Philips CM-20 manual iv Insertion and centering objective aperture

(5)Change camera length (by changing MAGNIFICATION knob) to 44.0 mm.

(6)Realign the diffraction pattern.

(7)Repeat steps 4 to 6 until the transmitted spot remains at (near) the center while changing camera length between 44.0 mm to 3.2 m.

(8)Deactivate ALGN, and depress D to get out of diffraction mode.

(iv)Insertion and centering objective aperture

(1)Move to the edge of the sample.

(2)Make sure aperture is out.

(3)Press D to get to diffraction mode.

(4)Put objective aperture in (lever to the left)

(5)Select aperture size (the second largest, 40 microns, is usually good)

(6)Center aperture around transmitted beam using knobs on aperture

(7)Press D to get out of diffraction mode.

(D)Astigmatism Correction of Objective Lens (section 2.5.4 in TEM manual):

Astigmatism of objective lens appears as an image blurring phenomenon which one image is accompanied by another different-focus image at right angles to each other. Namely, because of asymmetrical focus, an image does not become sharp and appears to drift in one direction. Under this condition, exact focusing is impossible. Hence, for recording a quality micrograph, it is indispensable to correct astigmatism of the objective lens.

Three techniques (for correcting objective lens astigmatism) are commonly used depend on the magnification and material.

First Method (for low magnification images, <100kx)

(1) Find a suitable size of hole in a holey carbon or any sample.

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