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Solar Telescopes, Solar Filters & Accessories

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Solar Eclipse

A solar eclipse is an incredible sight to behold, with the day taking on a twilight appearance as the Sun darkens to just a bright corona in the sky. An eclipse can feel like a once in a lifetime event, and indeed on average a total solar eclipse will only reoccur in the same location once every 375 years. However every year there is a minimum of two solar eclipses of one form or another. While most envision an eclipse as the phenomena described in the opening to this segement, there are several types of solar eclipses.

Different Types of Solar Eclipses

  • Total Solar Eclipse - A bright corona of light surrounds the Moon, an effect sometimes referred to as the "diamond ring". This is the most visually impressive eclipse, but is the least frequent type of solar eclipse and is only visible from a narrow area.
  • Annular Solar Eclipse - In this type of eclipse the Moon is still surrounded with light, however compared to a total eclipse this presents as thicker ring. These eclipses are more frequent than a total solar eclipse, but are still relatively rare.
  • Partial Solar Eclipse - This is the most frequent type of eclipse, and is visible from a much wider area. Unlike the other solar eclipses that show a ring, this type of eclipse has a crescent shaped swath of light.
  • Hybrid Solar Eclipse - This eclipse is a hybrid of the total and annular types. It is the rarest type of eclipse, and is considered a hybrid as it will either appear as a total eclipse or an annular eclipse depending on where the eclipse is viewed from.




Viewing A Solar Eclipse

The hybrid solar eclipse isn't the only eclipse that appears differently depending on location, as total and annular eclipses will look like a partial solar eclipse for those outside of a narrow area. This partial eclipse area only covers so much, and outside of this area a solar eclipse will not be visible at all. Luckily the areas from which an eclipse can be observed can be predicted in advance, allowing us to chart the path the solar eclipse will take and ensure that we're in the correct location to view the event. For example, we know that the next solar eclipse will occur on April 8th 2024, where it will begin in Mexico and then pass through the United States up to Canada. The time an eclipse will occur and how long it will take can also be calculated, so we can be in the right place at the right time too! Wanting to know the best places to view this upcoming eclipse? Our article, How to View the 2024 Total Solar Eclipse, breaks down the perfect locations for this unmissable event.

Solar Glasses

Solar eclipse glasses are quite likely the most popular way to view the solar eclipse. This is because you don't need magnification to observe the solar eclipse, so these simple glasses are a great accessible way to enjoy the event. Typically these are constructed similarly to a pair of old 3D movie glasses, with solar safe film in place of the red/ blue gels. This specialized film is what allows these to be used for direct solar viewing. While the tinted view the film provides can look sunglass-esque, you can not use sunglasses, 3D glasses, polarized lenses, or any tinted glass to safely view an eclipse. The film used in solar eclipse glasses needs to meet the ISO 12312-2 standard to provide a safe viewing experience. There are a wide range of glasses that purport to be solar safe on the market today, however as discussed in our Solar Eclipse Safety article purchasing solar products from well established brands should be a large consideration when selecting a pair of solar glasses. For your convenience and safety, High Point Scientific only offers solar glasses that meet the ISO standard and are from suppliers recommended by NASA and the American Astronomical Society for safe solar view. It is also important to note solar eclipse glasses are designed to be used standalone - they can not be used with binoculars, telescopes, or any other optics in place of a proper solar filter.

With their simple construction, solar eclipse glasses are incredibly easy to bring with you wherever you're viewing the Sun or solar eclipse! This design isn't meant to be disposable, however solar eclipse glasses can be a bit more fragile than some other solar viewing equipment, and accordingly should be inspected for damage prior to every use.

A pair of Celestron EclipSmart Solar Eclipse Glasses

Solar Glasses Benefits

  • Certified safe
  • Very affordable
  • Incredibly portable
  • Most user friendly option, as there is no knowledge of telescopes or equipment required


Disadvantages of Solar Glasses

  • Not the most durable option for those interested in reuse/ more frequent solar viewing
  • Can not provide a more "up close and personal" view of the Sun or a solar eclipse

Solar Filters

A solar filter is a great way to get in close to a solar eclipse or observe the Sun, as these make observing with a telescope or binoculars safe! These filters achieve this by blocking enough of the incoming light from the Sun to make looking at the Sun safe, while still providing a great view. These filters come in a number of different designs that attach in different ways, however when it comes to the filter material itself there are just two main types - solar safe film and solar safe glass.

Film filters are made of, as the name suggests, film which is coated on both sides. As with solar eclipse glasses, this film is ISO 12312-2 certified. Crucially however, this film blocks light before it reaches the telescope optics and is magnified. Solar eclipse glasses would need to block much, much more light to allow for safe viewing through telescopes or binoculars - and so while the materials are the same, a dedicated solar filter is needed.

Glass solar filters use glass that is typically coated on just the underside for their filter medium. This makes them a bit more durable and easier to maintain as compared to film filters, as the surface that is most exposed to the elements can be usually be cleaned without fear of damaging coating. This doesn't necessarily mean glass is better, as discussed in our article that explores the glass vs film debate in more detail.

An AstroZap solar film filter

When thinking about purchasing a solar filter, there are some important considerations. First is portability. While the filters themselves aren't particularly heavy or bulky the equipment they're used with can be, which can complicate travel. Next is safe fitment. To provide a safe viewing experience, the filter coating(s) need to be free of damage, no stray light should be able to enter the optics, and the filter should securely attached. Selecting a filter of the correct size is a large part of this, which you can read more about in our guide on measuring your telescope.

Solar Filter Benefits

  • Certified safe
  • Allows for a closer, more detailed solar viewing experience as compared to solar glasses
  • Designed for more frequent solar observation


Disadvantages of Solar Filters

  • Pricier compared to solar glasses
  • Requires additional equipment for use
  • Safe, secure, fitment needs special attention

Smart Telescopes

A relatively new type of product in the mainstream consumer astronomy community that has quickly gained popularity. Smart telescopes don't necessarily do anything new, but instead seek to combine mount, camera, WiFi control, software assist tools like planetariums and plate solving, and image stacking/ processing all in one self-contained product - and make all of it easy to use. These devices are generally very light and compact compared to more traditional rigs, making them easy to transport. These aren't just for observing the night sky either, with manufacturers like Vaonis, ZWO, and Unistellar bringing solar observing to their smart telescopes.



The Seestar S50

Just as with conventional telescopes this is mostly made possible thanks to a solar filter, which corresponding smart telescope software has then been designed around. Since smart telescopes tend to have their own unique designs however, at this time these filters are largely only available in proprietary options. Once the filter is place these devices make it simple to get pointed at the Sun for general solar observing, or to observe a solar eclipse! Since the Sun or eclipse is being viewed on a mobile device this may feel a bit less "personal" as compared to direct observation, but it is important to keep in mind that the view provided is still unique and reveals even more detail than is often visible with the naked eye - something that can be easily captured with smart telescopes for enjoyment long after the event has ended! An additional benefit to the mobile device experience is that it is inherently safer than using a solar filter or solar glasses and allows for multiple people to share the view; however when using solar glasses and filter products properly it is important to stress they are still incredibly safe.



Smart Telescope Benefits

  • Easy to use
  • Typically very portable, making it easy to travel with
  • Safest way to observe the Sun and solar eclipse
  • Great for solar viewing with a group
  • Dual functionality opens up night time observing and imaging


Disadvantages of Smart Telescopes

  • Pricier than solar glasses or solar filters, as these are entire telescopes, mounts, and imaging systems that need an often sold separately solar filter
  • Some may feel the experience is less personal
  • Brightness of mobile device can be a consideration for daytime viewing

Vaonis Hestia

Vaonis has taken a unique approach with the Hestia, which is the first smartphone-dedicated smart telescope. This type of smart telescope utilizes the user's smartphone for some of the features that would normally be built into the device itself, in the case of the Hestia both the computing power and the imaging capabilities. This approach allows the Hestia to be an accessible and very portable option for viewing the night sky - or with the Eclipse package, the Sun and solar eclipse!



The Vaonis Hestia

With the offloading of the "brains" of the smart telescope to a mobile platform, Vaonis has created their powerful Gravity to ensure users still receive the full smart telescope experience. This easily guides users through the setup process, so Hestia is ready to observe in no time. The Gravity app has solar observing functionality built in, which allows users to capture incredible views of the Sun when using the dedicated filter included with the Eclipse Package. This not only provides a view of the Sun in real-time, but also can capture images that can be viewed later and shared for further enjoyment! With this solar observing feature and the ultra-portable design Hestia is the perfect companion from viewing the solar eclipse, whether that be from home or an eclipse viewing destination!



Hestia Benefits

  • Easy to use
  • Very portable, making it easy to travel with
  • Safest way to observe the Sun and solar eclipse
  • Dual functionality opens up night time observing and imaging


Disadvantages of Hestia

  • Pricier than solar glasses or solar filters
  • Some may feel the experience is less personal
  • Brightness of mobile device can be a consideration for daytime viewing

Coronado H-alpha Solar Telescopes

Dedicated solar telescopes are great tools that can provide an even more stunning view of the Sun, by blocking certain frequencies to display the incredible H-Alpha features that lay beneath. Coronado is no stranger to these types of telescopes, producing our top of the list for Best Solar Telescope for 2024 - the highly regarded Coronado PST!

Coronado's H-alpha solar telescopes reveal the intricate detail on the Sun's surface by integrating two key types filters into their scopes. First is the etalon filter, which reject most all visible light that isn't on the Ha wavelength. These incredible filters focus the telescope on a section of the light spectrum that can start at just 1 Angstrom in width - that's 0.00000000001 meters! These filters are also stackable, which shrinks the already narrow bandpass even further. With a window that small, any shift in the wavelength these fine details appear on due to external conditions would result in a suboptimal image. To account for this Coronado implements a patented RichView system, which allow the etalon to be tuned for the best possible view.

While etalon filters do most of the "heavy lifting" in a dedicated solar telescope system, they still do let some extra frequencies through. This is where the second filter comes into play. Placed behind the etalon filter, often times in the diagonal, a blocking filter cuts down the incoming light even further to provide a highly detailed and safe view of the Sun or a solar eclipse. These filters don't make Coronado telescopes great for direct observing only - as these dedicated solar telescopes can also be used in conjunction with a camera for stunning solar images!



An image captured through a Coronado PST by valued High Point Scientific customer Ron P. Click to Enlarge Image

"Bought a lot of gear from HPS and I've never been diappointed. Really enjoying look at the sun and even doing some imageing using my Coronado PST. Great company to do business with." - Review and image captured through a Coronado PST by valued High Point Scientific customer, Ron P.



Coronado H-alpha Solar Telescope Benefits

  • Hands-down provides the most detailed and highest contrast view of the Sun and solar eclipses in a consumer telescope
  • Integrated RichView tuner allows adjustment for the best view
  • Double stack models and add-ons available for even more performance

Disadvantages of Coronado H-alpha Solar Telescope

  • More expensive than solar filters or solar glasses
  • A bit less user friendly than solar glasses, solar filter, or smart telescope options

Lunt Solar Systems H-alpha Solar Telescopes

Lunt is no stranger to dedicated solar telescopes, with their website proudly stating "Lunt Solar Systems, based in Tucson Arizona, is the world’s leading manufacturing and sales facility that designs, fabricates, assembles, and tests solar telescopes and solar filters." Indeed it is hard to argue that a company that has worked with NASA and Princeton University isn't one of the premier manufacturers of quality optics that provide stellar solar views. For an example of what their top-of-the-line telescopes are capable of, one need look no further than this image captured through their LS230THa Double Stack Solar Telescope with B3400 blocking filter.



An image captured through a LS230THa Double Stack Solar Telescope with B3400 blocking filter Click to Enlarge Image

Image through LS230THa from Lunt's website



Lunt quality isn't just for big institutions, with their excellent LS40THa 40 mm scope with B600 blocking filter coming in at a very competitive dedicated H-alpha telescope price. Like other dedicated solar telescopes, Lunt's offerings isolate the H-alpha details of the Sun and its chromosphere with advanced filters. To isolate the H-alpha light and reveal the Sun's chromosphere and other structures like solar prominences and granules, an etalon filter is used. These filters are nothing short of incredible - where standard astrophotography filters have bandpasses measured in nm an etalon's bandpass will be listed in Angstroms, which is equal to 0.1 nm! This window can be narrowed even further with the use of a second etalon, and it is for this reason that Lunt offers add-on etalons as well as models that come double stacked from the factory.

Were one to view the Sun in space, the Ha wavelengths emitted by the Sun would remain relatively constant. Here on Earth however these wavelengths can shift, which becomes particularly problematic when using filters that let very select light through. It is for this reason that etalons are tunable to shift the wavelength they isolate. There are a number of ways that an etalon can be tuned - mechanical compression to physically move the filter components, tilting the filter components to change the distance light travel between plates, or adjusting the barometric pressure inside the filter. The latter is what Lunt has implemented on a good number of their scopes, as this keeps the etalon components parallel and does not distort the optical surfaces like a mechanical compression might.

While Lunt's filters do a fantastic job at focusing in on the Ha wavelength, they do still pass some additional unwanted light through as all etalons do. To reject the rest of these wavelengths Lunt telescopes utilize a blocking filter in the diagonal to complete the Ha isolation and provide an amazing image of the Sun or a solar eclipse!



Lunt Solar Systems H-alpha Solar Telescopes Benefits

  • Hands-down provides the most detailed and highest contrast view of the Sun and solar eclipses in a consumer telescope
  • Tunable to provide the best possible view, with some models implementing Lunt's great pressure tuning system
  • Double stack models and add-ons available for even more performance

Disadvantages of Lunt Solar Systems H-alpha Solar Telescopes

  • More expensive than solar filters or solar glasses and can take time to produce
  • A bit less user friendly than solar glasses, solar filter, or smart telescope options

Solar Observing FAQ: What You Need to Know

Solar eclipses have been viewed as mysterious and ominous events in times long gone by, however here in the current day we've come to understand them as interesting and visually striking feats of astronomical physics. We've delved into the Top Ten Facts About Solar Eclipses within our Astronomy Hub, though to shed some light on just how the Moon blocks the Sun's, we've created a list of answers to this and many more commonly asked questions!

What is a solar eclipse?

A solar eclipse is a phenomenon that occurs when the Moon passes between the Earth and the Sun, blocking the Sun's rays. There are many chances for a solar eclipse to happen in a year, however due to the Moon's elliptical and tilted orbit the necessary alignment for a total solar eclipse often doesn't occur. In many cases the Moon misses the window for alignment entirely, in which case an eclipse will not occur. Other times it may only be partly in position as it passes in front of the Sun, which results in a partial eclipse. Even when the Moon passes directly in front of the Sun, a total eclipse still may not be observed. Owing to its elliptical orbit, the distance between the Earth and the Moon varies. When the Moon is farther away from us when passing through that key position needed for an eclipse, it fails to fully block the Sun and results in what is known as an annular eclipse. For more information be sure to check out our articles Why Do Solar Eclipses Happen? and Understanding The Different Types Of Solar Eclipses

What determines the path of a solar eclipse?

When the Moon passes in front of the Sun during a solar eclipse, it only blocks light in a relatively small region on the Earth's surface. This means that even parts of the Earth that are in daylight when the Moon passes in front of the Sun may not see the eclipse at all, or only see a partial eclipse. As the Earth spins on its axis and continues on its path around the Sun, and the Moon continues its orbit around Earth, the section of the Earth experiencing an eclipse also continues to shift. This creates a band or path where the eclipse will be visible over the course of the eclipse. Where this falls on the Earth's surface depends on the orientation of the globe and position of the Moon. As these positions are able to be predicted in advance, maps are available for eclipses far into the future.

How often do solar eclipses occur?

A solar eclipse can only occur when the Moon is in its new Moon phase, which occurs every 29.5 days. However as the orbit of the Moon is tilted about the Earth slightly, often times it is not in the right place to cause an eclipse. Taking this into account, an eclipse can happen as frequently as five times a year or as infrequently as only twice per year. There can not be less than two eclipses of some kind per year.

How often does a solar eclipse reoccur at the same location?

The frequency at which total and annular eclipses occur in the same location was approximated in 1982 by Belgian astronomer Jean Meeus. His analysis concluded that, on average, a total solar eclipse will reoccur at the same location every 375 years, with annular solar eclipses occurring every 224 years. Combining the two, he concluded that on average an eclipse of some type will occur every 140 years at a given location.

When is the next solar eclipse?


The next solar eclipse will occur on April 8th, 2024. This will be a total solar eclipse, and the eclipse will pass over Mexico, the US, and Canada. Be sure to check out our When Is The Next Solar Eclipse article for more information on the upcoming total eclipse!

How long does a solar eclipse last?

The duration of a solar eclipse varies, as the Moon's transit in front of the Sun changes from eclipse to eclipse; with a similar eclipse reoccurring only once every 18 years, in what is known as a Saros cycle. This variance means that total solar eclipses can last anywhere from a few seconds, to about as long as seven and a half minutes. An annular eclipse can last a bit longer than a total eclipse though still is only visible for a relatively short amount of time, often lasting less than ten minutes.

Is it safe to look at an eclipse with the naked eye?

Even though the Moon is blocking a lot of the Sun's rays, even a small amount of light can cause permanent damage or blindness. Never look directly at the Sun for any period of time without the proper viewing equipment. Dedicated solar telescopes, telescopes used in conjunction with solar safe film or solar safe glass filters, solar binoculars, or eclipse glasses should be used when observing the Sun during an eclipse. For more information on safe viewing of the Sun, please refer to our How To View A Solar Eclipse and View The Sun Safely With Solar Eclipse Glasses articles.

Can I view the solar eclipse with sunglasses, 3D glasses, polarized lenses, or tinted glass?

Absolutely not. While these do help shade your eyes from the Sun, none of them block enough light for direct solar viewing. Solar eclipse glasses use a specialized film that is designed to allow safe solar viewing. To ensure a safe viewing experience, you will want solar eclipse glasses that meet the ISO 12312-2 safety standard; which we've made easy by offering only eclipse glasses that meet this certification standard.

What can you use to view a solar eclipse?

There is a wide variety of equipment available now to provide a safe, spectacular viewing experience. Equipment suitable for observing the solar eclipse includes solar eclipse glasses, solar safe film and solar safe glass filters that can be used with telescopes or binoculars, specialized solar binoculars, and dedicated solar telescopes. In addition to these tools which allow for direct viewing of the Sun, indirect individual viewing has begun to gain traction thanks to the growing popularity of smart telescopes. These devices have begun to offer dedicated solar filters, which make imaging of the Sun and solar eclipses more accessible. For more information about solar viewing gear, read on in our Must Have Solar Eclipse Gear article!

Can you watch the solar eclipse with binoculars?

Without any additional protections in place, you can not safely view the solar eclipse with binoculars. However for those looking to view the Sun or solar eclipse with binoculars there is good news, as solar viewing solutions are available. To use a set of existing binoculars, solar safe film filters specially designed for this application can be placed in front of the lenses. These work well, however as with standard solar filters care needs to be exercised that these filters are kept securely in place during use. More permanent solutions are available in the form of dedicated solar binoculars, which integrate the solar viewing elements directly into the optics, eliminating the concern of secure attachment at the cost of regular viewing functionality.

Solar Terms To Know

Angstrom

The Angstrom is a unit of measurement, named after Anders Jonas Ångström. It is denoted by the Å symbol, and is equal to 0.1 nanometers (0.00000000001 meters). Classically the Angstrom has been used to quantify spectral lines, such as those important to astronomy like H-alpha, OIII, and SII. Currently the nanometer (nm) is the more common unit used for most applications, however the Angstrom is still prevalent when discussing solar viewing equipment such as the Etalons used in dedicated solar telescopes. It is easy to convert between the two by moving the decimal place one place. For example, Ha’s emission line can be expressed as 656.46 nm or 6,564.6 Å.

Annular Eclipse

“Annulus” refers to a ring shaped object, and is where the Annular Eclipse gets its name. As the Moon has an elliptical orbit around the Earth, it is not always the same distance away. An annular eclipse occurs when the Moon is at or near its farthest point from the Earth, making it appear smaller in the sky. Due to this, when the moon passes in front of the Sun it does not block it as entirely resulting in a thicker ring around the Moon than what is visible in a total eclipse. While still rare, this type of eclipse is more frequent than a total solar eclipse.

Astrophotography

This refers to photography of astronomical bodies and phenomena. Astrophotography is not new, for example the popular T threading still used today harkens from Tamron’s T-mount developed for their 35 mm cameras - however it has seen a notable increase in popularity with improvements in cameras, mounts, filters, and software making astrophotography much more accessible. This is not limited to celestial bodies such as nebulae, planets, or galaxies either, as solar imaging is now more within the reach of the average consumer than ever before.

Baily's Beads

This is the name for a visual phenomena that occurs during an eclipse, where dots or "beads" appear around the Moon. Due to the Moon being home to mountains and craters just like Earth, the outer edge of it is not a smooth circle but instead full of peaks and valleys. As a result when the Moon passes in front of the Sun these structures create sections where more or less Sun can shine through, forming the distinct beads.

Blocking Filter

Blocking filters are used in dedicated solar telescopes to reject all remaining light outside of the H-alpha wavelength. This is necessary as etalons pass harmonics of the desired wavelength. Luckily however these harmonics are far enough apart that while the filter in a blocking filter has a relatively speaking large bandpass, compared to that of an etalon, it can block them while still providing a clear view of the H-alpha light.

Chromosphere

Like Earth, the Sun has layers spanning from its surface down to its center. The chromosphere is one of these layers, and can be observed once some of the other visible light from the Sun is filtered out. As the chromosphere and its features are predominately only visible in H-alpha, a dedicated solar telescope is needed to isolate this wavelength that is otherwise washed out by brighter light standard solar filters pass.

Corona

The corona is the outermost layer of the Sun's atmosphere, extending far from the solar surface. This layer is usually invisible due to it being very diffuse and easily outshined by the other light emitted by the Sun, however is observable during a total solar eclipse. When a solar eclipse is at totality, where the Moon is directly in front of the Sun, the corona can be seen around the Moon.

Coronal Mass Ejections

Coronal mass ejections, commonly shortened to CME, are eruptions of high energy from the Sun. CME’s are similar to solar flares, and these ejections can occur after a flare, though these phenomena are different. In a CME there is a large explosion of energy, ejecting sizable amounts of electromagnetic energy and particles out into space. Again CME’s are similar to solar flares in that they interact with the Earth’s atmosphere and cause technological interference.

Dedicated Solar Telescope

While there are telescopes that permanently integrate standard solar films or glass which could be thought of as a dedicated solar telescope, typically the term is used to describe telescopes that use more specialized Etalon and blocking filters. These filters are designed to isolate a specific wavelength, H-alpha, from the wide spectrum of light that the Sun emits, revealing more detail and contrast on the solar surface. For more information, including what types of eclipses these are best suited for, check out the Solar Telescope section of our Must-Have Solar Eclipse Gear article.

Diamond Ring

Another visual phenomena that occurs during a solar eclipse, the "Diamond Ring" is related to Baily's Beads. As a solar eclipse progresses, Baily's Beads will begin to disappear. When all but one bead has disappeared, the eclipse can be said to resemble a diamond ring with one bright section "set" in an even band of light.

Eclipse

A solar eclipse occurs when the Moon passes between the Earth and the Sun, causing the Sun to be obscured by the Moon in the process. Since the Moon has to pass directly between the Earth and the Sun, a solar eclipse can only occur at the new Moon. While new Moons occur about once a month, the moon’s orbit is tilted resulting in many months where perfect alignment of the Sun, Moon, and Earth doesn’t occur. This means that even for those who land in the path of an eclipse (the band around the Earth where the Moon will pass in front of the Sun) may not see an eclipse; as this phenomena can be over in mere seconds. The longest a total solar eclipse can last is seven and a half minutes.

Etalon Filter

Etalons are the primary filters used in dedicated solar telescopes to isolate the H-alpha light emitted by the Sun. Contrary to standard astronomical filters that use coatings to block unwanted light, etalons use two reflective surfaces placed closely together that interact in such a way that only light of certain frequencies will pass through to the other side. The spacing between these surfaces and properties of the air inside determine which light is allowed through, features that are exploited in etalon tuning systems to adjust the wavelength being passed.

Hydrogen Alpha (H-alpha)

In very simplified terms, when atoms change energy levels, specific wavelengths of light can be emitted. Hydrogen has one of these wavelengths (or more specifically, spectral lines) around 656.46 nm, in the form of H-alpha. This is close to red and can be observed in nebulae - or more importantly for the subject at hand, in stars such as our Sun. When being written, H-alpha is commonly shortened to simply Ha in the astronomy community.

ISO Certified

The International Organization for Standardization (ISO for short) is an organization that, as the name implies, seeks to create sets of unified standards - most notably film speed standards. They created the ISO 12312-2 standard for products to be used in direct solar observing. Solar glasses, films, and other gear that have this certification, have been found to meet this agreed upon standard for safe viewing of the sun and solar eclipses.

Path of Totality/ Centerline

As the Earth is constantly spinning and orbiting the Sun, with the Moon continuously orbiting the Earth, the area on Earth's surface where the Moon appears to line up with the Sun shifts over the course of an eclipse. This traces a path across the Earth, where an eclipse will be visible at different times. This can have several "sections" or "lanes", to denote where a total solar eclipse will be visible vs only a partial solar eclipse. The section where a total eclipse is visible is known as the path of totality. Within this path, the line running directly in the middle of the path of totality is called the centerline. Locations that fall along the centerline will experience a total solar eclipse that lasts longer than areas further out in the path of totality.

Penumbra

Referring to a shadow, the penumbra is the lighter outer section that is only partly in shadow. When the Moon passes in front of the Sun, the section of the Earth that is only partly in shadow therefore is consider to be in the penumbra. From this zone, only a partial solar eclipse will be visible.

New Moon

The Moon goes through several lunar phases, from New Moon to Full Moon and then back again. This cycle takes about 29 days, meaning there is a New Moon about once a month. At New Moon the Moon appears completely dark or unlit, and it is in this phase that a solar eclipse can occur.

Node

When describing the Moon's position in relation to a solar eclipse, a node is a point along its orbit that would place the Moon in line with the Sun. This isn't necessarily where an eclipse will happen, just a location where it can happen as the Moon still needs to be in the New Moon phase when passing through this point for eclipse to occur. There are two nodes in the Moon's orbit, one where a solar eclipse can happen and one where a lunar eclipse can happen.

Single Stacked/Double Stacked

Single or double stacking in a dedicated solar telescope context refers to the number of Etalon filters used. Stacking of filters is done to further isolate the wavelength of light that passes through the telescope, or in other words reject more light that isn’t the desired wavelength. The range of wavelengths above or below the desired wavelength is commonly referred to as a filter’s “bandpass”. The smaller this range is, the more detail and contrast from the selected wavelength can be resolved. This is desirable for solar viewing, where there is a good amount of H-alpha detail.

Saros Cycle

While it may seem like every eclipse is unique, it has been determined that about once every 18 years the Earth, Sun, and Moon will return to a similar position resulting in a similar eclipse; though the locations through which this near identical path will differ. This period of 18 years is known as a saros, and is used to describe this cyclical nature of solar eclipses.

Smart Telescope

Smart telescopes are, relatively speaking, new in the consumer space. These telescopes seek to combine as much separate hardware as possible into one unit, and pair these electronics and optics with modern quality-of-life and computational advancements such as digital planetariums and target lists, plate solving, and image processing. Smart telescopes are often fully self-contained units that can be set down and then moved/ observed remotely from a mobile application. Most manufacturers now offer dedicated solar filters for these smart devices, allowing for general solar and solar eclipse viewing in addition to their night sky observing capabilities.

Solar Flare

A solar flare is an eruption of electromagnetic energy from the Sun. These eruptions occur in areas of high activity on the solar surface, and eject their electromagnetic energy far out into space. At times where the Earth is in the path of this energy, it interacts with the upper atmosphere and can cause radio wave interference.

Solar Granule

These “granules” give the surface of the Sun its textured appearance. These form when hot plasma pushes to the top, and cooler plasma begins to sink. The difference in temperature results in a bright center and dark edges. As the plasma sinks it begins to heat again, and the previously heated plasma begins to cool, resulting in an ever changing pattern across the solar surface.

Solar Maximum and Solar Minimum

The amount of activity on the Sun's surface is not static, instead increasing and decreasing over time. When activity is at its peak, this is known as the solar maximum. Intuitively, the solar minimum is when activity dies down and the Sun is at its most calm. This occurs cyclically, with the Sun reaching its solar maximum every 11 years on average. For solar observing, this is important as observable features such as sun spots, solar flares, and CME's are most prevalent during a solar maximum.

Solar Prominence

Solar prominences are “structures” that project from the surface of the Sun, and are sometimes referred to as solar “filaments”. These structures are loops of plasma that extend far out from the Sun, and can be seen during a total eclipse. These can be seen when generally observing the Sun (particularly in H-alpha) as well, as they contain comparatively cooler gas that gives them a darker appearance. These dark loops snaking across the solar surface is where they get their other name of solar filaments.

Solar Safe Film

Solar Safe Films are, as the name suggests, films that have been designed to provide a safe solar viewing experience. These films are covered on both sides in a coating that rejects most of the incoming light, only passing a safe amount through to be viewed. These films can often be referred to as Mylar films, due to part of their construction, or sometimes “white-light” films owing to the white hue they usually (but not always) give the Sun. For more information on solar films and how they compare to glass filters, read on in this article.

Solar Safe Glass

A popular option for safe solar eclipse viewing is the Solar Safe Glass filter. These filters use a piece of glass that is typically coated on the underside, as opposed to solar films which are coated both top and bottom. The solid construction makes them a bit more durable and easier to clean, as the uncoated outer surface can be cleaned without concerns of damaging the safety coating. These can sometimes be known as “orange view” filters, as the light they safely let through usually has a warmer tone as compared to solar films. To learn more information about these filters and how they stack up to the popular films, check out this article.

Sun Spot

When an area on the surface of the Sun has a strong localized magnetic field, a Sun spot is created. These spots are visible when viewing the Sun through a solar filter or dedicated solar telescope, appearing as dark dots on the solar surface. This dark appearance comes from a difference in temperature, as the power magnetic field impacts the ability of new hot plasma to flow which causes a cool spot to form.

Umbra

An umbra is the darker section of a shadow. When discussing a solar eclipse, this refers to the darker section of the eclipse which is the section that will observe a total solar eclipse. The path the umbra will trace along the Earth's surface as an eclipse progresses is what is known as the path of totality.

UTC

UTC is a standardization for time throughout the world. UTC 0 is the time zone at 0 degrees Longitude, with other time zones adding or subtracting an hour as they move away from this zone. For example New York City, NY in UTC -5 is 5 hours behind UTC time. As UTC does not observe daylight savings time, the difference from UTC can change in time zones that do. This standard is important to know as some events, such as solar eclipses that can travel through multiple time zones, and other astronomical data/ equipment may use this standard.