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 BAHTINOV string  focusing mask

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The steps for making a string Bahtinov mask are as follows.

  1. 1. Gather the materials
  2. 2. layout the lines on the mask
  3. 3. prepare the string supports
  4. 4. Cut out the center of the mask
  5. 5. mount the string supports
  6. 6. String the mask.

 It's a great project on a rainy day after the honey-do list is completed.

Here are the details:

For the material I used 1/4" plywood , 1/2x1/2 x1/16 aluminum angle (you can also use 1/2 flat stock), a spool of thin twine ( you  can use fishing line if you  want), 4 -  6/32 machine screws and nuts to  bolt the supports to  the plywood. (You  can click on the drawing at the right to get a larger image)

  • I started out cutting a 12" square of  1/4 plywood and marked the center. Using a compass, I drew a 8" diameter circle ( the diameter of my scope's objective). I drew another that  was 9" in diameter. This is where I drilled the holes for the string. and a third circle 10.5" in diameter. outside edge of the mask.
  • Draw a line representing  the Diameter of the 9" circle from top to bottom. This will be the location of the long support for the strings.
  • Draw a line representing the radius  of the right side to the center. This will be the location for the short string support.
  • Now I laid out the holes. I wanted the strings to be 1/4" apart.
  • Using the center point of the circle I measured 1/8th of an inch on either to get my starting point for the support holes.
  • Next I drew lines  1/4" apart on the side of the circle which doesn't have the Radius drawn. Each of these lines will be parallel with each other and intersect the Diameter line at 90.
  • Draw the angular lines at 25.5 to  the Short Radius support line. The angle isn't critical but is you  make it too wide the legs of the X in the image of the mask will be too far apart and make it difficult to judge when the center line is in the middle. These line must also be  parallel to each other  and 1/4 apart.
  • Use an appropriate size drill bit for the string you are using and drill holes where these lines intersect the middle circle.
  • To make the aluminum supports I cut a piece of aluminum angle to  9"  to match  the diameter and one to 4 1/5"  for the radius at the right.
  •  The tricky part is making the center diameter support since you  want an ear on each end to use as mounting points for the bar to the plywood. (see Picture on right)
     Cut a piece 9" long and one 5" long
    a. If you  used Flat stock
        I.   Make   four  1/2 "wide " L "  to  be used to mount the support bars to  the  mask
        II.  Take the 5" piece and bend a 1/2"  90  "L" on one end.
                This will allow you  to  bolt the short to  the middle of the long piece
    b. If you  used angle stock:
         I.    On the 9" piece mark 1/2 in from each end and cut down till you 
                intersect the other leg of the angle
        II.     Starting at the saw cut on one end remove the remaining material
                to the saw cut at the other end. You want to end up with a flat
                bar and an ear on each end for mounting.
        III.    Do the same same with  the 5" piece except that  you  only  need one ear on the outside end.
        IV.    Measure in 1/2" from the side of the %" piece that  is flat and bend it 90.
                This will allow you to     bolt it to the long support.
  •   Drill holes in each of the mounting ears to match  the screws you  have .
  •   Lay the long support on the mask and mark and drill mounting holes in the mask.
  • Do the same with  the short support , laying it on the Radius line and mark and drill a mounting hole in the mask and mark the position of the hole in the ear on the long support.
  •  Find the center of the long support and measure 1/8" on each  side. This is your starting point to  mark and drill the thread support holes. I marked each location and the center punched it so the drill would not wander around.
  • Fasten the short support bar to  the long bar at so that  the bar is over the radius line.
  •  Temporarily fasten the supports to  the mask making sure that  the holes drilled in the long support line up  with  the lines on the mask. The short support should overlay  the radius line.
  • Mark where the angular lines meet the short support bar. Center punch and drill this  also.
  • Now cut out the center of the mask and the excess off the outside using the outside circle as a guide.
  • At this point you  can string the mask. I started at the left side on the top and brought the string up  from the bottom. Came across to  the first hole in the long support bar and through  the top hole on the right side. The came back up thru the second hole on the right , thru the support and to the second hole on the left. I proceed like this all  the way thru the left side.
  • Looking at the drawing you will note that  there are line on the left side that do not go across the mask but go from the top of the left side to  the bottom half of the same side. These strings were laces separately.

This is the procedure I used for lacing the mask. Use what  ever method you  find best. The key thing is that  the strings  on the left MUST be 90 to  the long support bar and the strings on the bottom right MUST intersect the short bar at the same angle as the ones on the top.

 

This is an example of  the image thru the mask

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