Differences between revisions 23 and 24
 ⇤ ← Revision 23 as of 2010-01-01 20:08:02 → Size: 1727 Editor: penczek Comment: ← Revision 24 as of 2010-01-01 20:12:58 → ⇥ Size: 2431 Editor: penczek Comment: Deletions are marked like this. Additions are marked like this. Line 25: Line 25: = Description = . 1. The Contrast Transfer Function of electron microscope has the following form: . {{{`"CTF"(K)=sin(-pi*("defocus"*K^2-"Cs"*lambda^3*K^4/2.)-phi)*e^{(-1/4*"B_factor"*K^2)}`}}} . {{{`K=sqrt(K_x^2+K_y^2+K_z^2)`}}}  . {{{`phi="atan"("Q"/(1-"Q"))`}}} [See reference 1 and 2] . When {{{`K=0`}}}, {{{`"CTF"(K)=sin(-phi)="amplitude_contrast_ratio"`}}}  = Reference =1. J. Frank ''Optik'' (1973) 38:5192. R. Wade & J. Frank ''Optik'' (1974) 49:81.3. Z. Huang, P. R. Baldwin, S.Mullapudi, and P .A. Penczek, Automated determination of parameters describing power spectra of micrograph images in electron microscopy. ''J. Struct. Biol.'' '''144''' (2003), pp. 79-94.

# How to use CTF in SPARX

In SPARX, the EMAN CTF object are used to store and apply all CTF parameters.

• To generate a CTF object, use function [:generate_ctf:generate_ctf(p)], here p is a list of parameters [defocus, cs, voltage, apix, bfactor, ampcont]. Their definitions are:

defocus

defocus in `µ`m. Positive number means underfocus.

voltage
microscope voltage in kV (default 300 kV).
apix

pixel size in `Å`.

cs
Spherical aberration constant in mm. (Default 2.0)
amp_contrast
Amplitude contrast ratio, unitless, given as percentage of (0,100). (Default 10%)
bfactor

B-factor defining exponential decay of Fourier amplitudes decay `Å^2`. (Default 0.0, i.e., no decay).

Notice: In some other software packages, the defocus value is in Å and amplitude contrast ratio is in the range of (0, 1). This function will automatically determine whether these two CTF parameters are in this convention and convert them into our convention.

• To apply CTF object to an image, use [:filt_ctf:filt_ctf(img, ctf)], where ctf is a CTF object.

• To put a CTF object into the header of an image, it can be done in two ways:
1. use function set_ctf(img, p), where p is a list of parameters [defocus, cs, voltage, apix, bfactor, ampcont].

2. if the CTF object already exists, one can use img.set_attr("ctf", ctf) directly.

• To recover the numerical values of CTF parameters from CTF object stored in the header of an image, use function p=get_ctf(img).

• To access or set individual parameters in a ctf object, use:
• bfactor = ctf.bfactor
• ctf.bfactor = 200.0

# Description

• 1. The Contrast Transfer Function of electron microscope has the following form:
• `"CTF"(K)=sin(-pi*("defocus"*K^2-"Cs"*lambda^3*K^4/2.)-phi)*e^{(-1/4*"B_factor"*K^2)}`

• `K=sqrt(K_x^2+K_y^2+K_z^2)`

• `phi="atan"("Q"/(1-"Q"))` [See reference 1 and 2]

• When `K=0`, `"CTF"(K)=sin(-phi)="amplitude_contrast_ratio"`

# Reference

1. J. Frank Optik (1973) 38:519

2. R. Wade & J. Frank Optik (1974) 49:81.

3. Z. Huang, P. R. Baldwin, S.Mullapudi, and P .A. Penczek, Automated determination of parameters describing power spectra of micrograph images in electron microscopy. J. Struct. Biol. 144 (2003), pp. 79-94.

CTF_info (last edited 2017-05-07 11:51:17 by penczek)