Understanding How Mirrors Produce Magnified Images

When discussing the production of magnified images through mirrors, the most common mirror type is the parabolic mirror, especially in high-quality telescopes. However, there are other designs and types of mirrors that can also produce magnified images under specific conditions.

Types of Mirrors and Their Capabilities

In general, the typical answer to which mirror produces a magnified image is a parabolic mirror. However, the classification is more nuanced, particularly in certain catadioptric telescopes (a combination of mirrors and lenses), such as the Schmidt Cassegrain design.

In these telescopes, simple spherical surfaces can be used instead of parabolic mirrors, thanks to the corrector lens located in front of the mirror. This allows for a more corrective lens design that enhances the aberration-free field of view, making it comparable to the all-mirror design of the Newtonian Telescope.

Magnification vs. Focusing

It is essential to note that mirrors themselves do not inherently magnify the image. Instead, the incoming light is brought to a focal point. The magnification effect is achieved through a specialized lens, typically the eyepiece, located at the primary focal point of the mirror.

The parabolic mirror's curve and the spherical mirror's curve are actually very close, with the parabolic mirror having only a slightly deeper center than the spherical mirror. In some smaller, cheaper reflectors, a spherical mirror is often used instead of a parabolic one due to cost and ease of manufacture.

Grinding and Polishing Techniques

For individuals interested in making their own Newtonian Telescope, the process typically involves grinding the mirror to a spherical curve first, which is a simpler and less costly task. In the final stages of polishing, the mirror is deepened slightly in the center using Cerium Oxide on a specially shaped lap of pitch.

A crucial part of this process is the Foucault test, a simple test to check the parabolic shape of the mirror. The mirror is stood on its edge at its calculated radius of curvature, and a doughnut-shaped shadow is observed through the mirror. Corrections are then made if necessary.

The Role of Concave Mirrors

A concave mirror can also be used to produce a magnified and inverted image. However, for this to occur, the object must be placed between the center of curvature and the focal point of the mirror. The image produced will be real and magnified.

For those interested in learning more about the nuances of mirrors and their magnification capabilities, we recommend the following resources: Mirrors: Focus Inversion and Magnification Additional resources on telescope building and mirror making

Conclusion

While parabolic mirrors are the standard for producing magnified images in precision instruments like telescopes, other types of mirrors, such as spherical ones, can also be used in certain designs. The key lies in the specific conditions under which the mirrors are used and the additional lens systems employed for further magnification.

If you have any questions or need further information, please feel free to ask. Your interest and support are greatly appreciated!