The Importance of Solar Filters

Spectrum Solar Filter for Telescopes
Spectrum Solar Filter for Telescopes

While most of us assume astrophotography is a nighttime hobby, there is one object that is reliably visible during the daytime every day of the year as long as the sky is not entirely cloud-covered. You may have guessed this object is the sun. This is especially true in the summer when the sky cloud cover is less, and the days are longer than in the winter. The converse is also true about summer nights, which are shorter and offer less time for nighttime observing or photographing.

While solar viewing and photography can be extremely fun, there are some safety precautions that must be followed to prevent harm to the telescope or camera and irreversible damage to your eyes. It should be obvious that the sun is extremely bright, and one should not look directly at it without protection. The same concept is true for a telescope or camera, and they should never be pointed at the sun without a proper filter.

In general, there are two types of proper solar filters: film and glass filters. Film filters are usually cheaper in price than glass filters. You can often buy solar filter film and make your own filter to fit your camera lens or telescope. Film filters have one potential advantage in that the sharpness of the view can be a bit better than a glass filter. But one very important risk of a film filter is that it can easily be torn or develop small pinpoint holes that can allow the full light of the sun to pass. This can be extremely dangerous, and they should be inspected thoroughly every time they are used. Make sure you buy film filters from reputable manufacturers to ensure high quality.

My recommendation is that you should pay a bit more for a glass telescope solar filter since they are more durable and less prone to develop small imperfections that might allow full sunlight passage. I have seen advertisements for solar filters that fit onto an eyepiece for use with a telescope. In my opinion, you should NEVER try this since you are allowing the full intensity of the sunlight to pass through the telescope, which focuses it even more. This could potentially overheat the filter and cause it to crack, which could be catastrophic if you are observing at that time. Instead, use a filter that is fitted to your telescope aperture or camera lens. In this way, the light that enters the telescope or camera is already reduced in intensity, making it safe to observe directly or to photograph. Just make sure the filter fits snugly or is attached in some way, ensuring it does not fall or blow off during a session. Make sure you measure the outer diameter of your telescope or camera lens before purchasing a solar filter, or buy one specifically designed for your camera or telescope. A proper match is essential for safety.

Thousand Oaks Camera Lens Solar Filter
Thousand Oaks Camera Lens Solar Filter

For telescopes, Spectrum makes a fine glass filter of varying sizes, for example, the Spectrum Glass Solar Filter: 6.5” Cell Inside Diameter #ST600G. It blocks 99.999% of the sun’s light, leaving a yellowish-orange look. For camera lenses, Thousand Oaks Optical makes high-quality screw-in film solar filters of varying sizes for camera lenses, such as the Thousand Oaks Thread-On SolarLite Filter. Before you buy, make sure you check the front of the lens for something that looks something like this: ΦXmm. The “x” represents the thread diameter of accessories for the lens, including filters. Make sure you buy a filter with a size that matches the “x” on your lens.

If solar observing is more than just a passing interest. I recommend buying a dedicated solar telescope. They tend to be expensive, however, the quality of the image and the safety of the solar telescope will be exceptional. Lunt and Coronado are two well-known manufacturers of excellent solar scopes. Check out the Coronado SolarMax III 70mm Solar Telescope for $3799 (may require special order), or check out my review of the Lunt Solar 80mm f/7 Modular Telescope.

FAQ about viewing solar eclipses

Q: Is it safe to look at the sun during the partial phases?

A: NO!! Even though the sun is covered partially leading up to totality, it remains very bright until more than 99% is covered. In fact, even 30 seconds before totality, the landscape still looks like it is daylight. The ONLY time it is safe to look at the sun directly without protection is during the few minutes of totality.  I actually recommend shedding your glasses during totality since the sun’s corona is only visible during totality, and you won’t see it with solar filter glasses on.

Q: What type of filter is safe?

A: There are many safe filters for solar observing. Usually, in the months leading up to a solar eclipse, you will see advertisements for solar viewing glasses. As long as they are made by reputable companies, they should be safe to use. Make sure there are no obvious tears in the filter and inspect it for tiny pinhole defects. If there are any imperfections, discard them and don’t use them. Keep in mind that as the date of the eclipse nears, it will be harder to find quality filters since the demand will rise sharply.

Q: I don’t live in the path of totality, but the sun will be 95% covered in my location. Do I still have to use protection?

A: Yes, absolutely. As in the previous question, the sun remains dangerously bright right up until the total phase. Even if 99% of the sun is covered by the moon, it will still be dangerously bright.

There will be a great interest in solar filters every time a total solar eclipse will be occurring. It really is a spectacle worthy of traveling to see at least once in your lifetime. But let’s not ignore the beauty the sun offers even when it is not eclipsed. Just remember to use high-quality protection at all times.

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