Baffle Placement for a Refractor
Minimum Baffle Method

First, draft out a scale drawing (full size is better) of your proposed
scope. Include all items which limit the light path such as the focuser
tube lens cell. Make sure to account for the focuser tube's extension into
the tube when the image is at focus. Also, don't forget the star diagonal
if it will be used.

Select a desired size for the illuminated field and draw it at the
focal plane. Draw the light cone by drawing a line from each edge to the
respective edge of the objective.

Next, draw a line (red, above) from the bottom edge of the
field spot to the tube wall at the back edge of the objective. Where this
line crosses the light cone is the first baffle position.

Now draw a line (green in the illustration) from the bottom
of the objective to the first baffle position and extend it to the tube
wall.

Draw a line (red) from the bottom of the field spot to the
place where the previous line hits the tube wall. Where this new line
intersects the light cone is the position of the second baffle.

Do the same thing repeatedly until you run out of room. In most small
refractors, two or three baffles will be all that is needed. In the Okapi,
only two were used because the focuser tube acts as the third one.

NOTE: Using this method makes portions of the tube wall visible to the
field spot, but only those portions shaded by the nearest baffle. Thus,
flat black paint is necessary. In Okapi, the entire interior of the tube
is ridged with small cuts to act as mini-baffles down the length of the
tube.

If you want to make sure no tube wall is visible, at the expense of
perhaps using more baffles, skip the green lines from the objective and
draw the red lines to the base of each baffle
itself.