So with the upcoming total solar eclipse on August 21st 2017, I decided to get my hands on a solar filter for my 8" Dobsonian telescope so I can grab some video and pictures of that rather infrequent event. Since I had bought my telescope from Orion, I checked for a compatible solar filter on their site and found out that it was priced at $145! There was no way I was going to pay that much for something like that, and after some thinking decided to create my own. After all it's really a pretty simple design...
First I checked online and was able to find good quality solar filter sheets on Amazon, with a 10"x10" sheet for $20. Incidentally, as the solar eclipse date approaches, I have noticed that the prices are going up as well. You've got to love free markets...

I use OpenScad as my 3D design program. It is open source and is parametric, which means that your objects are not drawn directly on the screen, but rather described using simple code. This allows for easy modification and high precision particularly when important parameters are defined in variables. You can download OpenScad here: http://www.openscad.org/

I started by creating a standard filter cap which could be adjusted for any telescope or camera objective main tube outer diameter by simply entering that diameter in the code in millimeters and letting OpenScad do the rest. Here's the code:

// Telescope solar filter cap scope_Outer_Diameter = 242; //in millimetersinnerR = scope_Outer_Diameter/2;outerR = innerR+2;difference() {    cylinder(20,outerR,outerR,false,$fn=300);    translate([0,0,3]){        cylinder(17,innerR,innerR,false,$fn=300);    }    cylinder(20,innerR-10,innerR-3,false,$fn=300);}

And here's what it looks like:



However, there was a problem in my case: the size of the filter was too large for my printer bed. But not to worry, I figured I would just divide it into 4 smaller equal sections and print one section 4 times then glue the pieces together. I added a tongue and groove feature to the design so that it would interlock easily for accurate alignment and structural stability. Here's the code:

// Telescope solar filter cap quarter sectionscope_Outer_Diameter = 238; // in millimetersinnerR = scope_Outer_Diameter/2;outerR = innerR+2;difference() {    cylinder(20,outerR,outerR,false,$fn=300);    translate([0,0,3]){        cylinder(17,innerR,innerR,false,$fn=300);    }    cylinder(20,innerR-10,innerR-10,false,$fn=300);    translate([0,-outerR,0]){        cube([outerR,outerR*2,20]);    }    translate([-outerR,0,0]){        cube([outerR,outerR,20]);    }    translate([-innerR+3,-3,0]){        cube([4,3,3]);    }    translate([-outerR,-3,9.5]){        cube([3,3,4]);    }}translate([0,-innerR+3,0]){    cube([3,4,3]);}translate([0,-outerR,9.5]){    cube([3,2,4]);}

And that's how it looks:



I printed 4 quarter sections and glued them together with super glue, with the end result looking pretty good:



Finally, I cut the filter sheet to size after marking it to the appropriate diameter using a protractor and glued it to the inside of the filter cap assembly. Remember that the shiny side faces the sun!

Here's the final result ready to use. I have to say that I'm pretty pleased with it