of dark frames (about 50 should do).

3.  In K3CCDTools, I extracted the Y component from both sets of frames.

4.  In Iris, the dark frames are averaged and then subtracted from the flat frames.  The dark-corrected flat frames are summed and exported as a RAW file to Photoshop.

5.  In Photoshop, some positive curve stretching and then a slight negative curve stretch is applied to the flat to bring out the banding.  This is what it looks like.  OK, you can hardly see the banding - but that's the point.  It needs to be pretty subtly done.

6.  This is then converted to 8 bits, and copied as a layer on top of the luminosity image of M51.  Set the blending mode to Difference, then do a Free Transform on the banding flat layer.  Nudge it around with the arrow keys until the banding on both images tend to cancel out.  Adjust the brightness and contrast of the banding flat layer to get the best effect.  The result is shown here.

7.  This resulting luminosity image is then used as described in the main Image Processing pages.