May 21, 2018

A Home-made Solar Filter for Cheap.

Solar observing is just plain cool. While some celestial objects such as the Andromeda Galaxy will look exactly the same on the day you die as when you were born, the face of the Sun can change day to day, or even minute to minute. As we are currently in the depths of a solar minimum, now is the time to construct a white-light filter and prepare for those sunspots and faculae that will start to creep across the face of our nearest star in the next few years.

Of course, a word of warning is due here. NEVER look directly at the Sun through a telescope, even momentarily. I know, those old time astronomers used crazy things like smoked glass filters, but the only truly safe ways to observe the Sun is via projection , or an approved filter that fits snugly on the front of your instrument. Inspect it prior to every use by holding it up to the Sun and looking for pin-holes, and if it doesn’t pass, throw it away immediately. The filter fit around the tube should be tight, to assure that the wind (or passers by!) do not knock off the cover while you have “eye ball to eye piece.” Never leave a ‘scope, filtered or no, aimed at the Sun and unattended, as the temptation is always there for someone to take a peek. Never use a ‘scope that places the filter as a screw on attachment to the eyepiece, as these can overheat and crack. I doubt even the department store style scopes dare to sell these anymore, but they probably still lurk at the occasional yard sale… can you believe I actually had one of these as a kid? I’m glad my eyes survived the experience…

But enough with the scare warnings. Trust me, solar observing is indeed fun. This week we will show you how to construct a safe white light filter with card board, tape, glue, and Baader Safety Film solar filter paper, which is the heart of this high tech system. You could build this filter holder out of wood or even metal if you have access to the means to precession cut it, but I’ve made and used a simple cardboard ones for years. We have a Celestron C8 telescope, which has a Schmidt-Cassegrain configuration. Light travels through the tube three times, and thus its not recommended to aim it at the Sun for projection, as it can overheat very quickly and damage the optics. The filter will simply be a 60mm offset, as you simply don’t need all that aperture looking at the Sun. Plus, you can then use all that extra Baader Safety Film to make filters for your binocs, cameras, glasses, etc. This filter will enable you to view the dazzling photosphere of the Sun, an area where sunspots, faculae, and sometimes even white light flares can be seen. The filter paper has a neutral density rating of 5, and will only let through less than 1/100,000th of the visible light, giving the Sun a pinkish-purple tint. All told, this will give you the capability of constructing an inexpensive (competing manufactured filters run about +100$) white-light filter at a fraction of the price; 20$-40$, the price of the film!

Anyway, on to the construction. All that’s required to hold the Baader Safety Film in place is card board, glue, staples, and the handy man’s secret weapon, duct tape.

The cardboard should be fairly thick, and not flimsy or translucent. The glue, as I found out this past weekend, is optional, as heavy staples and duct tape work just fine! You’ll be constructing two off-set templates, in which to sandwich the Baader Safety Film in between. Make sure the filter paper is securely fixed with tape or glue. Next, you’ll want to construct a collar. This must be snug enough that gravity or the wind will not pull the filter off while observing! I like to mold these onto the scope itself, sort of a custom fit.

Now, all that’s left is to join the collar with the mask. My only advice here is tape, (or glue), staple and tape again; the filter cannot have any gaps around the edges. Make sure the filter is taunt when fixed in position but take care not to rip it. If possible, the mask should be slightly larger that the aperture. Heck, it doesn’t even have to be round, as long as it is attached and covers the entire aperture.

How do you check for gaps? I like to simply hold it up to the Sun and visually inspect it each time before use. This filter should last for years; if any gaps are found, simply trash the cell and construct another.

So, what can you see with this contraption? Aiming the telescope is pretty straight forward, although you need to cover any telrads/finders for the same reason you cover the main optics. I like to look at the shadow of the tube itself on the ground, aligning it until the shadow is smallest. As mentioned, sunspots and bright faculae will soon be part of our daily solar menu in a year or so. These filters are also handy to observe total and partial solar eclipses, as well as transits of the Mercury, Venus (in 2012) and just perhaps, the Shuttle or International Space Station! These homemade filters are a safe, effective and cheap way to get into solar observing… it may even whet your appetite to get a pricier Hydrogen Alpha ‘scope to complement it, as well. And best of all, you don’t have to worry about light pollution!


  1. Georgr says:

    I used a Floppy Disk (diskette) as a sun filter. I cut it into small pieces and stacked them onto the Telescope’s eyepiece. It works great and you’ll be able to see the cold spots.

  2. David Dickinson says:

    Thanks for the reply; as a clarification, however, we do not endorse such a modification; a filter eyepiece can get dangerously hot and crack. A safe filter affixes tightly to the aperture of the telescope.

  3. Gonzalo Contreras says:

    Floppy disk could be harmful. Dangerous waveleghts pass thru the floppy disk material and could affect your eyes. This material is not intended to work as a solar filter. Don’t use it. And also. don’t place solar filters at the eyepiece. Heating will crack the eyepiece.

  4. David Dickinson says:

    Sage advice… thanks for the comment. Also, something that recently came to my attention is to be sure your not using Baader lower density photographic filters for visual work, as dangerous UV still gets in. Safety Film for visual solar use is packaged and clearly marked as such.

  5. Tim says:

    I have read that the off-set opening should “touch” the edges of primary and secondary mirrors. Is that true, and if so, why?

    Also, I have an unrelated question. Has the “tip” of the crescent moon ever appeared to touch Venus? If so, any idea when it will happen again?

  6. David Dickinson says:

    Never heard that; I made the off-set opening for the 3 filters that I’ve built for my 8″ SCT about 60mm wide with about 10mm on either side and they work fine. In fact, I would suggest leaving a margin on either side so no spaces appear that could let light in; much easier to seal around.

    Any time there’s a lunar occultation of Venus, its possible to have it appear on the horn of the Moon if you’re along the graze line; the Moon always appears as a crescent near Venus as it never strays far from the Sun. Such an event does occur on August 13th on this year:

    Although it looks like you’d have to journey to the North American Arctic to see a graze!

  7. Tim says:

    Thanks David!


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