Celestron #44301 Instruction Manual page 9

Measuring Function — DMS has a simple measuring function which measures
the distance between two points in millimeters. Click the "Measure" button
on the bottom right of the screen. Click where you want your starting point
and then click again to set the ending point and it will display the distance
between the two points. You can adjust either point by dragging the pin. You
can click "Clear" to start over.
The measurement function works accurately when the microscope is directly
on the object.
You are now ready to begin your digital adventure!
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