Determination of Curvature of Image.

TM 9-258

crossline of the instrument. Now, remove the mask and

9-7. Determination of Curvature of Image.

place the cutout circle on the center of the objective.

Sharply focus the instrument on a point in the center of

Examine the image of the object. Any blurring of the

the image and note whether or not the points at the edge

image may be attributed to unfocused rays in the outer

of the image are clear and well defined. If the edges of

zones of the objective. Adjust whatever focusing devices

the image are hazy, continue focusing the instrument

there are available to sharply focus the image. The

until the edges of the image are sharp and distinct. The

amount of movement required is an indication of the

amount of focusing required is an indication of the

amount of spherical aberration present.

curvature of image.

9-2