How to build an Off Axis Mask

  1. Measure the distance from the thumb screws to the end of your telescope tube. Draw out two long rectangles on the poster board, whose height is the distance you measured.
Measurements. A = distance from thumb screws to end of tube; B=outside diameter of filter.
Draw strips and circles on the posterboard.
  1. Cut out the strips of cardboard. These two strips double up to form the sidewalls of the mask.
  2. Using a compass, measure out one circle the diameter of the filter on the posterboard.
  3. On the circle, use the ruler to draw out lines indicating the vanes of your spider assembly. Then use the compass to draw a circle in the center representing your diagonal. Finally, draw a large circle the diameter of the filter aperture.
  4. Using the compass, draw an off-axis circle within the diagonal, spider vane and filter lines. For a 10 inch scope with a four vane spider and a 1.85" diagonal, a 3.5-inch off-axis aperture fits perfectly.
Cutting the aperture hole. A = outside diameter of filter. B = diameter of filter aperture. C = diameter of center mirror. D = spider vanes. E = aperture hole.
  1. Cut the circle from the posterboard. Cut out the inside of the aperture hole. Sharp scissors and/or a sturdy razor blade are useful to cut through the heavy posterboard.
  2. Take the strips of cardboard, and arrange them into a circular collar the same circumference as one of the circles. At one end where the strips overlap, join them with tape into one long strip. Temporarily tack the other side of the collar so that it forms a cylinder.
Join strips together, so the length is the circumference of the telescope tube.
Join remaining ends into a cylinder.
  1. Test fit the collar against your telescope tube. Adjust the diameter of the collar for a snug fit. Then seal the other end of the collar with packing tape, to permanently form the cylinder.
  2. Attach the large circle to one end of the collar. Cut duct tape into six-inch strips, then cut notches about every 3/4 to 1 inch along the tape. Cut the notch halfway across the width of the tape.
Cut notches halfway through the tape.
Fold the tabs of tape to match the curvature of the filter collar.
  1. Place the uncut end of the tape along the edge of the collar, then fold the tabs over the circle to follow the curve of the top.
  2. Hold the cap up to a bright light, and check for small pinpoints where light may be showing through the tape. Cover any holes with small patches of duct tape.
The mask is now complete. Place the mask over the end of your filter and enjoy.