Materials needed

  • Baader AstroSolar (available in the US from Astro-Physics)
  • foamcore (usually found at Staples or Office Depot, you'll need 2 or more for larger telescopes)
  • double sided tape, masking tape
  • very sharp knife
  • pencil
  • fine sandpaper
  • compass

Solar FIlter

Astronomy, the hobby of night owls! But there is one really easy celestial target that often gets overlooked: the Sun. Observing the sun however is not as easy as one would think. You do need a solar filter that effectively filters the light to make it safe and comfortable to look at the sun through a telescope. Glass solar filters are available from several companies. I've used filters made by Thousand Oaks in the past. But many years ago, a new material came on the market called Baader AstroSolar Safety Film. The problem was that it was sold in sheets, so how do you cover the front end of your telescope? Well, some folks literally would take a sheet and rubberband it around the end of their scope. This is not a good option as that quickly destroys the film and makes it hard to reuse. So people started making their own filter cells. The Baader Planetarium now also has instructions on their website for building a filter cell. And below is how I built mine. The next time I make one, I will probably do a combination of the two methods.


  1. Measure the front end of your telescope.
  2. Cut foamcore into squares that are about 2-3 inches wider than the front end of your telescope. For example, if the front end of your telescope is about 3.5 inches in diameter, you should have squares that are at least 5.5 inches wide.
  3. Protecting the center of one square, carefully draw out a circle with a diameter of the front end of your telescope. Here its better to err on the side of being slightly smaller.
  4. Using the very sharp knife and protecting your work surface, cut out the circle.
  5. Slide the square with the cutout circle onto the telescope end. Trim out as needed so that you have a snug fit but can still slide it on and off.
  6. Use that piece as a template to trace and cut out circles on 3-4 other pieces. Save at least 1 square without a circle.
  7. Stack the cutout squares together and slide onto the end of the telescope. Trim the pieces as needed using the knife or sandpaper (away from telescope!).
  8. Cut out 1 piece of Baader AstroSolar film that is not as wide as your squares but bigger than your circles.
  9. Lay the piece flat on a clean work area.
  10. Attach doublesided tape to one side of one of the cutout squares and then place it down on the film essentially picking it up.
  11. Put doublesided tape on another cutout square and attach it to the other side of the film sandwiching the piece of film between the two pieces.
  12. Use doublesided tape to attach remaining cutout squares on one side. So the stack should be boxlike with the film sandwiched between two pieces of foam core at one end.
  13. Slide the filter cell onto the scope. If it is loose, you can use masking tape applied to the inside edge of the cutout pieces to tighten the fit.
  14. Take one square (without a circle) and stack it on the end near the film. Use masking tape and tape just one edge so the piece looks like a flap and protects the film.

Well, there you go, a simple filter cell.

Let's get fancy!

Sometimes, you might not want a full aperture solar filter. You can modify the above instructions to build an off-axis filter. The inner foam core pieces will have to be cut full aperture so that they can slide onto the telescope, but the outside piece can be trimmed with just the one small aperture. My first one, I built with two off-axis apertures and then a cover that rotated to reveal 1 or both openings and could rotate and cover both openings. Why two? When the telescope was out of focus, I would have a double image. As I focussed, the images would converge. But sometimes, particularly when I observed during the middle of the day, the sun was still too bright so then I could observe through just one opening which reduced the light and made the view a little bit dimmer.