What To Look For When Buying A Telescope – Astronomer Advice

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I said it on this blog many times before; there is not just one best beginner telescope for everyone. Each of us buying the telescope are looking for a specific thing to do with it. Anyway, if you have no clue how you want to use your first telescope, read my article about the telescope that I generally recommend for beginners.

But for all of you, who know or have some clue how you want to use it, continue reading. I will show you what to look for when buying a telescope, so you can decide on a specific one that will suit your needs. 

There are many features of the telescope you have to consider before buying one. Do you want to use it for astrophotography or just visual astronomy? Or maybe, you want to build an observatory on the backyard. Some of you might be planning to travel with it on every camping trip under the dark skies, and it doesn’t end here. It is also essential to decide if you want to observe the Moon and the planets or just deep-sky objects. 

You have to consider all of this to buy a perfect telescope for you. Many amateur astronomers will end up owning a few telescopes for each purpose anyway because you can’t do everything with only one telescope. I will list and explain all important specifications to look for before buying a telescope, and at the end, you should know what telescope will suite you best. I will also throw some recommendations for each category to make it easier for you.

Aperture

The aperture of the telescope is the diameter of the optical tube. The aperture is, I think the most important factor to settle on before buying the telescope. Big aperture has more light gathering capabilities than smaller aperture; hence, you will be able to see more stuff. Distant galaxies and nebulas require large aperture. These objects are very dim, and you need to gather a lot of light with the telescope to see details and good contrast. It also depends on your seeing conditions, but basically, the bigger aperture is the way to go here. 

On the other hand, if you are planning to observe mostly planets and the Moon, you don’t need a large aperture. These objects are very bright, so there is no need to gather a lot of light to see nice details. Also, the smaller aperture telescope is more portable. Moreover, if you are under dark skies with great seeing conditions, you can observe some brighter deep-sky objects too.

If you plan to dive deeper into the astronomy hobby, I recommend going for the biggest aperture your budget allows. Large apertures also have advantages while observing planets and the Moon because you will get better resolution and you will see much more details.

The aperture is connected with the magnification too. It determines the maximum useful magnification for the telescope. The rule of thumb is that the highest useful magnification is the aperture size in mm. So, the telescope with an aperture of 80mm have 80x useful magnification, and the telescope with 200mm aperture have 200x highest useful magnification. But in real-life experience, you won’t go over 150 because the earth atmosphere and seeing conditions limit this. So, if you have 400mm aperture, you won’t be able to focus on anything while using 400x magnification. You are limited to maximum 200x (maybe) in the best conditions possible. It really depends on your location. The only advantage of aperture this big will be the contrast and details of the image.

You might be asking now, why the heck are we building huge, few meters long aperture, expensive telescopes in observatories when the earth atmosphere is limiting the image quality and the magnification? Well, you can find the answer in my article why are observatories build on the mountain tops where I also explain the technology used to battle atmospheric conditions.

Refractor vs. Reflector

Ok, so you know you want the big aperture to enjoy deep-sky objects and our solar system in full details. Now you have to decide what type of optical design you want in your telescope. There are two main telescope designs. Refractor and reflector. The main difference is that refracting telescopes are using lenses to produce and magnify the image, and reflecting telescopes are using mirrors. Let’s take a look at the advantages and disadvantages of both designs.

Refractor

The first telescope in history was the refracting telescope. It was invented by Galileo Galilei in 1609. It was a simple design with one convex lens and one concave eyepiece. Nowadays, refractors are more complicated using more lenses with different shapes, but the main feature is the same. Refracting telescopes are using lenses to refract light. They are very common telescopes amongst the hobbyist astronomers.

The main problems with refracting telescopes are a spherical aberration, chromatic aberration, and lens sag.

Spherical aberration is the problem of the spherical lenses because they don’t bring parallel light rays to the same focus point. Each part of the lens refracts light under a different angle, so the light rays are not crossing in the same spot. The parabolic lens can correct this. However, this problem appears only in very cheap basic refractors that are more toys than real telescopes.

Chromatic aberration is a little bigger problem. Single-lens element refracts short-light wavelength(blue spectrum) more than longer-light(red spectrum) wavelengths. It creates color distortion around the object when observing. The problem can be solved with a compound lens, each with a different type of glass and different refracting properties. The telescope with this type of lenses is called achromatic refractor, but this solution is not correcting the chromatic aberration completely. 

The more advanced and more expensive apochromatic refractors are almost free of chromatic aberration, but they come with the higher price. They also use a special coating on the lenses to produce the best image possible. Apochromatic refractors are very common in astrophotography because they are basically huge, high-quality lenses for the cameras.

Lens sag happens only in large aperture refracting telescopes. When the lens is becoming larger, the weight of the lens is also increasing. It is not practical, and the problem is that lens shape is deforming under its own weight. So, making a big lens aperture is not only pointless but also very expensive. It is the biggest problem with the refracting telescopes so if you are already decided that you want a big aperture for your backyard observatory, forget about refracting telescope. Refracting design is used mostly in small spotting scopes, binoculars, or small portable telescopes. But don’t worry because sir Isac Newton got you covered. He invented reflecting telescope in 1668.

Reflector

Reflecting telescope is using concave mirrors to reflect light into the eyepiece. There is no refraction in the design, so all light rays are in perfect focus, and there is no chromatic aberration. And because the mirrors can be supported from behind, they can be made much larger than lenses. This is why all large telescopes in observatories use mirrors rather than lenses. But nothing comes without some issues. The reflecting telescopes suffer from a few problems as well. The problems are coma or comatic aberration, and they require more maintenance than reflecting telescopes. However, these problems are not as significant as in refractors, and the reflecting telescopes are always cheaper too.

Coma or comatic aberration is usually present around the edge of the image in the telescope. In my opinion, it is not a big deal because the center of the field of view is clear and sometimes you won’t even notice it. Anyway, it is a big issue when you want to use the reflector for astrophotography. But it can be corrected with correcting lens that you can buy if the coma is irritating you. However, I ‘m not recommending it for visual observations because implementing another optical element in the telescope will decrease the performance of the telescope. So it is better to learn to live with that.

Maintenance of the reflecting telescope is not difficult. Because it is an open optical design, you have to be careful not to drop anything on the primary mirror. And before every stargazing, you need to leave the telescope outside to cool it down. Rising heat from the inside will cause image distortion. Also, I would not recommend trying to clean the mirror in your telescope unless it’s very dirty- I’m talking a few years of use.

The main part of the maintenance is the collimation. The reflecting telescope is using two mirrors, one primary at the back of the telescope and a small, secondary one at the front. These mirrors need to be perfectly aligned. During transportation or frequent using mirrors can become misaligned. The process of the alignment is called collimation, and it can be done with different methods and tools. Most people are turned off by the collimation procedure and scared to buy a reflecting telescope. But trust me, it is a piece of cake after you do it a few times, and it will take you only 5 minutes before every session.

Reflecting telescopes are made in much larger apertures then refractors so I recommend going for the reflector if you want to enjoy astronomy. And of course, here applies the rule the bigger is better.

Cassegrain Reflector

Cassegrain reflector is a combination of the convex primary mirror, and concave secondary mirror positioned parallel to each other, so the focal point is conveniently placed behind the primary mirror. Secondary mirror reflects the light into the eyepiece through the hole in the middle of the primary mirror. This type of design is creating a longer focal length of the telescope in a physically small optical tube. 

We have two main types: Schmidt Cassegrain and Maksutov Cassegrain. These types of telescopes are very portable. I’m not going to explain in-depth because I already did in the article Schmidt Cassegrain vs. Maksutov Cassegrain. There are advantages and disadvantages to Cassegrain design scopes, but they are excellent telescopes in general. They have all the properties like reflecting telescopes but in a closed small optical tube and they come with a higher price tag of course.

Focal Ratio

We get the focal ratio by dividing the focal length of the telescope by its aperture. For example, one of my telescopes has an aperture of 130mm, and the focal length is 650mm, so the focal ratio is F5. What is it used for? The focal ratio defines how “fast” the telescope is. Telescope with the low focal ratio is fast, and one with the high focal ratio is slow.

The fast telescope is mostly used for observing deep-sky objects like nebulas because it has a wider field of view, and the image is brighter with any given eyepiece or camera. Fast telescopes are also generally best for low magnification wide-field observing, and they are better for long-exposure astrophotography. It’s because the time to collect light for the image is only one-fourth the time with fast F5 than with slow F10 telescope.

Slow telescopes are usually better for lunar or planetary observing with high magnification. The objects are much bigger in the eyepiece because they have a narrow field of view. Slower telescopes can also be really long, so they are not very practical for manipulation or transportation(except Cassegrain design). 

Magnification

Magnification of the telescope may seem like an important factor, but in reality, it is not. As I said before, the magnification is limited by the earth atmosphere. And the significant factor of the magnification is not the telescope but the eyepiece you are using. The magnification is calculated by dividing the focal length of the telescope by the focal length of the eyepiece. For example, a telescope with a focal length of 1000mm and 10mm eyepiece focal length will give you 100x magnification.

Usually, all telescopes come with a set of 2 or 3 eyepieces with different focal lengths. So getting different magnifications is easy because you can always upgrade your eyepiece set later to get the magnification you desire with your telescope.

The magnification is essential only with planetary and Moon observing where you want to see small details of the object. Deep-sky objects are generally observed with low magnification in the broader field of view because they occupy a larger portion of the sky. Magnification is also the power of the telescope. When you see the power of the telescope, it is usually the magnification with a given eyepiece. You can also check my complete Telescope Eyepiece Guide article.

Telescope Mount

Last but not least factor what to look for when buying the telescope is the mount. The telescope mount is the mechanism on the tripod that holds and move the telescope itself. There are two main types of the mount: altazimuth mount(AZ) and an equatorial mount(EQ). They can be motorized and manual. We also have one unique mount called Dobsonian mount, which is basically altazimuth mount but without a tripod.

The telescope mount is sold mostly with the telescope, but you can build your own rig buying the optical tube and the mount separately. Keep in mind that you have to pick mount strong enough to carry your telescope. Buying mount that barely holds your telescope will cause stability problems, and the stargazing can end up as hours of frustration and disappointment. Therefore, never cheap out on the mount because it is a significant part of the telescope.

Alt-Azimuth Mount(AZ)

The altazimuth mount is a simple mount that moves in altitude direction and azimuth direction. Simply said, it moves up and down and rotates parallel to the ground. The mount is easy to operate, and the setup is quick. The only thing you need to do is to level the tripod properly. It is also much cheaper than the EQ mount. 

The advantage of the AZ mount is that the eyepiece will be almost always in a convenient place to look through it. AZ mount is best for visual observing, but it is not the best choice for astrophotography. But still, it can be used for planetary and lunar imaging or short exposure astrophotography. You can read more about AZ astrophotography in my article Budget Astrophotography.

Dobsonian Mount

Dobsonian mounts have all the properties of the AZ mount, but they are without a tripod. It is only a base that you put on the ground. The advantage is that it can carry heavier loads, so it’s great for big aperture telescopes. Dobsonian mount provides steady support for the telescope, and it is very portable. It is also the perfect choice for beginners.

Equatorial Mount(EQ)

There are a few different types of EQ mount, but the most common is the German Equatorial Mount. Unlike AZ mount, the EQ mount rotates parallel to the earth axis. It allows easier tracking because when you target an object, the tracking is in one direction only, unlike the AZ mount. With AZ mount, you always have to adjust both directions to compensate the earth rotation and keep the object in the field of view of the eyepiece. 

Therefore EQ mounts are the best choice for astrophotography. The tracking is more accurate, and there is no field rotation that you experience with AZ mount. However, setting up the EQ mount is more complicated, but it can be mastered over time. Because the EQ mount is rotating parallel to the earth axis, you have to align it with the Polaris, and for the astrophotography, it has to be done very precisely.

Motorized Mount

For visual observing and learning the night sky, the manual mount is completely sufficient. But if you plan to do astrophotography in the future or you want to save time during the stargazing session, then I recommend buying a motorized mount. All types of mounts also have motorized variant. The motorized mount is controlled by electric motors and small handset computer that will guide you through the night sky. It will also track the object for you, so you don’t have to adjust the telescope to keep the object in the field of view.

Finding and tracking the objects is easy with the motorized mount. At the beginning of the session, you have to perform the star alignment and then just type or find the object in the handset. The mount will automatically point the telescope in the right location and start tracking the object for you. The motorized mount database contains thousands of objects to look at, and it also provides the guide of all brightest object on the sky for the given night.

Telescope Recommendations By Category

Now, let’s take a look at the recommendations that I promised. First of all, it is tough to recommend something so complex like a telescope, so I hope that some of you will be able to choose for yourself. After reading my article, you have more information and specifics about what you need and what to look for. Anyway, if you are still unsure, I chose for you the most commonly used telescopes in each of the following categories.

Beginner Telescope

Beginner telescope must be easy to use, and it has to be big enough to satisfy you when stargazing. Never buy a very cheap telescope as your first telescope because it will disappoint you for sure and you will give up on astronomy. Because I expect that you will be using it only for visual observations, I recommend the Dobsonian telescope.

I recommend Orion Skyquest XT8, and you can read more about this telescope in my review article.

Astrophotography Telescope

Astrophotography is a challenging but rewarding hobby with a steep learning curve. Because it is also a very expensive hobby, I recommend starting with a cheaper setup as I recommend in articles: Best Telescope For Astrophotography and Best Telescope Mount For Astrophotography.

But the most commonly bought telescope setup for astrophotography is motorized German Equatorial Mount and Apochromatic Refractor.

Solar Telescope

You can transform any telescope into solar telescope by using a proper solar filter. This setup will allow you to see sunspots on the sun. For a more advanced image like solar prominence, you need specialized filters or telescope that is build for solar observing.

The best entry-level solar telescope is Cornado PST.

Conclusion

I remember buying my first telescope. It was hours and hours of research and a lot of information that I didn’t understand. My last recommendation is: don’t overthink it, roughly decide on what you want, and buy it. Take your first telescope as a learning experience. Believe me; you will be more certain of what you need and especially, what you like just after your first telescope. 

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