Solar observation is the only area of astronomy where a mistake can result in injury or permanent bodily harm. And even if you think you’re safe from buying the right equipment, there have been numerous cases of fake eclipse glasses sold resulting in permanent eye injuries in 2017 and even cases of fake ISO certification labels, meaning the ISO certification can no longer be trusted as the sole guideline to guarantee a safe solar filter or pair of eclipse glasses.
This is not to say solar observing should be avoided, however. It is no more dangerous than many outdoor activities. However, care should always be taken while observing the sun and solar eclipses.
With that said, let’s first get to some basic safety information and to choosing equipment.
Approved Brands for Solar Filters
The American Astronomical Society and others have a list of approved brands. Here is a condensed list of the brands they and others recommend for completely safe solar filters and eclipse shades:
- Adorama
- Alpine Astronomical
- American Paper Optics
- American Paperware
- APM Telescopes
- Astro-Physics
- Baader Planetarium
- Celestron
- DayStar
- Explore Scientific
- Galileo Optics
- ICSTARS
- iOptron
- Halo Solar Eclipse Spectacles
- Jaxy Optical Instrument Co., Ltd.
- Kendrick Astro Instruments
- Lunt Solar Systems
- Marumi/Agraph Corp.
- Meade Instruments
- Orion Telescopes & Binoculars
- Rainbow Symphony
- Seymour Solar
- Solar Eclipse International
- Spectrum Telescope
- Thousand Oaks Optical
- TSE 1/110th.De
- Vixen Optics/Mr. StarGuy
Using a Solar Filter Safely
Unless you are using a dedicated solar telescope or solar binoculars with a built-in filter, you should always make sure of the following when using your filter on telescopes, spotting scopes, and binoculars.
- NEVER use a solar filter that threads onto the eyepiece end of your telescope
Many older (particularly Japanese-made) refracting and reflecting telescopes come with these, as do (sadly) some cheap department-store telescopes. Being so close to the focal plane of the telescope they are subjected to an intense, nearly focused pinpoint of light and heat which can crack them (and in turn let the unfiltered sun into your eyes) in under a second. I have met folks who have had permanent eye damage from using these devices or ones who walked away from their telescope only to come back and find the filter cracked!
- Always make sure your filter is securely attached to your telescope
Even if sunlight is not directly shining into the objective lens or mirror of your telescope, it can easily bounce off the tube walls and find its way to the eyepiece.
- Don’t use truss/collapsible tube telescopes for solar observing
This seems obvious, but with the advent of low-cost collapsible-tube reflectors, people seem to be doing solar observing with them more and more. Even with a light shroud, all it takes is a slight breeze to potentially let sunlight in to hit the primary or secondary mirror and blind you.
- Don’t use a large Dobsonian telescope for solar observing
A big Dob not only is pointless for solar observing (daytime seeing always limits a scope above 6-8 inches); it’s also dangerous. Going blind is bad enough, but a large Dob could easily start a fire on a sunny, dry day if sunlight strikes the primary mirror, igniting its own wooden components or nearby grass. I recall a story of a man whose shroud slipped off his 25” (yes, 25”) Dobsonian during a gust of wind and the scope proceeded to ignite both its wooden upper tube assembly as well as the area around it. All that was salvageable were the mirrors and a few other parts, and there was a large scorched patch of ground next to where the scope was.
- Check for pinholes
Unless your solar filter is a solid glass design, it probably uses a thin, Mylar-like safety film. Hold the filter/glasses up to the Sun or a bright light and look at the ground to be sure there are no pinholes that can compromise your safety.
- Do not use welding glass
Only certain types of welder’s glass absorb enough light to be used safely for solar viewing – others let in too many harmful UV rays (or just too much light altogether), and some can even provide a too-dim image. The risk is overall not worth it, and in any case, the image is an ugly green, and welder’s glass doesn’t always have a good optical quality which would result in a less-than-sharp image of the Sun anyway.
- Do not use eclipse glasses or other naked-eye solar viewers on a telescope
A telescope amplifies light and if many eclipse glasses are used with a telescope the resulting transmission is too high to be safe, and in any case, you would have to cover up a lot of the usable aperture and the image quality would be poor anyways.
Choosing & Using Solar Viewing Equipment
Hydrogen-Alpha vs. White Light – What’s the Difference?
White-light solar filters use similar filtering to eclipse glasses and welder’s glasses. A tiny fraction of a percent of the light hitting the filter is let past into the telescope objective, while the rest (including especially harmful UV rays) is reflected or absorbed. The result is usually a white, yellow, or orange view of the Sun.
White-light solar filters will show you sunspots and the grainy solar surface known as the photosphere. As of the time of this writing the Sun has been at a solar minimum for the past couple of years, with few observable sunspots and overall low activity, so don’t be surprised if you point your scope at the Sun and see nothing at all but a blank disk.
Hydrogen-Alpha solar telescopes use a Fabry-Perot etalon to narrow the band pass of the telescope to no more than a few angstroms of the hydrogen-alpha wavelength of light, a fraction of the bandwidth transmitted by a nighttime hydrogen-alpha filter. This also dims the image enough that less additional filtering is needed. In dedicated solar telescopes, the etalon is located at the front with an additional blocking filter located inside the telescope or diagonal, while in the DayStar Quark the etalon is located within the device and depending on what telescope you’re using you may or may not need an energy reduction filter for additional filtering. The Sun in Hydrogen-Alpha appears reddish and in addition to sunspots and granulation, it also shows solar flares, prominences, spicules, and plages. These features can change by the hour, making H-Alpha solar astronomy an extremely entertaining “live” experience, albeit a pricey one.
Choosing A Solar Filter
If you are looking to buy a filter for your telescope or binoculars, you can get one (either glass or film) from Baader Planetarium, Celestron, Orion, Seymour Solar, Spectrum Telescope, or Thousand Oaks Optical. While some other AAS-approved brands make solar filters, the aforementioned companies provide the best optical quality out there. My personal favorite is Baader Planetarium, which also make filters for pretty much every sized spotting scope, camera lens, or even binoculars out there.
Make sure to measure the inner and outer diameters of the front of your telescope tube/dew shield. Some manufacturers make filters for a fixed size; others make filters that have some adjustability inherent in their design.
You can also buy the film from Baader Planetarium or Thousand Oaks and make your own mounting for it to secure it to your telescope out of cardboard or wood. If you have the skills and confidence to do so it can save you a lot of money. However, you must be more vigilant about pinholes and light leaks to ensure safe operation.
In addition to regular white-light solar filters, DayStar makes a device called the Quark which can be used with some refractors with no additional filter, and with almost all refractor, Maksutov-Cassegrain, and Schmidt-Cassegrain telescopes if an additional Energy Reduction Filter is used. The Quark is expensive, but various models allow you to view the Sun in both Hydrogen-Alpha and Calcium-K wavelengths.
Herschel Wedges – Avoid Them
The Herschel Wedge, invented by John Herschel in the 1830s, is an old design for a solar filter that replaces the star diagonal in a refracting telescope. It uses a prism to redirect almost all of the light out the back end of the telescope, then a few neutral density filters to filter down the remaining light to a safe level.
I do not recommend buying a Herschel wedge. For one, they are expensive compared to solar filters and do not give significantly better images, despite what some people may claim. Second, in the interest of keeping pricing low (or at least making it appears to be that way) the manufacturers can sometimes make you buy the filters separately which can cause confusion and risk the consumer not obtaining the right/enough/any filters to save money (or simply due to being unaware), which is, of course, a safety hazard. Third, the beam of light out the back of the wedge is a fire and eye safety risk – some wedges have a ceramic cooling system to disperse this beam but some do not. Lastly, because UNFILTERED sunlight is entering your refracting telescope and being focuser by the objective lens, the risk exists that you could accidentally scorch the internal light baffling of the scope if you are pointed slightly away from the Sun.
Choosing a Solar Telescope
While you can obtain dedicated or semi-dedicated white-light solar telescopes, if you have at least $600 to spend you can buy a hydrogen-alpha telescope. We recommend reading our Best Solar Telescopes guide to learn more.
Viewing the Sun with a Telescope or Binoculars
- Getting Aligned
The first step to viewing the Sun with your telescope is to get it pointed there in the first place. Surprisingly, this process can be trickier than it sounds! You can use the scope’s shadow to line it up with the Sun, but this can prove difficult and annoying, especially with catadioptric instruments. The best solution is to buy or make a dedicated pinhole projecting solar finder like the Tele-Vue Sol-Searcher.
With solar binoculars, try to resist the urge to sight along the barrel with your unshielded naked eye to acquire the Sun; while not as dangerous as viewing through an optical device it is not great for your eyes to look at the Sun unfiltered for any length of time for any reason.
- Aperture, Eyepieces, & Magnification
The Sun, of course, produces an immense amount of heat. As a result, the area of sky directly around the Sun typically has poor to mediocre seeing, and you will probably not gain anything by going above 6 to 8 inches of aperture besides the ability to use slightly higher magnifications without dimming the image too much.
Your pupil isn’t going to be dilated during the day, so you want to use eyepieces with an exit pupil of 3-4 mm or less, which corresponds to at least 6x-9x per inch of aperture. At the same time, magnifications of over 25x/inch or below a 1mm exit pupil will result in a too-dim view. So try to keep your magnification somewhere between that range.
Field of view is relatively unimportant when viewing the Sun (particularly with hydrogen-alpha scopes which are usually rather small), so I recommend getting the sharpest, highest-contrast eyepieces available for solar viewings, such as Plossl or preferably Orthoscopic eyepieces. A zoom eyepiece (provided it is sharp) can also be handy for solar viewing.
Solar Photography
- DSLRs
You can buy a solar filter for your camera lens, but it is probably best to connect your DSLR to a telescope if possible.
Try to shoot video at the highest frame rate possible to counter atmospheric turbulence, which will smear and blur a single exposure.
- CCD/CMOS Cameras
A CCD/CMOS “webcam-style” camera that inserts directly into the eyepiece holder of your telescope and plugs into a computer via USB is best for solar imaging. The extremely high frame rates of these cameras allow you to combat atmospheric turbulence for the best possible final image. Keep in mind that you may have trouble reaching focus without a Barlow lens, particularly with some of the smaller H-Alpha solar scopes which have limited focus travel. Otherwise, the process is basically the same as lunar/planetary imaging.
- Cell Phones
A cell phone camera really hates the brightness of the Sun through the eyepiece, and will refuse to autofocus and adjust brightness – especially in H-alpha. I would not recommend attempting afocal solar photography.
Viewing Solar Eclipses
A solar eclipse is a special, typically once-in-a-lifetime event. However, making the most out of it can be tricky.
Naked-Eye Eclipse Viewing
Many people make do with a simple pinhole projector. This allows you to view the Sun indirectly and 100% safely, and you can see the Sun slowly appear to have a bite taken out of it, become a crescent, and disappear entirely. If the eclipse is a total one, you can safely look up and view the Sun with no protection during and moments before/after totality and see the “diamond ring” and “Baily’s Beads” effects caused by sunlight passing through craters and valleys on the limb of the Moon just before and after totality, and the beautiful solar corona.
Pinhole projectors are great because they cost essentially nothing, can be made or improvised almost anywhere and with anything, and a group of people can use them. The downside is that the resolution is essentially zero. Even a huge sunspot group that is viewable with eclipse glasses will be invisible.
The next step up from a pinhole projector is eclipse glasses and handheld solar viewers. These are all basically the same thing – a cheapened, less optically smooth version of the film from a telescope solar filter put into cardboard or plastic frame. Some of the more fancy handheld viewers have 2x or 3x magnification using really weak lenses, but I’ve not heard much about them. Eclipse glasses and handheld viewers generally provide an orange, white, yellow, or green view of the Sun.
Eclipse glasses and handheld viewers are great because they’re extremely cheap and can generally be bought in bulk. However, they still do not show you everything.
Viewing Eclipses with Binoculars or a Telescope
Viewing an eclipse with a telescope or binoculars is basically the same as normally viewing the Sun. However, just before totality you can and should take off your solar filter(s) if possible – it’ll allow you to see prominences on the edge of the Moon’s disk that has not been eclipsed, again normally only visible to H-Alpha telescopes. Be sure to put your filter(s) back on as soon as totality ends, though.
Thanks for this informative article.