Solar activity is starting to pickup for Cycle 24.  Preparations to view the sun with a 10" Dobsonian telescope and share this information with other Amateur Astronomers is the reason for developing this website.  This filter assembly was fabricated for well under $40 in less than 2 hours and is much more rugged than solar filter frames made from posterboard. Please help support providing this free information by visiting our sponsors.


The following describes how an off-center solar filter was built for a 10" Dobsonian Telescope on a Saturday afternoon in January 2007.

Please note the following safety precautions:

• Never look directly into the sun without the proper eye protection or permanent eye injury can occur.
• Do not use photographic film, tinted glass, plastic or metal film.
• Always inspect the solar film for cracks and pinholes before use.  
• If there are any of these signs then do not use the solar filter or permanent eye injury can occur.
• Use only material that is designed, approved and test for viewing the sun directly.

    Material listing:

• 8" x 8" piece of Black Polymer Solar Filter Film   (With an 8" x 8" piece there is enough left over to make a filter for your finder scope.)
• Another solar film type that is readily available
• Another solar film type "Baader Solar Film" 
• One sheet of 1/2" thick Elmer's Foam Board
• One sheet of 3/16" thick Elmer's Foam Board
• One sheet of poster board
• Rubber Cement
• Elmer's Wood Glue
• White Duct Tape

    Recommended tools:

• Drawing Compass
• Pencil
• Scissors
• Hobby recipricating saw (if available) 
• or a very sharp single edge razor blade with lots of patience
• A magic marker
• Straight edge or Ruler

1. Measure the outside diameter of the telescope.  In my case this was 12 1/4"
2. Take the outside diameter measurement and add 3/4".  This will be the size of the outside diameter of the solar filter frame.
   
3. Using a compass I marked a 13" diameter circle on the piece of poster board and cut it out with scissors.  This will be used for a template to markup the 3/16" foam board to make the top of the filter frame.
4. Using a compass I marked a 12 1/4" diameter circle on the piece of poster board and cut it out with scissors.  This will be used for a template to markup the 3/16" foam board to cut out the inner filter frame which holds the solar filter film against the top filter frame.
5. Using the 13" diameter template I then marked a circle on the 3/16" thick foam board.
6. Then cut out the 13" and 12 1/4" diameter foam circles using a reciprocating hobby saw, bandsaw or a very sharp single edge razor blade.  (If using the single edge razor blade use extreme caution to avoid personal injury.)
7. Now make the outer frame which is a ring that should fit snuggly over the tube of  your telescope.
8. The ring is cut out of the 1/2" thick foam board.
9. Then measure the clear opening between the supports for the secondary mirror.  In my case a 4" opening that was free from obstructions.
10. I then marked a line using a pencil which was 1 1/4" from the outside diameter of the 13 1/4" circle.
11. Using a straight edge I then marked a line that intersected the mark that was made 1 1/4" from the outer diameter of the 13 1/4"  diameter foam circle and the two points on the circumferance of the 13" foam circle where that line intercets the outer diameter.
12. Using a drawing compass draw a 4" circle where the outside diameter of the 4" circle is tangent to the line drawn in Step 9.
13. Then cut out the 4" circle in the 3/16" thick foam boards.
14. Using a straight edge I then marked a line that intersected the mark that was made 1/2" from the outer diameter of the 12 1/4"  diameter foam circle and the two points on the circumference of the 12 1/4" foam circle where that line intercets the outer diameter.
15. For assembly use rubber cement to hold the solar film to the top frame.  The 8" x  8" solar film will be enough material to make a filter for both the telescope opening and the spotting scope objective.
16. Use wood glue to hold the outer frame ring to the top frame.
17. Use rubber cement to hold to sandwich the solar film between the outer and inner frame components.
18. Once the parts are assembled use white duct tape around the circumference of the frame to give it a nice finishing touch.