Celestron Advanced Biological Microscope 500 Microscope Basics - Page 15

Focus Systems A focus control allows you to adjust the focus of the microscope. Every microscope includes a focusing control (knob) for quick (coarse) focusing of the image. More expensive compound microscope models include a coarse (quick) and fine focusing control. The fine focus is particularly advantageous in high power applications and required for 400x and higher but is not available on stereo microscopes since they are only low power. Coarse Focus Knob (Top) Fine Focus Knob (Bottom) Coaxial Coarse/Fine Focus Knob Coaxial controls (focus) are where both the course and fine focus knobs are on one large control (on top of each other). The larger knob is typically for the coarse focusing and the smaller knob for the fine focusing. The smaller knob is usually centered on the inside of the larger one. Focusing mechanisms used in microscopes are rack and pinion type. This is a design involving the intermeshing of a geared wheel and matching grooved rack. The rack is a track with teeth and the pinion is a gear that rides on the teeth. By turning a knob, the pinion gear moves along the track. A safety rack stop (rack stop or stop screw) is an adjustable screw located at or near the base of a focus mechanism. When adjusted properly, it prevents the body tube from lowering too far and potentially causing damage to both the high power objective and/or the specimen. Most microscopes have a slip clutch which allows for continuous turning of a focus knob when it is all the way up or down without damaging the gear system. Focusing can take place in one of two ways. The stage moves vertically to bring it nearer or further from the objective which remains fixed in place. The other method is the drawtube moves vertically with the objective and nosepiece while the stage stays in place. Head (Body) The head is the upper part of the microscope that connects the eyepiece to the nosepiece or turret. Some heads are fixed in place and allow you to tilt them from angles of 0° up to 60°. More expensive microscopes usually have heads that can be rotated 360° allowing two or more users to see the specimen without the microscope itself having to be picked up and rotated. There are several types of heads: Monocular - this is a microscope with a single eyepiece. These types are the more economical models and are very satisfactory for their usage. A monocular head with a second vertical viewing port is called a teaching head (dual view head) which can be used by a second person (or teacher) to observe the same image as the first person. Or, the second port can be used with various cameras. Binocular - this is a microscope with two eyepieces, one for each eye. They are generally used on high power compound microscopes and all low power stereo microscopes and are generally more comfortable to use than a monocular type. There are different types of heads for adjusting the interpupillary distance (IPD) - see description below -- and the more common types are (1) seidentoff - IPD is adjusted by twisting the eyepieces in an up and down arc motion similar to most binoculars and (2) slider - IPD is adjustable side-to-side by sliding the eyepieces toward and away from each other. 15