Removing patterned interference from images with FFT

[smerral]

Sometimes our images are spoilt by annoying lines. This can be caused by having the gain level set high, which itself might be necessary when we are trying to capture deep sky objects. They are difficult to remove without affecting the underlying image. Here's where the FFT filter comes in. It is an ideal tool for removing regular patterned interference. For those with photoshop, this tutorial shows how to use FFT as a free plugin. First, download the zip file:

http://home.planet.nl/~ber03728/4N6site/improc/fftplugin/getit.htm

Unzip it and copy the file FFT.8bf to C:/program files/adobe photoshop C5/plug-ins/filters.

For this tutorial I am going to use Dion's original image of M82. You can see the diagonal lines superimposed on the galaxy and also on the amp glow top left.


Open your image in photoshop. Go to filters, forensic filters and FFT:


(Ignore the Inverse FFT for now, we'll use that later)

Now you will see this:


Go to the channels tab in the right hand pane and select the red channel:


You now have this:


UPDATE: If you cannot see the stars in the red channel, try the green channel. It will work just as well:

Now using the paint brush paint out anything that looks like a star or a white point. DO NOT TOUCH THE CENTRAL STAR.


Click on the RGB channel again under channels:


Now go to filters again and this time INVERSE FFT:


You can see the lines are gone but we have a black and white image. Select all of this and copy to the clipboard. Then close WITHOUT saving (or save as a separate filename if you want it for future reference). Open up the original image. At this stage you can go to filters and apply a small amount of gaussian blur. It's up to you - it won't affect the final sharpness as the detail is in the luminance of the clipboard image.

Next, click edit, paste special and paste in place. Now the image is monochrome again.
One final step. Click the layers tab and set the drop down menu to luminosity:


And finally:


That's it! Now just save your image and you're done!

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