But each of the previously stated rules can be made irrelevant if Rule 4 is not followed:  

 

 

Rule #4       A telescope should have quality optics.

 

Needless to say, if the optical quality of the lenses and/or mirrors in an instrument is poor, it will perform poorly.  But how can you tell how good the optics are without a degree in physics?

 

You don’t need to run a battery of optical tests on a telescope yourself, there is one manufacturer’s specification which should insure optical quality.  Look to see if the manufacturer says that the optics are “guaranteed diffraction limited” or “corrected to 1/8 wave or better”.  Most manufacturers with this guarantee will actually state it in their advertisements.

 

In case you didn’t know, a beam of light travels in the form of waves as shown in Figure 1.

 

                                                            

 

Figure 1: A light beam is composed of waves

 

 

Defects in optical surfaces are measured in terms of the length of a wave of yellow light.  An optical surface corrected to 1/8 wave indicates that there is no defect (that is, an irregular bump or hole) on the reflecting surface which is greater than one eighth of a wavelength of yellow light or 0.0000026 inches.  This amount of correction insures that the telescope is capable of  resolving down to Dawes’ Limit.  While a correction of better than 1/8 wave won’t increase the resolution of a telescope, it may help you see more clearly by heightening contrast in an image.

 

Rule #5:      The type of telescope you get should depend on what kind of observing you will do most of the time.

 

Some observers prefer to look at planets, some especially like stars, while others prefer galaxies, etc.  Many prefer to gaze directly at objects, while others wish to photograph them.  Still others may wish to attempt to discover a comet.

 

 

 

copyright 2004 Singularity Scientific

 

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