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