The objective lens

The objective lens is the most important lens in the whole microscope.

All the lenses below the specimen serve to magnify the image of the specimen. Lets just think about the ray diagram shown below.

The magnification of objective lens is determined by the ratio of the distances between its object plane (the specimen) and its image plane (what we have called ‘the first image plane’). Think of the levers in that were introduced in section 3.

But we see that if the magnification is large (and in practice, it is generally a factor of 20-50), then the range of angles of rays impinging upon the first image plane is very small relative to the range the angles entering the object lens from the specimen. Of course, other lenses below the objective lens (which are in the projector system) also magnify, and as the magnification increases, then the range of angles that each subsequent lens must deal with is reduced and reduced.

Now, the argument goes like this. Because aberrations are worse at high angles than at low angles (with haven’t proved this, but it is generally true), then the lens which will be most affected by aberrations will be the objective lens, because it has to deal with biggest range of angles in the whole microscope. That means that our biggest single concern in electron microscopy should be to make sure that the objective lens is perfectly aligned and stigmated. In fact, if the objective lens is mis-aligned, the errors it introduces will so huge that no matter what we do with the other lenses, we will never be able to correct the error.

(In fact, this argument is not the whole story. A little thought will show that the final lens in the projector system must be dealing with a rather large range of angles if it is going to be able to illuminate the whole of the phosphor screen. However, all of these beams are pencil- thin rays – they are called principal rays – and the effect of aberration in the projector system is to distort the image but not to affect resolution. In a modern microscope, the projector system is balanced to minimise distortion. To all intents and purposes, only the performance of the objective lens really matters.)

Remember: The objective lens is the most important lens. The condenser lenses are the next most important, because they determine how the specimen is illuminated with electrons. The first lens of the projector system (sometimes called the 'intermediate' or 'diffraction' lens) is the next most important, and matters a little bit. We can forget about all the other lenses. Even though they provide a huge amount of magnification, they have virtually no influence on our scientific results.

(We assume here that the projector system is aligned OK: this alignment is important for defining the centre of the phosphor screen, but is usually only undertaken by the site engineer and is usually pretty stable over months or years. If features move from the centre of the screen as a function of magnification or camera length, then this implies that the projector system needs alignment.)

So, how do we align the objective lens?

Well, since the objective lens is the most important lens, we never choose to align its shift. Instead, we define the centre of the lens as being on the optic axis. So, that’s easy: by definition the objective lens is always on-line!

Now all we have to do is line up the rest of the microscope around the objective lens. To define the line of the optic axis, we draw an imaginary line between the centre of the objective lens and the centre of the phosphor screen. Because the phosphor screen effectively images the first image plane (via the projector system, which we have agreed is not very important), the centre of the phosphor screen is also, by definition, on the optic axis. The shrewd reader will observe that it must be necessary to align the objective with the projector system. This is done in a full alignment (both mechnanically and with the image shift coils), but we don’t have to worry about it under normal circumstances.

So, what’s left?

All that remains is to get the condenser system shooting the beam right through the centre of the objective lens and parallel to the optic axis.

It turns out that the easiest way to do this is to change the excitation of the objective lens and see if the image moves. We said it before, but we’ll say it again:

Remember: If you want to test the alignment of a single lens, alter its setting (i.e. its strength or excitation) and see if anything moves.

Actually understanding why the image moves if the illumination is not coming straight and parallel into the objective lens is not completely easy. Several things are going on. At this stage, you don’t need to understand this, so feel to skip to the next ‘Ask the demonstrator’.

Aside: We so far haven’t discussed yet another very important difference between electron lenses and optical lens. Electrons lenses rotate their images as well as focussing and magnifying them. If we adjust a lens back and forth in strength (which is called ‘wobbling’ a lens), the image will move unless we are looking at the optic axis of that lens. As a first approximation (but see next paragraph), this is a good way of steering the incoming beam onto the optic axis of the objective lens. What we find is called the ‘current centre’ of the objective lens. (‘Current’ because we wobble the current going through the lens.)

Tilting the illumination has other effects. The direction of the incoming beam makes the pattern of radiation downstream of the specimen be at an angle relative to the optic axis. As the objective lens is changed, we focus on different layers of this radiation pattern, which will appear to shift laterally because of the tilted inclination of the illumination. In fact, to correct this misalignment, it is best to tilt the illumination back and forth between opposite degrees of tilt, and try to make the image of something like a thin carbon film roughly identical at both illumination angles. What we find is called the ‘coma- free axis’.

Another thing we could try doing is wobbling the voltage of the incoming electrons. When their voltage is changed, their velocity is changed, and so is the strength of their interaction with all the lenses in the electron microscope. This is good way of getting the condensers and projectors all lined up with most important lens, the objective. What we find, as we tilt the beam to minimise any image shift, is called the ‘voltage centre’ .

Yet another important axis is defined by the what’s called the ‘reversal centre’ of the lens, which is that point in the image plane that stays stationary when the current flowing through the objective is reversed. You would think that the reversal centre would be the same as the current centre, but in fact if the polepieces of the objective are not perfectly aligned to the optic axis (and in practice, they never are) then there is always a slight discrepancy.

In an ideal world, the current centre, the coma-free axis, the voltage centre and the reversal centre would all coincide. In fact they quite never do. But you don’t need to worry about these things, unless you are really pushing for the very highest resolution.

:end of aside

For normal imaging, what we actually do is ‘wobble’ the strength of the objective lens (make it stronger and weaker periodically) and adjust the tilt of the illumination beam (see the figure below) until the image appears stationary.

Ask the demonstrator: To check the condenser alignment and pivot points and to identify the rotation centre alignment, or objective wobble, and whatever alters the beam tilt – probably the same old multi-function knobs. Also ask to be shown the objective lens stigmator control – probably also on the wretched multi-function knobs.

Experiment: Looking at a good contrast bright-field image, say of gold islands on a holey carbon film, focus the image as well as you can. Try altering the objective stigmator. The objective stigmator works just like the condenser stigmator, but because you are now looking at an image, and not a nice sharp single object, like the filament, it is not so easy to see what you are doing. Remember to alternate between focus, stigmator x, focus, stigmator y, focus, stigmator x, and so on. Try to make the image as clear as possible. It won’t look good because we haven’t learnt about the objective aperture yet, which is also important, and which will also affect the image quality. Just try to see a reasonably sharp picture.

When the objective stigmators are roughly okay (i.e. there is no obvious blurring in one direction or another as you change focus), try wobbling the objective lens. That is to say, correct the rotation centre. The multi-function knobs will now affect the tilt of the beam by adjusting the double- deflection coils between C2 and the specimen. This is the error we are trying to correct:

It is as if we are physically tilting the whole condenser system until the illumination is parallel with the optic axis. Practice turning the knobs until the image appears stationary. It may help to do this using the binoculars on the small phosphor screen. Ask the demonstrator if you don’t know how to use the binoculars.

At the beginning to this section, the demonstrator checked the ‘pivot points’ before we tried to adjust the rotation centre. This is an important detail that we shortly have to learn about, but first lets just think about the position of the specimen...

Copyright J M Rodenburg