There are a few drawbacks to the reflector

There are a few drawbacks to the reflector, though they are normally far outweighed by its advantages. Next I shall explain the disadvantages and ways in which they may be minimized.

First, light can only reach a reflector’s objective mirror after passing by the secondary mirror, thereby causing a special kind of light scattering called diffraction. Diffraction causes your view to loose some of its sharpness. The larger the secondary mirror is relative to the objective, the more diffraction there is. It is possible to buy a reflector where the secondary’s size has been kept to a minimum. There are easy methods to identify a telescope with a minimum sized secondary and I shall explain these methods to you soon. It is the diffraction of a secondary mirror in reflectors which keeps these telescopes from matching the image quality of the apochromatic refractor.

Second, the secondary mirror blocks some of the light from the objective. This is not really a big issue given that even an oversized secondary will typically block less than 10% of incoming light.

It is the two above mentioned drawbacks which allow an apochromatic refractor to always out-perform a reflector of the same size. Though in many situations the extra performance edge of the refractor will be of little importance, in some applications it may make a big difference.

Lastly, reflectors require periodic adjustment of the objective mirror. The manual which comes with the telescope will usually tell you how to perform the adjustment which is called collimation. In contrast, the objective lens of a refractor is typically supposed to be set into its proper position at the factory and may never need changing. However, it is my own opinion that the collimation of a refractor's objective should be checked at least once, right after it is purchased, and corrected if necessary. I say this as one who has noticed too many refractors arriving from the factory improperly collimated. Since refractors do not loose their collimation as easily as reflectors, that one adjustment may be the only one ever needed. Never-the-less, adjustment of a reflector telescope’s objective mirror is a relatively simple matter requiring NO advanced equipment.

Let us continue our discussion about types of telescopes.

I mentioned earlier that apochromatic refractors excel in showing details on the planets of our solar system and on other objects which are relatively close to Earth. But the reflector has one BIG advantage. It shines in its ability to view deep sky objects. Why? Because you can get a reflector with a HUMONGOUS objective for much less money than you would pay for a small apochromatic refractor! In illustration, let us compare two telescopes: a four inch apochromatic refractor and an eight inch reflector.

The smaller apochromatic telescope will usually cost three to four times the price of the reflector and give you razor sharp views of the planets. On-the-other-hand, the reflector will collect four times as much light as the refractor (Not twice as much light! Remember Rule #3!). In other words, because it is collecting so much more light, the reflector will allow you to see objects you could not even glimpse through the refractor!