Some of the brightest heavenly objects are among the easiest and most interesting to observe. Before setting up your telescope to view them, it’s important to take eye safety into account.
A quick answer to whether a telescope can hurt your eyes
A telescope will not hurt your eyes unless you observe the Sun without proper filters. Even solar viewing, like during an eclipse, is safe if you use an appropriate and high-quality solar filter. You may find other celestial objects (like the full moon) a little uncomfortably bright, but they are not dangerous.
But why can looking through a telescope be dangerous, and what is the safe way to do it? Let us first discuss how light can damage our eyes and what happens when we look directly at the Sun.
Background: how solar light damages your eyes
Looking directly at the Sun is very dangerous even with the naked eye, so observing through a unfiltered telescope would be far more damaging still.
There was an experiment in 2016 when a TV presenter showed what the sunlight could do to your eyes when looking through a telescope with no filters. He used a dead pig’s eye and placed it at the eyepiece of a telescope. After about 20 seconds, smoke started to come out of it. It seems that the telescope works as a magnifying glass, focusing the light and heat into a single point. After dissecting the pig’s eye, they concluded that even the retina was burned.
The pain would prevent most people from burning our eyes to the point of smoking, but the vision damage would be debilitating. It’s likely to take a full year to recover, at best.
So, if you find yourself wanting to observe the Sun, make sure that you are using the right filters to reduce the light and its impact. That’s an important topic that we’ll cover farther down.
Do telescopes actually make things brighter?
Although it may seem that telescopes make things brighter, they don’t actually increase brightness since they don’t affect how much light the source emits. They just magnify its dimensions so that we perceive it as brighter in some cases. The maximum brightness that you will get is the one that you can see with your naked eyes.
As you set the magnification higher on your telescope, the surface you see is broader, and the light your eyes receive is the same. That is why you now actually see it darker because the light is no longer focused on one point but is distributed in a larger area. With that being said, let us discuss the safe way to observe the Sun and other stars.
The safe way to observe the sun through a telescope
If you are interested in the Sun or wish to see a solar eclipse, you will need to block at least 99% of its light to avoid damaging your eyes. First of all, you have to make sure you are using the right filters.
Objects such as smoked glass, candy wrappers, or compact discs are not safe filters because although they block most of the Sun’s light, harmful radiation can still get through and hurt your retina. Remember to avoid filters that cover only the eyepiece end because the sunlight will destroy it, along with your eyes.
The best way to go are the filters specially made for telescopes, such as a sheet of solar-filter material. This type of filter is placed and covers the entire front end of the telescope. In case the telescope is bigger than the filter, you must protect it with some sort of mask and place the filter in the middle hole.
These solar filters are usually made out of metal-coated glass or Mylar and can block at least 99% of sunlight. We must emphasize the fact that you must place these filters in the front end of the telescope. If you put them at the eyepiece, these filters can be burned, or they can crack from the intense heat of the Sun, and of course, damage your eyes permanently, even causing blindness.
Using these filters, you will be able to see the Sun’s surface in a pale yellow, blue, or orange color, depending on the type of filter you have. You can stare at the Sun without any worries for as long as you like, since there are no risks for your eyes with these types of filters (when properly designed and installed, of course).
You should also look out for how well you place the filter so that it won’t fall off while you’re gazing at the Sun. This advice works just as well with binoculars.
A better way to see the Sun in detail, although pricey, is with small refractors with apertures that have built-in interference filters.
You also can buy these filters separately and attach them to your refractor’s tube. This equipment allows you to view different layers of the Sun, such as the hydrogen-alpha line at 656.3 nanometers and the calcium-k line at 393.3 nanometers.
These aspects cannot be seen in white light and are an exciting aspect to gaze at, even when the Sun is not eclipsed or transited.
Is it safe to look at other stars through a telescope?
Besides the Sun, it’s safe to look at other stars with a telescope. For example, Sirius is the brightest star in the sky, with an apparent magnitude of -1.46. It is quite close to Earth, at 8.6 light-years away, and though it appears as a single star, it’s made up of two.
Even though it is a star system with a star twice the mass of the Sun and one that is approximately the same size as this celestial object, it is safe to look at it through a telescope.
Stars will not damage your eyes, even with a giant telescope, but might have an unpleasant dazzle. If you find this bothering you, you can buy a neutral density filter, which is great for gazing at the Moon as well. A variable polarizing filter can be adjusted to the exact level at which you feel comfortable looking at.
When it comes to other bright celestial objects, Mercury is more comfortable to view during the day. However, you have to be careful not to accidentally swing the telescope toward the Sun or watch objects near it. A good idea is to stay in a building’s shade so that the Sun is out of reach.
Some have experienced some ocular soreness from looking at Venus during the night. Although Venus appears brighter during the night, which is why most astronomers prefer viewing it during the day, it will not cause any damage to your eyes.
The uncomfortable sensation could appear from the fact that the eyes need a while to adjust from the sky’s darkness or from Jupiter’s lower brightness to the shining Venus.
Is it safe to look at the moon through a telescope?
The Moon does no damage to your eyes, even when it’s full. The light emitted by this celestial object is far less than that of the Sun. However, if you know you have rather sensitive eyes, it might feel quite uncomfortable, but even so, it won’t cause any harm regardless.
The Blood Moon is safe to view through a telescope as well, so you don’t have to worry when observing the Moon. If you were wondering about the safety of watching a Lunar eclipse, then enjoy the view, whether it’s by using your telescope or your naked eye. It will do you no harm whatsoever.
You can also try watching the Moon during the daytime for a spectacular view. As long as you don’t look directly into the Sun, it’s presence won’t damage your eyes.
You can gaze at the Moon’s beauty in peace. But don’t try this when there is a solar eclipse because that would mean exactly what we advised you not to do, meaning you would be directing your telescope towards the Sun as well.
When to use a moon filter
If you find the brightness of the Moon overwhelming, you can always use a neutral-density Moon filter that will lower its light, or you can try to place a stop-down mask at the front-end of the telescope.
For this, you will need to cut a circular piece of cardboard with a hole in the middle of about 2 or 3 inches. You will then place this cardboard in front of the objective’s lens. Don’t forget to make sure it is well placed so that you won’t have any unpleasant bright surprises.
Keep in mind that Moon filters are various, and you need to choose the right one for you. For example, some filters let only a certain percentage of light pass through them. This is also related to your telescope’s diameter. The smaller it is, the less light passes through it, and as such, you won’t need a high percentage filter for it.
There are also variable Moon filters that let you choose the amount of light that comes through. If you enjoy watching the Moon frequently, then this type of filter is more useful for you because it lets you adjust the light according to the Moon’s phase and the light it emits.
The scale from which it varies is usually between 1% and 40%. These lenses also have the option to be adjusted while in place so that you can fix it quickly to the desired level.
Some filters can be neutral and not alter the real color of the Moon’s surface. This type of filter only dims the light rather than change it. Others can lower the Moon’s brightness without altering your view of other near celestial objects or double stars.