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Posted by Rick Crockett on April 19, 2000 22:09:24 UTC

Funny that S&T formula looks exactly like the one I posted ten months ago on the astromart forum. I do not like the newt25 program as it tends to promote diagonals that are too small. I always recommend diagramming your system. If you do not want to buy a roll of drafting or drawing paper, you can use butcher paper, kraft wrapping paper or the inside of gift wrapping paper. Make your drawing full sized and draw the primary optical path. Widen the point of focus to the width of field you plan to illuminate. 1 1/8" for 1 1/4" eyepieces. This is a line purpendicular to the line of focus. From each edge of the field draw a line down to the corresponding edge of the mirror. This is the cone of light for the entire field. From each edge of the field, add the radius of the mirror and the draw a line from that point down to the mirror and that is the unobstructed field of view. I like to clear that by 1/2 to 1" with tube walls and baffles.

Baffles should be drilled with plenty of holes right where they meet the tube wall so tube currents are minimised.

When using the diagram to determine diagonal monor axis. measur the cone of light across the bottom or primary side. For fast systems this can make a lot of difference in light not wasted. Why get a larger primary and pay for enhanced coatings if you are willing to lose 30 to 50% illumination of your AFOV to subsized diagonals?

Have Fun

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