Proper Solar Safety

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Dec 29

2019

by Norman Marigza

Activities like the annular solar eclipse last December 26 often get a lot of media attention. Astronomers and telescope manufacturers always caution against directly viewing the Sun, but sometimes the increased media attention brings a certain 'fear of missing out'. Several free telescope viewing setups were even deployed across the Philippine archipelago just to provide the public with a chance to view the eclipsed Sun safely. But since the activity falls right after Christmas and there is only so much ground that local astronomy groups can cover, people attempted to view and image the eclipse through unsafe methods. While events like these are well promoted to the public we should also promote solar safety to the same degree, if not even more so.

Unlike most astronomical event that are safe to view, solar observation MUST follow strict safety precautions as unsafe practices can lead to permanent damage to one's eyes or equipment. Aside from filtering out the bright glare of the Sun the other harmful rays in the ultraviolet (UV) and infrared (IR) wavelengths must also be filtered. Direct and unsafe viewing of the Sun results to the over-stimulation of the rods and cones in our eyes as the eye transmits most of the radiation between 380 to 1400 nm to the light-sensitive retina. This results to the burning up of our retina that creates a permanent blind spot that does not heal. The kicker is these retinal injuries are not perceived by the observer as it happens since there are no pain receptors in the retina and effects only occur at least several hours after.

In order to safely observe the Sun one must either use projection methods, use proper solar filters, or use solar telescopes. The safest, yet the most expensive view of the Sun is through a dedicated solar telescope that only views the Sun in a narrow band of light. Each band reveals different features of the Sun. The two most commonly used are the Hα and the CaK line.

The next option is the use of proper solar filters. I emphasize on proper as there are counterfeit filters being sold (mostly online), as well as individuals who use and promote DIY filters. Solar filters are designed to reduce the visual glare as well as filter out harmful UV and IR. The safety of the filter material is checked by accredited laboratories in compliance to ISO 12312-2 based on maximum transmittance of: luminosity, solar UVA, solar UVB, and solar IR. Only accredited laboratories after PROPER testing can guarantee if a filter is safe. They can't simply be tested by shining a UV flashlight and TV remote as some DIY filter makers propose since these only shine at specific wavelengths and are not comparable to the total output of the Sun. Filter material must be tested properly for their maximum transmittance.

See list of ISO certified filters for visual use: https://eclipse.aas.org/resources/solar-filters

Some of the most used material for filters are made from black polymer and aluminized polyester film of a specific optical density. Another popular option are glass filter materials. The choice of filters also vary in densities depending on the use – visual, photographic, used with optical aid (telescope/binoculars). An example of which is the Thousand Oaks Type 3 plus filter which should be used for photographic use only. Since these filters are not as easy to come by a safe and accepted alternative is a welder's glass of shade 14 used in heavy duty industrial welding. Some sources though have stated that NASA allows a shade 12 so some verification might be needed for shade 12&13 (especially since the shade 12 might be uncomfortably bright).

A lot of the so-called “old school” methods are actually UNSAFE. Included among these unsafe methods are the use of photographic film, photographic neutral density filters, polarizing filters, x-rays, floppy disks, compact disks, and many more. Some of these materials are very good in reducing the visual glare but have very high transmittance of near-infrared radiation. Smoked glass on the other hand has very good levels of radiation transmittance but remain unsafe due to inconsistency in the coating of the soot. Doubling on the unsafe material still does not make a filter safe except for two layers of fully exposed and developed silver-bearing black-and-white negative film (don't use non-silver-bearing). Reflection on water is not necessarily safe since it has to be done in a particular way.

The last alternative is the projection method which can be done with or without optical aid. For projection done using optical aids the light from the eyepiece is cast on an external surface so the observer is looking in a direction away from the Sun. Small telescopes are ideal for projection as they do not gather too much light. Catadioptrics are not too ideal since their designs can build up too much heat inside the OTA with a lack of openings for ventilation. The finder must also be removed or capped for safety. Note also that plastic eyepieces can be subject to melting under prolonged exposure to the Sun. There are many ways in which a projection can be done. My personal favorite is a Hossfield Pyramid after Casper Hossfield of the AAVSO Solar Division. For projection without optical aids you can simply use a pinhole projector, or even look at shadows cast from gaps in the leaves. Some even use holes on a strainer or on a cracker.

Events like a solar eclipse is indeed a remarkable thing to observe, however in the interest of promoting astronomy let us also promote proper safety. Let's not risk damaging our eyes for a single event, when there are a vast number of things and events to enjoy in the sky. There will always be a solar eclipse somewhere in the future (like the annular in June and total in Dec – depending where you are observing of course). Let us all be responsible in promoting solar safety for us to enjoy solar astronomy.