Build a Solar Filter for Your Telescope
You can build a solar filter for a large aperture scope for less
than $40, including the solar filter film. This is a great project to
build with your kids, or perhaps as part of a Project ASTRO classroom
I describe how I built a solar filter for my 10" F/7 Dobson scope,
but you can easily adapt these instructions for other scopes.
For daytime use, we build an off-axis filter smaller than the
scope's full aperture. For convenience sake, an off-axis mask on a
large Newtonian is easy to make, takes less material and the off-axis
size is often more than enough given the daytime seeing.
My solar filter in use. Photo: Jane Houston Jones,
San Francisco Sidewalk Astronomers
- Heavy posterboard, 20 x 30 inches
- Clear packing tape
- Duct tape
- Scotch tape
- Fine sandpaper
- Drawing compass
- Scissors, razor and/or x-acto knife
- Newsprint or tissue paper
- Baader Astrosolar safety film. An A4 (7.9 by 11.4 inch) sized
sheet is $29 not including shipping. A 1 meter by 1/2 meter sheet is
$69. It is available from one of these dealers:
Adirondack Video Astronomy
26 Graves Street
Glens Falls, NY 12801 USA
Fax : 1-518-745-4114
11250 Forest Hills Road
Rockford, IL 61115 USA
- Measure the distance from the focuser 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 focuser to end of tube; B=diameter
Draw strips and circles on the posterboard.
- Cut out the strips of cardboard. These two strips double up to
form the sidewalls of the filter.
- Using a compass, measure out two circles the diameter of the
telescope tube on the posterboard.
- On one of the circles, 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 your mirror.
- Using the compass, draw an off-axis circle within the diagonal,
spider vane and mirror lines. For my 10-inch scope with a four vane
spider and a 1.85" diagonal, a 3.5-inch off-axis aperture fits
Cutting the aperture hole. A = diameter of tube. B = diameter of
mirror. C = diameter of center mirror. D = spider vanes. E = aperture
hole for solar filter film.
- Cut both circles from the posterboard. Cut out the inside of the
aperture hole from the one circle. Sharp scissors and/or a sturdy
razor blade are useful to cut through the heavy posterboard.
- Place the circle with the aperture hole atop the other circle.
Trace out the aperture hole outline onto the other circle. Cut the
second aperture hole. You may need to smooth out the edges of both
aperture holes with some fine sandpaper.
- 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
Join strips together, so the length is the circumference of the
Join remaining ends into a cylinder.
- 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.
- Attach one of the large circles 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
Cut notches halfway through the tape.
Fold the tabs of tape to match the curvature of the filter collar.
- 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.
- 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.
- Take the remaining circle, and place it inside the cap. You may
need to trim or sandpaper it a bit to get it to just fit within the
cap. Line up the aperture holes of the two circles. On both the
sidewall of the cap and the circle, make some alignment marks and
label them A, B, C, D, E.
- Arrange the newspaper or tissue paper on a table. Cut out a
square of solar film with sharp scissors or a razor, carefully
following the instructions which came with the film. You do not want
to scratch the film or create pinholes.
- Take the free circle, and use Scotch tape to attach the solar
film over the aperture hole. Do this on the side opposite the
alignment marks. Do not stretch the solar film tight; this will ruin
its optical properties. It should be loose and maybe a little
- Insert the free circle into the filter, and realign the alignment
marks. Seal into place with packing tape.
- Hold the filter up to a bright light to look for pinholes in the
film. Dot the pinholes with a black or blue permanent marker. You can
use a halogen lamp, but do not get too close to the lamp! The heat
from the lamp will stretch or melt the film, ruining the filter.
The filter is now complete. Place the filter over the end of your
telescope and enjoy.
Using the Filter
Here are some tips for using the filter:
- Inspect the filter every time you use it, to ensure there are no
holes or scratches in the film.
- Make sure the aperture isn't obstructed by a spider vane.
- To aim the telescope, do not use your finder or Telrad! Instead,
point the telescope until it casts the smallest shadow possible
- If your scope has an open tailgate, you will need to cover it
with some type of shroud to prevent daylight from leaking in the
scope and overpowering the view.
- You may need to counterbalance your scope against the added
weight of the filter.
You can use these plans to build an off-axis aperture filter for
Moon viewing. A Moon filter lowers the brightness of the Moon through
medium and large aperture scopes. Just omit the inner circle, and the
solar safety film.
Copyright © 2001 Michael Portuesi, All rights reserved. This
article originally appeared in Above the Fog, the official publication
San Francisco Amateur Astronomers.
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