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Focal point focus and baffling


Andrés Esteban de la Plaza

Focal point focus and baffling

Almost all vintage refractor with 0.965in eyepieces have baffles positioned according to a cone of light ending in a focal point instead a focal area. When 1.25" eyepieces are used, even with a 0.965"-1'.25 adapter, we will not have the full eyepiece area (diameter=27mm) illuminated. Instead, original baffles will act as diaphragms resulting in a light cone of reduced diameter. The solution is to put new baffles or to adjust the position of the original ones in order to accept a light cone with grater diameter ending at the focal zone as a circle of 27mm. In this picture a 76.2mm FL=1250mm (f/16.4) OKKK refractor.
But sometimes the focuser drawtube, when using a diagonal, may intercept part of the light cone, thus making the focal area smaller. In this picture we see that the inner diameter of the drawtube cuts part of the light cone making the actual focal area diameter a circle of 22mm. Although baffling position and diameter was calculated with a 27mm focal area, the focuser drawtube position when using a 90º diagonal, will act as a new final baffle, cutting the light cone and producing an affective focal area of 22mm. The only way to have the 1.25" eyepiece fully illuminated requieres to change the focuser for one with a drawtube with bigger diameter.

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