Astronomy
The Good, Bad, and the Ugly Telescopes Now for the Bad.  Bad telescopes pose as fine instruments, and demand the same high price you would expect to pay for a fine instrument.  They are sold in department stores and camera shops.
Why are they bad?  They sacrifice image brightness for magnification that goes well beyond the limit of USEFUL MAGNIFICATION.   Useful Magnification always brings out new detail in an image as the image grows larger.  Useless Magnification exceeds the resolving power of the telescope and only makes the image larger without bringing out any new detail in the image.  In other words they are bad because they give you a big, dim, image that reveals little detail.
Longer focal lengths yield greater magnifications than shorter focal lengths do, and require much less work to make, as they don't need as much grinding.  The longer the focal length, the higher the Focal Ratio or F: number.  Thus the F: number of a telescope for a given aperture is a good clue as to the telescopes quality.
On the next page I will show you how to use that clue.