Vignetting

Vignetting: A reduction of the amount of light reaching the focal plane by objects intruding into the light path.

Vignetting occurs when the cone of light strikes an object in it's path before it reaches the eyepiece. The most common problem area is the inside diameter of the focuser. To see where the light cone might strike the focuser, use the Create Focuser Window option in the VIEW menu.

A tall focuser increases the probability of vignetting. A narrow focuser does the same thing. Many telescopes are built with the standard 1-1/4 inch by 3-1/2 inch focuser. At any focal ratio less than about F 10, this can cut off a fair amount of the light cone. Using a low profile, wide focuser is one of the easiest ways to improve a telescope.

Even if the 75% zone light cone just passes through the focuser, it can help to alter the focuser. What the eyepiece sees is all of the 100% zone, all of the 75% zone, and then a very sharp falloff of light to zero. No 50% zone is available at all. This will effect a low power, wide angle eyepiece the most, because this eyepiece has a very large field lens. If the lens is wider than the 75% zone, it will get very little light near the edges of the field of view.

Another place vignetting occurs is at the front end of the tube. Notice in the example file WHITE.NWT that the 75% ray strikes the tube wall near the front. This means that the 75% zone is reduced in size, and there is a sharp cutoff of light at it's edge. To solve this problem, increase the diameter of the tube. See the example file "WHITE.NWT" for a 6 inch F8 telescope (this is the default telescope which is displayed when Newt is started).