Caoimhín's Content

All of the videos I make use Stellarium, we’ve seen how it can transport you through time and to other planets, and we’re going to play around with one of its features in this video, the ability to simulate the view from a telescope. Telescopes are simple in principle but of course the details can get a little complicated. This can especially be an issue if you are buying a telescope, and given the time of year I’m sure many telescopes are about to be gifted. It’s especially hard to buy a telescope for someone if you yourself aren’t into astronomy, so this video aims both to show off what Stellarium can do and give some hints to potential shoppers.

The telescope and ocular plug-in is the feature we’re focusing on and its accessed in the top right of the Stellarium screen. If you want to zoom in on something you need to have something selected, so we’re going to start with Saturn. Once we’re zoomed in, you’ll see a bunch of numbers just under the ocular symbols. These are the oculars or eyepieces that you are using, if it’s a telescope, the telescope or binoculars you are using and any extra lenses you have attached. Starting with the telescope itself, Stellarium offers a big range of models and sizes, so we’ll start with something small, 60×700.

As I mentioned, there are a lot of numbers, but these two are some of the most important and are used to describe most telescopes. The 60×700 is 60 millimeters wide by 700 millimeters long. The width, 6cm here, is the aperture, the width of the hole at the front that the light comes through, while the length is the focal length, how far back the light is actually focused. The aperture of the telescope is usually pretty close to the width, where as the focal length can often be much longer than the actual tube of the telescope, usually thanks to mirrors bouncing the light back and forth, essentially folding it into a smaller space. Smaller telescopes are usually refractors, which only use lenses, rather than reflectors with mirrors, so for a telescope like this, the focal length and the actual length are probably pretty close.

These two numbers can tell you even more about the telescope. If you multiply the first number by 2 (if it’s in millimeters, by 50 if it’s in inches), you’ll get roughly the maximum magnification. So for a 60×700 telescope, you can expect to max out at around 120 times magnification. Usually, if the telescope claims it can do more than this, then it is lying.

Telescopes usually have an eyepiece, a little lens at the back to look through, and in many telescopes this can be swapped out and changed. These eyepieces are called oculars, and they can effect how much magnification you can get, as well as how much of the sky you can see. The amount of sky visible through your telescope is known as your field of view or FOV. It’s usually measured in degrees of arc, or hours, minutes and seconds of arc. As we are looking at the sphere of sky around us, we measure things by how many degrees of a circle they take up in the sky, and how much of the sky your view takes in is measured the same way. Usually, oculars that have wider fields of view have lower magnifications, but there are various types of oculars with varying effects. Different eyepieces can even alter the “relief” or “eye relief”, that is, how far away your eye needs to be from the eyepiece to see correctly. Eyepieces usually have a little eye relief at least, so you don’t have to actually put your eye on the eyepiece, as is the case for most binoculars and microscopes.

There are of course ways to squeeze a little extra magnification out of a telescope, known as Barlow lenses or teleconverter lenses, these are extra lenses that slot in between the telescope and the eyepiece. They effectively increase the focal length of the eyepiece, increasing the overall magnification. These lenses will allow you to zoom in more, but the atmosphere around us will always limit the amount of detail we can see, as will the quality of the lenses you are using. You can get more out of a telescope with the right combination of oculars and Barlow lenses, but there will always be upper limits.

Sometimes, you want less magnification! This may sound strange, but a telescope with a narrow field of view might not be able to see all of the Moon or the Andromeda Galaxy at one time. A focal reducer does the opposite of a Barlow lens, decreasing your magnification and usually increasing your field of view. With options like this, even a rather small telescope can satisfactorily observe a variety of different targets. However, some things are still better suited to some targets.

A small field of view is best for planets, and magnification doesn’t need to be incredibly powerful. For very distant objects, ones that appear small in our sky, a small field of view is still fine, but magnification becomes more important. If you are looking at nearby nebulae, clusters of stars, or very nearby galaxies, a wide field of view is more useful. This can be achieved with a telescope, but sometimes a binoculars is even better suited.

I’ve mentioned in previous videos that the Pleiades are a wonderful target for a telescope and that is demonstrated in the video here. Binoculars also come in various sizes and magnifications, but they are far less adaptable, you usually can’t change the eyepieces on binoculars. They are usually much smaller, which makes them far more portable. Very big binoculars however can be heavy, and this can be very taxing on the arms if you are looking high up into the sky. Binoculars don’t usually come with a tripod, but many can still be mounted on one, and this can be worth it if you are planning on using a large one to look high up in the sky. Looking almost straight up to the Andromeda Galaxy for example, a big pair of binoculars can offer a wonderful view, but it’s a had position to maintain without some support.

With big telescopes, very wide ones in particular, you will often have mirrors or a combination of mirrors and lenses to help make the telescope shorter. Combination telescopes are often called catadioptric, and a lot of bigger commercial models will be of this type. For this reason, their focal length will often be very different to their actual length, so a different number may be used, the focal ratio. The focal ratio or f-number is the aperture divided by the focal length. This is an important number in photography, and some camera lenses, telephoto lenses, are practically small telescopes, they are just as appropriate for astronomy and especially astrophotography.

If you want to take photos of the sky, you do have options, and Stellarium can help you look through those as well. Besides letting you test out the view you would get through various telephoto lenses, Stellarium lets you see how much of the sky a camera sensor would pick up. Camera sensors can be mounted directly onto telescopes, and adapters are available to mount cameras on telescopes as well, essentially turning the whole telescope into a telephoto lens. Camera sensors are often rectangular, so knowing exactly what will be in the frame of your picture can be quite useful. With these tools, Stellarium gives you a chance to test out different arrangements on different targets, which can be especiallly useful in astrophotography. With so many options and variables, astrophotography can take a bit of practice, and this is a way you can do some practice indoors.

I hope that this will give you some idea as to what telescope you might like, either for yourself or as a gift. Very big telescopes may seem better, but unless they have a wide field of view you might be limited to very distant objects. A nice pair of binoculars is great for some targets, less so for others. All of this demonstration was made possible by the Ocular plugin for Stellarium, a piece of software that was written and released for free. You can check out the end of the video to see the creators credited or head over to the Stellarium website. If this video and write-up didn’t give you the information you need to buy a telescope, you can download Stellarium for free and have a look around yourself.

Even if you don’t get a telescope this year, I’ll keep showing close-ups of interesting things in these videos, so I hope you’ll continue to join me for them in the future.