3.6 – CTF Correction
Point spread function – PSF
-If you had a single point and imaged it, due to inherent issues, point is blurred, maybe surrounded by a halo
This function is true for each point before CTF isues
You can use stuff like gold beads which are single points and then are surrounded by halos and peaks
PSF is a Fourier transform of the CTF (and vice versa)
Remember, the Fourier transform of a gaussian function is another gaussian function
A broad G function because a narrow one, vice versa
A wave becomes a curve with bumps afterwards
PSF is identical for all points of high density in the image
--Called convolution
Image = Object (convolved with) PSF
Image = inverse fourier transform(Fourier transform of the object * CTF)
Here we go
Object = Inv F tfm ( F tfm (image)/CTF)
So, if the CTF is known, then math can be used to pull it back out
basically, take the FT of the image, divide by CTF, and then inverse FT and you will get the object
well, a closer version
Called CTF correction
Can’t multiply by zero when CTF crosses axis
So, can just exclude those
Or, can “phase flip” elements that have a negative phase in the CTF
To do all this, you need to fully understand the CTF
