Last updated: 8 September 2002

Solar Filter

Subject:	guide to making cheap solar filters - with pictures
Sent:	Tuesday, September 3, 2002 15:52:37
From: (Brian Anderson)
I've just completed a project to create 4 ETX-90 solar filters using
Baader AstroSolar film.  The total cost of the project was $48, which
works out to $12/filter.  I took digital photos of each step in the
project.  If you think your readers might like to view a step-by-step
how-to guide detailing this process I'll send the pictures and
descriptions to you.  I just need to know if I should send all the pics
in one email, zipfile them together, or send multiple emails with some
type of size limit on each.

Two ways to create a really cheap dew cap for the ETX-90:

First, go to Home Depot (a great but generally unknown resource for
telescope parts) and buy a 4" PVC pipe coupler (~$1).  Unscrew your
ETX-90 lens cap and take it with you if you need a size guide.  If you
want to get sophisticated clamp the pipe sleeve in a vise and use a
sanding tool attached to a hand drill (~$4) to grind away the small rib
in the middle. Then spray paint the tube flat black inside and out (2

solar filter
Buy a set of sticky felt pads (~$1.50) and place at least 3, preferably 6, around the inside of one end of the dew cap. Then slowly twist it onto your ETX-90. It almost looks like it was designed for the scope.
solar filter
If you don't want to bother with spray paint you can buy a black plastic drain pipe adaptor ($1.49) and cut it down to fit the ETX-90. Wrap some black electrical tape around the inside and outside of the dew cap where it will touch your scope tube. Usually the tape provides enough thickness to make the cap slide on snugly. If not, you can buy a 4 inch hose clamp ($0.89) and use that to tighten it down. It looks hideous, but the parts and labor are about as minimal as one can get.
solar filter
How to create 4 Baader AstroSolar filters for the price of one: If you like to look at the sun with your ETX-90 but want to keep the cost down for those somewhat fragile (but optically excellent) solar filters you can build 4 for the price of one. The process takes some time, but it's not a bad weekend project. First order the A4 size Baader AstroSolar Density 5 film (available from for $30) Then buy the following: 4 * 4-inch PVC pipe caps, $1 each 4 * 4-inch PVC drain plugs, $1.89 each 1 * flat black spray paint, $4 1 * small tube glue, $2 The total comes to about $48, including the AstroSolar film. This is enough to make 4 filters for your ETX-90. Start with the pipe cap and drain plug combination. The drain plug should fit inside the pipe cap without the need to use much force. You may have to try out several combinations at Home Depot before you find 4 pair that work well together.
solar filter
Next cut the drain plug down the middle, creating 2 parts; a ring and a drain.
solar filter
Don't worry about cutting this cleanly, you have one good side to the ring already which is all you need. Then insert the ring (cut side up) into the plug and mark the inside diameter using a felt pen.
solar filter
Remove the ring and insert the cap into a vise. I created a simple jig to hold the cap from 2 pieces of scrap wood but this is not required.
solar filter
Drill out a series of holes using a 1/4 inch bit along the line you draw with the marker. Be careful not to exceed the diameter of the line with any of your holes, since that would eliminate part of the lip that will hold your filter in place. Flip the cap over and put it back in the vise.
solar filter
Then get out your sabre saw and (using a fine tooth blade) cut out the top of the cap.
solar filter
Do not attempt to use the sabre saw directly and skip the hole cutting stage. The PVC is thick and it will melt, gumming up your blade and making it impossible to actually remove anything. Pop out the top of the cap and discard it, leaving a jagged hole.
solar filter
Flip the cap over again, reinsert the ring (cut side up) and mount a sanding bit to your drill.
solar filter
The ring will act as a guide to help you grind away the bits of the cap the sabre saw left behind. Don't rush this step, keep working it until you get a smooth hole. Take the pieces apart and hand sand them around the edges to remove any remaining material. Test fir the cap and ring They should fir very closely. If your cap has a beveled inside edge you may want to sand down the edge of your ring to give it a closer fit. Clean all the parts with paper towels and windex, let them dray and give them 2 coats of flat black paint, inside and out.
solar filter
After everything is dry open up your AstroSolar package and put the film onto a set of overlapping tissues.
solar filter
Lay out the rings (cut side up) on a similar size piece of paper and position them as you will on the actual film. Don't make the rings touch, you will need some room to cut them apart later. Pick up each ring, flip it over (exposing the good side), apply a thin layer of glue and place it down onto the AstroSolar film.
solar filter
When you have done all 4 rings place the AstroSolar cardboard mailer on top and put the 4 black caps on top of that.
solar filter
This will provide a small but useful pressure to the rings as the glue dries. When the glue has dried cut apart the rings, placing each one on a new tissue.
solar filter
Using the cardboard insert that came with the film trim each ring of excess film.
solar filter
Place each ring in its corresponding cap.
solar filter
Save the 2 large pieces of unused film for use on another project (binocular filters, solar sunglasses, finderscope) Then use the leftover drain piece as a plunger to press it down to the bottom.
solar filter
Add some glue to 3 points around the inside of each cap to hold the rings in place and leave the 4 filters to dry.
solar filter
Finally, add some stick-on felt pads around the inside edge of each cap to create a snug fit with your ETX-90 tube. Keep the leftover drain plugs to use as protectors when transporting or storing your solar filters.
solar filter
I use blue painters tape because it leaves no residue and won't peel off the black paint from the cap. Carefully test out each filter on a sunny day. You should see nothing except a nearly pure white sun with a few spots. You might not know what to do with 4 filters, but they are easily damaged so having a backup isn't a bad idea. You can always mail one to a fellow ETX-90 owners as a really cool gift. Be sure to send along a copy of the safety instructions that came with your AstroSolar film. Brian Anderson database geek and sometime astronomer Long Island, NY (where the light pollution, automobile traffic and property values are all too high) Brian Anderson PS Great site, it's helped me a lot. Now if you could only convince New York City to turn down the lights at night :-)

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