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Safely observe the sun

7 - Venus transit in filtered scope

2012 Venus transit and sunspots in a 70mm telescope with a solar film filter

Solar eclipses, transits of Mercury and Venus across the face of the sun, and sunspots are all wonderful celestial phenomena, and you can observe them safely with just a little caution and forethought (a list of upcoming events, and observing reports from past events, are on this page). However, it is very easy to injure yourself or others if you are careless, especially if magnifying optics are involved.

Here is a list of SAFE ways to observe solar phenomena, in rough order of expense or effort required. Each is explained in more detail below.

  • pinhole projection
  • direct observation using solar film glasses or #14 welder’s glass
  • front projection using binoculars or a telescope
  • rear projection using binoculars or a telescope
  • direct observation using a front-filtered binocular or telescope
  • direct observation using a telescope equipped with a Herschel wedge
  • direct observation using a purpose-built solar telescope

Most of the unsafe ways to view solar phenomena involve naked-eye observing through inadequate materials. The sun radiates energy across almost the entire electromagnetic spectrum, from gamma rays to radio waves. Just because you cut the incoming visible light to tolerable levels does not mean that your eyes are safe; ultraviolet and infrared rays can still injure you. Here’s a list of UNSAFE materials for viewing the sun:

  • smoked glass
  • camera film
  • x-ray film
  • stacked pairs of sunglasses
  • photographic or astronomical neutral-density or polarizing filters
  • any filter that screws into the eyepiece of an optical instrument

A word of explanation about that last entry: in past decades, some telescopes were sold with “sun filters” that screwed into the eyepiece. These were horribly stupid and dangerous. The concentrated sunlight delivered through the telescope could overheat the filter and cause it to crack, at which point a blindingly intense beam of collected sunlight could shine through the crack and into the observer’s eye. Fortunately these filters are no longer made or sold with modern telescopes, but they sometimes turn up in older telescopes from garage sales, eBay, etc. If you find one, please smash it and then throw away the bits. In general, when viewing the sun through any kind of filter, the filter goes in front of the light-gathering aperture, whether that is the pupil of your eye or the front end of a telescope (the only exception is the Herschel wedge, which does not work with all types of telescopes).

Now, back to the safe ways to observe the sun.

eclipse pinholes cropped

Pinhole projections

Pinhole projection

  • Good for: eclipses, planet transits
  • Not good for: sunspots (unless they are huge)
  • Risks: none

This is dead easy, and free, or at least very cheap. Make a small hole in something and use it to project an image of the sun, using the same principle as a camera obscura. Almost anything will work: your crossed or curled fingers, a cracker, a strainer or collander, holes punched in index cards, paper plates, etc. Trees do this automatically as long as they have leaves or needles – during an eclipse, the dapples of sunlight will be in the shape of the eclipsed sun.

9 - London sunwatching

Observing the 2012 Venus transit with #14 welder’s glass

Direct observation using solar film glasses or #14 welder’s glass

  • Good for: eclipses, planet transits
  • Not good for: sunspots (unless they are huge)
  • Risks: remote

Solar film glasses and eclipse viewing cards with pieces of #14 welder’s glass are now widely available and cheap ($1-5 right now, cheapest if you buy in bulk). I can only think of a couple things to watch out for: (1) if you get solar film glasses, make sure the solar film is intact and doesn’t have any pinhole defects, and (2) be aware that polycarbonate welding filters do not provide the same protection as glass unless they are gold-coated.

UPDATE: In advance of the August 21, 2017, total solar eclipse, at least some unscrupulous online vendors have been selling inadequate and unsafe eclipse glasses, some even with fake ISO certifications copied and pasted from the real thing. The American Astronomical Society has a useful page on how to tell if your eclipse glasses are legit here, and a list of trusted vendors here.

Front projection using binoculars or a telescope

  • Good for: eclipses, planet transits, sunspots
  • Risks: instrument can overheat, don’t get between the instrument and the projected image, cover or remove finderscope

You can aim a telescope or binocular at the sun and use the eyepiece to project an image on a nearby surface (the ground, a fence, etc.). I’m not wild about this method, however. The concentrated light going through the instrument still converges behind the eyepiece, where your eye would normally be. So it’s still possible to burn yourself or start a fire if that concentrated beam of light falls on your body or on something flammable. And for the love of all that is good, don’t leave such an instrument unsupervised for even a second, or some unsuspecting kid may wander over and try to look through it and fry their retina.

Along the same lines, cover – or better yet temporarily remove – any magnifying finderscopes; a 25mm finder will fry your eyeball or hand the same as the telecope’s objective lens or primary mirror. This goes for all of the following methods that involve a telescope.

1 - Venus transit in Astroscan

2012 Venus transit in an AstroScan and Sun Funnel

Rear projection using a telescope and a closed-loop device

  • Good for: eclipses, planet transits, sunspots
  • Risks: instrument can overheat, aim instrument away from the sun before installing or removing the projection device, cover or remove finder

As of this writing, I know of two instruments in this class: the Sun Gun and its offspring, the Sun Funnel. Both of these involve using the eyepiece of a telescope to project an image of the sun on a sheet of rear-projection screen material. I like these methods because the light path is completely enclosed (as long as you use a solid-tube telescope rather than a strut- or truss-tube), so you can’t inadvertently stick your hand or something flammable into the light path. Used correctly, the sun funnel creates an image about four inches across on the screen, so as long as you use a telescope of four inches diameter or smaller, the projected image will have the same energy density as unmagnified sunlight.

Incidentally, the Edmund AstroScan should be better than most other reflecting telescopes for use with a Sun Funnel because the front of the tube is closed off with an optical window. Although you’d think that would make the scope more prone to overheating, the optical window actually absorbs most of the infrared light that would otherwise be collected and focused by the primary mirror. And if you unscrew the shoulder strap, the tube has two openings for cooling the interior.

6 - shooting the sun

Photographing the 2012 Venus transit through a 70mm telescope with a solar film filter

Direct observation using a front-filtered binocular or telescope

  • Good for: eclipses, planet transits, sunspots
  • Risks: make sure filter fits snugly and cannot fall off or be blown off, check filter for defects, cover or remove finderscope

You can buy filters made from coated glass or solar film, or buy a sheet of solar film and make your own. Whether you buy or build, the main threat here is of the filter falling off while someone has their eyeball behind the eyepiece, so make sure the filter fits snugly and cannot easily be dislodged.

Direct observation using a telescope equipped with a Herschel wedge

  • Good for: eclipses, planet transits, sunspots
  • Risks: buy a good one that won’t overheat, and know what kind of telescope to attach it to, cover or remove finderscope

A Herschel wedge is the exception to the “filters go on the front of the telescope” rule. They’re for refractors only, and best with simple achromats. Don’t try to use one with any fancy designs that have a rear lens element (e.g., Petzval), because the concentrated sunlight passing through the scope can overheat and destroy those elements. Herschel wedges still require the use of a neutral density filter, and on the best wedges such a filter is a permanent part of the design.

Direct observation using a purpose-built solar telescope

  • Good for: eclipses, planet transits, sunspots
  • Risks: depend on type, use an appropriate, non-magnifying finder and some common sense, read the instructions

Purpose-built solar telescopes include both sun projectors and telescopes with hydrogen alpha or calcium K line filters. In general, these are all subject to the same common-sense safety guidelines as the projectors and filtered telescopes listed above. If you buy one, actually read the instructions and the manufacturer’s safety information before you risk your vision.

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7 comments

  1. […] map is excellent. And if you need safe, inexpensive ways to observe the sun, check out my page on safe solar observing. Clear […]


  2. Question , using an eyepiece out of a telescope , do you think it possible to project an image of the sun, have you tried this.
    thanks.
    Dave.


  3. I love the image of creative ways to use pinhole projection, like the cracker! May I use that image in free planetarium shows at the National Air and Space Museum, and educational handouts there? And what credit line should I use?


  4. Thanks for the kind words, Genevieve, and please forgive me for taking so long to reply. The cracker image is one I found online – I’m not sure who to credit for it. But you are welcome to use the similar photos I took using holes punched in index cards, which are in this post.


  5. Hello Matt, and thanks for your reply! May I use the other three images of pinhole projection – on the wall, through the sieve, and through the fist? Are those credited to you? They are great photos too!


  6. What focal length eyepiece did you use in the photo captioned: “2012 Venus transit in an AstroScan and Sun Funnel” to get that good sized image of the sun?

    I dug out an Astroscan tonight that hasn’t been used in about 25 years. The eyepiece is 28mm and the recommended is 2.4mm. (105mm÷43) The closest that I can find is about 7mm.

    Thanks,
    Ira


  7. […] I wrote up an observing guide (link) for people on the Western University of Health Sciences campuses in Lebanon, Oregon, and Pomona, California, but really this stuff applies for everyone on the path of totality (Lebanon) or off (Pomona). Except for the timings, and you can get local eclipse timings here. My more complete page on safely observing the sun is on the sidebar (link). […]



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