|Noise is the bane of all astrophotos, and like photos in
general it is more prominent in the dimmer areas of an image. Examine
the image below and the two crops, one from the upper left corner and
one from a brighter section of the nebula.
|The crops clearly show that the core has a much higher SNR
than the dimmer outer sections of the image. For the noise reduction
I'll use the frequency filter tool in Images Plus to apply a lowpass
filter to the image.
As you see the image noise is now gone, but the detail is
pretty much gone from the image. Next we make an inverted luminance
mask from the blurred image. When we apply the mask to the blur
operation it allows the blur to show through where the mask is light
and where the mask is dark the original unblurred image is visible.
|Using the mask surface tool in Images Plus to apply the mask
to the blur produces the masked noise reduced image. The same technique
can be applied using Photoshop or most other image processing packages.
|This produces the noise reduced image shown below with a
smoothed background and an unblurred core. Compare the final image
below to the initial lowpass filtered image.