Identifying Astigmatism in your microscope

After all of the engineering and testing which goes into a microscope, you’d imagine that any manufactured scope would have perfect alignment of the optics. Believe it or not, in many cases things can be off! So how can you tell this? A simple test is to view a bead slide, or a grid slide, and run the focus up and down through the sample’s best focal plane. What you should see is clear defocused rings or bars which expand away from the object as you drive away from either side of the focal plane.

Running through the focus quickly will reveal astigmatism:

• Does the defocused haze seem to move from right to left as you drive into the focal plane, and continue traveling in the same direction as you focus past the focal plane? This can indicate astigmatism or sample alignment problems.
• Do your out of focus regions seem to defocus into horizontal lines on one side of the sample, and then defocus into vertical lines on the other side? If so this can indicate astigmatism.

In either case this is a limitation to the amount of clarity your optics can resolve and should be addressed.

You can further isolate the issue by switching objectives and running the test again.

• If the problem remains, the problem is not related to a single objective, but is either due to the optical components inside the scope, or due to the stage not installed at perfect level (this is actually quite common!).
• If the problem is only visible in one objective and not in any others, the problem is in the objective affected. (If it’s an oil lens first try giving it a good cleaning! old oil can build up on the lens and cause this type of behavior!

-Austin

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