Section 1: Why not mirrors?
Many amateur astronomers believe that mirrors are better optics than lenses, because all the problems resulting from the dispersion of light in glass (i.e. "color error") are completely absent in the case of mirrors. Light does not penetrate the mirror, instead, it is reflected from the glass surface (coated with some highly reflective coating, like aluminum), so there is no false color in a telescope containing only mirrors.
Unfortunately, we believe that even if telescopes using mirrors are essentially color free, they are not the ideal telescopes for the following reasons:
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- They usually have some central obstruction, blocking the light in the middle area of the main mirror.
- Most of the systems have a „spider” in the light path, and its legs cause diffraction spikes in the image.
- They are very sensitive to virtually any mechanical or thermal problem that a telescope must face under the starry sky.
- Mirror surfaces are due to degrading as time goes on, reducing reflectivity
- Mirror surfaces usually have lower reflectivity rate than the amount of light that goest through a lens
- Cleaning a mirror is much more difficult then cleaning a lens, so in average, mirror scopes have much more dirt on their optics, ruining fine planetary detail
- Handling stray light in a mirror telescope is very difficult, nearly impossible.
Lets’s examine some of the problems in details!
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