Reflecting Telescopes

(or reflectors) Many of the difficulties with refracting telescopes are avoided with reflecting telescopes. In reflectors, the objective lens is replaced by an objective mirror. With a mirror, there is no problem of chromatic aberration, since light of all wavelengths is reflected at the same angle. The mirrors are made by shaping and then polishing a large piece of glass. While the polished surface has some reflective ability, it is not enough for a good mirror. Therefore a thin layer of reflecting material is deposited on the surface (aluminization).

Since the light doesn’t pass through the glass, the requirements are for a good surface, not a good volume. Moreover, the glass can be supported from behind. It is therefore possible to make reflectors larger than refractors.

One advantage of the wave nature of electromagnetic radiation is that the radiation is essentially unaffected by objects much smaller than the wavelength. When electromagnetic waves reflect off a metal surface, they do it by inducing an oscillating current in the surface. This oscillating current then produces the reflected wave.

If the surface is much smaller than the wavelength, there will not be enough room to produce a reflected wave at this wavelength. This means that to have good image formation, the surface of the mirror must be perfect to within approximately λ/20, where λ is the wavelength of the light being observed.

Various shapes are possible for the mirror. Since that the spherical mirrors are the easiest to grind, and to benefit of the properities of the parabola in ray cocentration, this makes the paraboloid is perfect for this purpose.

Certainly, I can provide the advantages and disadvantages of using a prime focus arrangement in telescopes based on the provided text.

Advantages:

1. No Additional Mirrors: no need for additional mirrors beyond the objective mirror. This means there are no extra surfaces for light to pass through, reducing the potential for image distortion and light loss.

2. Fast System: is considered "fast" because they have a small focal ratio. This allows for shorter exposure times in astrophotography and provides a wider field of view, which is advantageous for capturing a larger portion of the sky.

3. Large Field of View: The prime focus setup can offer a large field of view, making it suitable for capturing wide-angle shots and observing larger areas of the night sky.

Disadvantages:

1. Blockage of Objective: the eyepiece and observer need to be placed between the stars and the mirror. This positioning may partially block the objective, leading to some obstruction of incoming light. While this might not be significant for smaller telescopes, it can reduce the effective aperture for larger telescopes.

2. Observational Limitations: the observer and eyepiece are positioned near the mirror, which can restrict the angles at which you can observe celestial objects. This may limit the types of objects that can be effectively observed with this setup.

3. Complexity for Large Telescopes: For very large telescopes, the blockage of the objective can become significant, impacting the telescope's light-gathering capability. In such cases, additional optical elements or design considerations may be necessary to minimize the impact of this blockage.