A ronchigram is essentially a CBED pattern of your sample.
In terms of STEM usage, the most important part of it is the centre part which is 'flat' looking. External warping of the ronchigram is due to aberrations in your microscope. (Which can be reduced using aberration correctors)
During STEM, the beam is rastered across your sample, and the intensity of the CBED pattern (at a particular angle) is taken and an image is formed from this.
A smaller condenser aperture can be used to select the part of the ronchigram which has no aberrations, making your image look better.
However, smaller apertures also lead to diffraction limited imaging, meaning you will lose some resolution that way.
A ronchigram is essentially a CBED pattern of your sample.
In terms of STEM usage, the most important part of it is the centre part which is 'flat' looking. External warping of the ronchigram is due to aberrations in your microscope. (Which can be reduced using aberration correctors)
During STEM, the beam is rastered across your sample, and the intensity of the CBED pattern (at a particular angle) is taken and an image is formed from this.
A smaller condenser aperture can be used to select the part of the ronchigram which has no aberrations, making your image look better.
However, smaller apertures also lead to diffraction limited imaging, meaning you will lose some resolution that way.
When we put the electron microscope in the STEM mode,with the scan switched off,and put the condenser lens,aperture and stigmator,objective rotation centre in control.Now examine the phosphor screen at the bottom end of the microscope.The rays coming out of the sample are conical in shape formed by the condenser aperture.this cone hits the phosphor screen which forms a bright image called as ronchigram.You may find detailed answers for the size of apertures on the following link.
As Jyoti wrote, you may find a quite nice description of TEM/STEM and the Ronchigram on the site of Rodenburg: http://www.rodenburg.org/STEM/t200.html. I started with this page several years ago and found it quite useful.
Basically, I use the Ronchigram for the alignment when doing STEM.