The easy solar filter: Baader Astrosolar Film on aperture mask cover.

March 15, 2015 // by ecuador

Example of DIY filter for a 5" Maksutov.

Example of DIY filter for a 5″ Maksutov.

One of the best and least expensive solutions for observing the sun with a telescope is to use the Baader Astrosolar Safety Film. There are many places that will sell you a pre-mounted filter for your telescope, or you can make it yourself and save money. For example, by gluing strips of cardboard around the end of your aperture you can make a thick ring, on one end of which you can glue an Astrosolar Film, sandwiched between two cardboard holding rings (example on the right). It is not hard, but it does take some time to build, so if you don’t like DIY projects but you would like something quick (e.g. for observing the March 20 eclipse), you can take advantage of two facts:

  • You don’t necessarily need your scope’s entire aperture for solar observation. Even a 40-50mm aperture would provide enough resolution for decent results when observing/photographing the sun. Yes, a full aperture filter will give you better views, so go for it if you can do the extra work.
  • Many telescopes (especially refractors and Newtonian reflectors) come with covers that have an aperture mask (or “aperture stop”). That’s the smaller hole with the smaller cap you might be wondering about.

If you have an OTA with such a cover, you are all set to convert it quickly for Solar viewing. You still need to purchase some Baader Astrosolar Safety Film. It comes at around £18 per A4 sheet, but since you will need less (a square with a side at least as large as the aperture hole diameter), you can find various retailers (e.g. Agena, ebay) that will sell you a square piece for less.

Here are my Skywatcher 200P-DS and Evostar 80ED Pro covers that both feature a 2″ aperture mask:

Skywatcher 200P-DS and 80ED Pro front covers.

Skywatcher 200P-DS and 80ED Pro front covers.

You just turn them around and use thin stripes of duct tape (in my experience duct tape will hold it well for a very long time) at the four corners to end up with something like this:


Now, if you want to continue using the cap regularly for covering the telescope, it is a good idea to add some protection that you only remove when you want to view the sun. Otherwise, you risk touching the solar film at night when you are reaching to grab the cover – it doesn’t break easily, but it does not clean easily either, so add something like a piece of plastic binder as a protection making sure that for this you use tape that is not very adhesive:


You are all set. For observing the Sun, you take off the protective plastic and the cap (and for a reflector make sure the aperture hole is not over one of the secondary mirror holding vanes), point at the sun and enjoy the view:

Sun. Canon 550D on SW Evostar 80ED Pro w/Baader Astrosolar @ 50mm aperture.

Sun. Canon 550D on SW Evostar 80ED Pro w/Baader Astrosolar, #12 yellow @ 50mm aperture.

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