Your Day in the Sun: A Guide to Solar Filters

Latest Revision: May 12, 1999

DISCLAIMER: All this information is accurate to the best of my knowledge; if there are any omissions or errors, please let me know. This document is intended to be an overview, not the end-all-and-be-all of a given topic. If you want to find out more about a specific gadget, accessory, or thingy, consult the references listed in each section.

Solar Filters -- Contents:

1. What solar filters are safe?
2. How can I tell if a filter is safe?
3. What should I use to view a total/partial/annular/etc. solar eclipse?
4. What do the numbers in welder's glass specifications mean?
5. Given that I have a safe filter, is it suitable for naked eye/binocular/telescopic use?
6. I have one of those in-the-eyepiece solar filters. What should I do with it?
7. I saw an ad for a "hydrogen alpha" filter. What does it do?
8. Who makes 'em?
9. References

What solar filters are safe?

The simplest rule: If you don't know its transmission characteristics in the ultraviolet and infrared spectrum (as well as the visible!), don't use it.

A solar filter is safe if it blocks out a reasonable (see section 2) fraction of all solar radiation wavelengths that can damage the eye. Remember that IR and UV can be just as dangerous as visible light. IR, in particular, poses a severe hazard since too much IR, focused on the retina, will cause a thermal burn consisting of permanent damage to the cells therein. UV is generally less immediately dangerous to the retina -- largely because it tends to get absorbed in places like the lens, where it too can cause damage.

How can I tell if a filter is safe?

Easy -- find out if it violates the rule above. The simplest way to do this -- if you have the requisite equipment -- is to measure its transmittance over an appropriate wavelength range. In general, the fraction of light transmitted should be well under 1%, and preferably under 0.1%, over a wide range including the visible and both the near UV and near IR.

Most people can't do that, so the next best thing is to stick to filters which have either been deliberately designed to pass such tests (i.e., filters specifically designed for solar work), or which have not been so designed but nonetheless give suitable protection.

Safe filters include the following:

• Filters designed for solar observation with telescopes that mount on the sky-facing (not the eyepiece) end of the scope. These "over-the-aperture" filters include aluminized Mylar filters (e.g., Solar-Skreen) and metal-coated glass filters (e.g., Thousand Oaks, Questar).
• Sufficiently dense shades of welder's glass. Shade #14, the darkest shade readily available, is recommended for general purposes, though shade #12 is probably safe as well.
• Some kinds of black and white film, completely exposed and developed. The film must have a silver based emulsion (not dye) and in general, more than one layer will be needed. Because of the inherent variability in composition, quality etc. of film, and since it (unlike the previous two options) is not required to pass light transmission tests, this is a riskier choice than the other two. I don't recommend using this unless you know exactly what you are doing.

Unsafe filters include anything that is either highly transmitting in UV or IR over a wide range (out to ~250 nm in the UV and out to ~2 microns in the IR), or whose optical properties are not consistent from sample to sample. Materials with one or both of these problems include:

• crossed Polaroid sheets
• color film (of any type, no matter how exposed or processed)
• dye-based black-and-white film
• magnetic floppy disk medium
• compact discs^*
• non-optical grade aluminized Mylar (e.g., Pop-Tart wrappers)^*
• smoked glass^*

^* These filters are considered "unsafe" principally because of variations in preparation from sample to sample. Individual examples of these materials can be safe if their transmission characteristics can be determined and shown to be sufficient for solar viewing (e.g. < 0.001% transmittance at all wavelengths). In general, though, trying to make "homemade" filters is a bad idea.

The optical properties of some of these materials have been characterized in detail. The February, 1998 Sky & Telescope cited in the references gives some detail. Several years ago, a direct transmittance measurement of 5.25-inch floppy disk material was posted to sci.astro. The material is highly transparent to IR with wavelengths greater than 800nm.

What should I use to view a total/partial/annular solar eclipse?

You need to use a safe filter whenever a portion of the photosphere, the brightest part of the Sun, is visible. The photosphere is the part of the Sun that is visible through a good filter as a disk about half a degree across; it's also the part that you see during sunsets through haze, or when projecting the solar image using a telescope onto a screen. As far as eclipses are concerned, cases in which some photosphere is visible are the following:

• all partial eclipses
• all annular eclipses
• total eclipses before and after the total phase (i.e., before second contact and after third contact)

Even a small amount of photosphere (such as that present in a deep partial or annular eclipse) is enough to be damaging, since it's the intensity of the light that determines its potential for damage.

Contrary to widespread belief, viewing the total phase of solar eclipses does not pose a threat to vision -- the intensity of the corona and chromosphere is much too low to cause damage. The total light output of these outer regions of the sun is comparable to that of the full Moon.

What do the numbers in welder's glass specifications mean (e.g., #14)?

The shade number (SN for short) is based on the optical density of the glass in the visible region. If the fractional transmission of light through the glass is T, then

 OD = -log T
 SN = 1 + (7/3) OD

Consider, for example, a filter that blocks 99.9% of the visible. T = 0.001 and hence OD = 3, and SN = 1 + (7/3)*3 = 8. This would thus be typical for a #8 welder's glass.

If you plug SN=14 into the formula above, you can show that the OD is 39/7, or 5.57... A #14 welder's glass transmits about 3 millionths of the light hitting it.

Note that this formula only applies to transmission in the visible; for IR and UV the specifications are a little more complicated, and not given by a nice neat formula like this. In the US the requirements for welders' glasses are given in the ANSI publication listed in the references. In general the tolerances for UV and IR are not quite as strict as those in the visible, which is part of the reason astronomers recommend the darkest shades of welders' glass for solar viewing.

Given that I have a safe filter, is it suitable for naked eye/binocular/ telescopic use?

All safe filters can be used as naked eye filters.

Welder's glass is not optically flat. It is good for naked eye viewing, and possibly low-power binoculars. Used at high powers, it tends to produce undesirable distortions. The largest instrument I've seen anyone use a welder's glass with is an Edmund Astroscan, which is a small (4.25" aperture) reflector optimized for low powers (typically 15x to 50x). Additionally, the small size of welder's glasses (the largest sizes are about 6 inches square) makes them awkward on large telescopes anyway.

Welder's glass gives the Sun a greenish color, which some people find aesthetically unappealing.

Both the mylar and the metallized glass telescope filters work well for telescopic use at a wide range of magnifications. There are two main differences, for practical purposes. The Mylar filters tend to be cheaper, and there is a difference in the color of the image -- aluminized Mylar often renders the Sun blue, whereas metal-coated glass gives it a dull orange-red color. The difference is mainly cosmetic; some observers use normal orange colored filters in conjunction with aluminized mylar to give the Sun a more natural color.

I have one of those in-the-eyepiece solar filters. What should I do with it?

Take a large sledgehammer and make filter confetti out of it. Since these filters absorb nearly all of the energy passing through your scope, they can get dangerously hot and (if you're not fortunate) crack while in use.

Sunlight collected by a telescope is particularly powerful. People who use telescopes for solar projection -- pointing a telescope at the Sun and projecting its image onto a screen -- sometimes damage their eyepieces, when the Sun's heat melts the cement holding the lenses together. Eyepieces are designed to be nearly transparent and thus to absorb very little solar energy -- imagine what must happen to a filter that is nearly opaque.

Most scopes that come with such filters are small (60mm) refractors. This is a common size, and replacement filters that go over the aperture of such a scope should be available from major filter manufacturers. In particular Celestron offers a line of over-the-aperture filters; since this company also makes a number of 60mm telescopes, one of their filters ought to fit.

Over-the-aperture filters, already described as the preferred method of solar filtration, go on the sky end of the telescope, thus rejecting most of the solar energy before it even gets into the scope. Additionally, they work principally by reflection, rather than absorption, so even in full sunlight they don't get dangerously hot. (Again, so long as you use them as intended. Just about anything will get dangerously hot if placed near the focal plane of a telescope pointed straight at the Sun.)

I saw an ad for "hydrogen alpha" filters. What do they do?

A hydrogen alpha (Ha) filter is used for special kinds of solar observing. The name comes from the alpha emission line of hydrogen, which is in the red portion of the visible spectrum. Many solar features are most prominent in the red light of this line, and get drowned out by the intense yellow-white light of the photosphere. With an Ha filter, many solar features, notably prominences, that are invisible normally become easy to see (without an Ha filter, about the only way to see prominences is to be fortunate enough to see a total solar eclipse).

The passband around the Ha line must be extremely narrow for such details to be visible -- typically 0.1 nm (compare 8-20 nm for narrowband light pollution filters). Because of this extremely tight tolerance, Ha filters are very expensive. Most professional and some club-owned instruments have them, but they're less common among ordinary amateurs.

Both Lumicon and Daystar make some Ha filters that can be used with common commercial telescopes such as Celestron and Meade SCTs. They are still quite expensive, and don't work well with all telescopes, but may be worth investigating if you're interested in Ha. See David Knisely's Hydrogen Alpha WWW page for more information on amateur astronomy with hydrogen alpha filters.

Who makes them?

Lots of places.

For welders' glasses, visit a local welder's supply store. Note that #14, being an extremely dark shade, is rarely used by welders. At the shop I got my #14 filters, the cashier remarked that the densest shade they normally get requests for is #10, with very occasional requests for #12. Consequently, you will probably have to order #14 filters specially. Cost is about $2 for a rectangular filter about 2" by 4".

For telescopic filters there are a number of major manufacturers and resellers; your best bet is to get an issue of Sky & Telescope or Astronomy and check the ads. The February, 1998 issue of S & T gives a number of representative manufacturers. Some common names include Solar-Skreen, a Mylar based filter produced and sold by Roger Tuthill, Inc.; DayStar Inc. (which also makes hydrogen-alpha filters); and Thousand Oaks, a company that makes metal-coated glass filters. Celestron also makes a number of solar filters. Also, Questar ships its telescopes with a metal-coated glass solar filter, designed specifically for use with the Questar.

Some suppliers that normally sell "finished" aluminized Mylar filters will often sell pieces of filter-grade Mylar that can be used as is or mounted into a homemade filter. Finally, the next time there's a solar eclipse (of any depth, even a partial eclipse) nearby, visit a local planetarium or science museum. They'll probably have inexpensive cardboard "sunglasses" with safe Mylar filters as "lenses".

References

• Sky & Telescope, February 1998, p 36. Extensive information on a wide range of filters including transmission measurements on over 20 materials used or proposed as filters. Covers new filter designs as well as old standbys like welder's glasses.
• Sky & Telescope, August 1981, p. 119. An article by the author of the February, 1998 article, describing the kinds of damage sunlight does to the eye, and the transmission characteristics of several kinds of filters available at the time.
• Sky & Telescope, July 1991, p. 79. An article describing the solar eclipse of that month, with an emphasis on safety precautions for viewing the partial phases, and hence for solar work in general.
• ANSI Standard Z87.1, 1968 The formal US standard for safety of certain welders' equipment, including filters. A more recent version than 1968 may be available by now. Keep in mind standards outside the US may (probably will, in many cases) be somewhat different.
• Sliney, D. and Wolbarsht, M. Safety with Lasers and Other Optical Sources. Summarizes the ANSI specifications and may be easier to find than the ANSI document itself. Also covers case histories of eye damage from intense light sources and similar useful nformation.