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