If you are planning to buy your first telescope, you can come across some terms that you don’t understand. Actually, if you’re a total beginner and it’s the first time for you to come across astronomy and especially telescopes, there will be many terms. It is a good practice to learn what they mean before you buy something. They can help you with what telescope you decide to buy in the end. I have made a list of basic terms that you can encounter, and I’m going to explain them as easy as I can.
OTA is short for Optical Tube Assembly. It is the actual telescope tube. The reason we need this term is that many telescopes are sold as OTA only, so keep this in mind and read the description of the item carefully. Some resellers are using images of a complete telescope with a mount, but they are selling only the OTA.
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Telescope mount is an instrument that is carrying the OTA. We have two main types of mounts. Altazimuth mounts and Equatorial mounts.
Beginners mostly use altazimuth mounts(AZ) for visual astronomy. They work on basic altitude and azimuth movements. These mounts are very easy to use and setup.
Equatorial mounts(EQ) are more complicated and usually used by more experienced individuals. These mounts are necessary to do professional Astrophotography. The movements are in the right ascension and declination, and they have to be polar aligned before the use.
Polar alignment is used only on EQ mounts. Low-cost EQ mounts have to be positioned to the north and then adjusted according to the latitude, so they point to the Polaris star. More advanced EQ mounts include polar scope inside the mount. So, you have to align them with the north and look through the polar scope to align it with the Polaris. This is more precise polar alignment compared to low-cost mounts. Although, there is no need for perfect polar alignment if you are doing visual astronomy only. On the other hand, it is crucial for astrophotography because you need to track the sky with the highest precision possible.
Altazimuth mounts don’t need the polar alignment.
GoTo Systems can be used with both AZ and EQ mounts. It is an upgrade from manual operation of the mount to motorized, computer-controlled movement. GoTo system is using motors to move the mount with the telescope, and it is controlled by the handset. The beauty of the GoTo mount is that you can choose any object you want and the telescope will point and track the object automatically. I recommend this for beginners because it will save you a lot of time during the stargazing. GoTo database contains more than 40 000 objects.
The focuser is an instrument that you can find on every telescope. It is an essential part of the telescope because it is used to focus the image in the eyepiece or camera attached to the telescope. Focusers have two rotating knobs on each side. By turning them, focuser tube is moving up and down to reach the focus. You can find telescopes with dual speed focusers as well. These focusers have one small additional knob on the right side for fine-tuning the focus. It is a very handy feature, but it usually comes with more expensive telescopes. You can replace the focuser on most telescopes if you want to change it for dual speed one.
Refractor telescopes are using only lenses to provide the image of the object. If you tell someone that you have a telescope, they usually imagine the refracting telescope- the tube on the tripod. These telescopes have the focuser mounted on the back of the tube.
Reflector telescopes, also called Newtonians, are using mirrors to provide the image of the object. They don’t have any lenses in the tube. The design consists of one big primary mirror at the back of the tube and a small secondary mirror mounted at the front of the telescope. The focuser is positioned on the side at the front of the telescope, above the secondary mirror.
Dobsonian telescopes are exclusively reflectors mounted on the Dobsonian mount. Dobsonian mount is very portable, and it can carry very big and heavy telescopes. But even with a small OTAs, Dobsonian telescopes are the best choice for beginners because you get a lot of telescope for your money. They are easy to use and good for transportation. The Dobsonian mount is just a platform on the ground without the tripod, and the movements are exactly the same as with AZ mount because Dobsonian mounts are AZ mounts.
Collimation is the most important procedure you have to perform on every reflecting telescope. The Collimation is a special alignment of both primary and secondary mirror to get a clear and focused image. If the mirrors are not collimated, you will get a blurry image in the telescope. Some people are doing collimation before every observing session. But, if you do it right and you don’t move or travel with the telescope, you don’t have to do it very often.
Refracting telescopes don’t need the collimation because they don’t use mirrors.
Aperture – The Bucket Of Light
Aperture is the diameter of the lens or the primary mirror, depending on the type of telescope you have. It is a critical number because the aperture will determine what you will be able to see. The bigger the aperture, the more light the telescope can collect. And that means brighter objects with better resolution. For example, the aperture of 130mm will collect 30% more light than 114mm aperture. In result, you can see dimmer objects like nebulas or galaxies with a bigger aperture.
Focal length is sometimes referred to the length of the telescope tube which is not entirely true, but we can live with that. The correct answer is that the focal length is either distance between the primary mirror and focal point in reflector or the distance between the lens and focal point in a refractor. The focal point is a point in the focuser tube where parallel light rays come together bouncing of the primary mirror or passing through the lens. If you are trying to focus and turning adjusting knobs on the focuser, you are searching for the focal point. Focal length is important for the magnification of the telescope. The longer focal length is, the more magnification can be achieved.
Focal Ratio or F Number
Focal ratio is the ratio between the aperture and focal length of the telescope. It is measured by dividing the focal length by the aperture. For instance, the telescope with a focal length of 650mm and aperture 130mm has focal ratio f/5. The F-number is significant because it’s telling us how fast is the scope. Fast scopes have low F-number, and slower scopes have high F-number. The example of a fast telescope is the f/5 number. And why is this important? It’s because faster scopes can give you a brighter image than the slow scopes. The f/5 will show you 4x brighter image compared to f/10. You need fast telescopes to observe dim deep sky objects. The focal ratio doesn’t play a big role with planets and the Moon.
Resolving power of the telescope is the magnification that we all love. Magnification of every telescope must be calculated. You have to divide the focal length of the telescope by the focal length of the eyepiece you are planning to use. So, the scope with a focal length of 650mm and the eyepiece with focal length 10mm will give you 65x magnification. This means you will get different magnifications with different telescopes and eyepieces. But, you have to keep in mind that the magnification is limited by the 2x aperture of the instrument. In our example, the aperture is 130mm, so the highest possible magnification is 260x. If you exceed the maximum useful magnification, the image will be blurry with no chance to focus.
The eyepiece is a small lens instrument that you have to put in the focuser to see the image through the telescope. Eyepieces come in many different types, sizes, and focal lengths. Remember that the focal length of the eyepiece is determining the magnification of the telescope. You can read my article Telescope Eyepiece Guide to learn more about them.
If you don’t have a short focal length eyepiece and you need higher magnification, Barlow lens is a handy tool. Barlow lens is multiplying the focal length of your telescope. If you have a Barlow lens with 2x multiplier, it will double your magnification. Barlow can be 1.5x, 3x, etc.
The focal reducer is the opposite of the Barlow lens because it is shortening the focal length of the telescope. 0.5x focal reducer will reduce the focal length to half. It is mostly used if you need a wider field of view with a long focal length telescope.
Filters are specially coated glass accessories that you can screw on the end of the eyepiece or camera. A specific filter will allow only specific wavelengths of light to go through the eyepiece. This will let you see more details of the object. Moon filter can help you see the full moon better because it works like sunglasses. The full moon is very bright, and it’s uncomfortable to look at it through a telescope without the filter. Light pollution filter will allow you to see more objects on the sky in highly polluted areas like cities. Then we have very special and expensive filters called narrowband which allow only a small spectrum of the wavelength to go through. It is used mostly in astrophotography to get the best details from nebulas.
Magnitude is the number that astronomers use to categorize the objects on the sky by their brightness. The limiting magnitude is the highest possible magnitude of the object you can see with the telescope. Every telescope has a different limiting magnitude which depends on how fast is the scope and how big is the aperture. For example, the moon has a magnitude of -12.6 and Pluto has a magnitude of +13.7. The faintest object you can see with your naked eye in the urban area is +3.
The astrophotography is photographing the objects on the night sky with the telescope. It can be achieved by attaching a special astronomy camera, DSLR camera or simple webcam to your telescope. The easiest type of astrophotography is to use a modified webcam to photograph the Moon or planets. For more advanced astrophotography of deep sky objects, you need DSLR or astronomy camera specifically made for it. Astrophotography is complicated and expensive but also a very rewarding hobby. You usually need specialized telescopes and mounts to get professional results.
Video Astronomy or EAA
Video astronomy or Electronically-Assisted Astronomy(EAA) is something between visual astronomy and astrophotography. It also means that you use special cameras with the telescope to provide a near-live image of the deep sky objects that looks close to what you get with astrophotography but in real time. You will see colors and details you won’t be able to see by looking through the eyepiece of your telescope. It is also much cheaper to do compared to astrophotography. You need some kind of motorized mount to track the object and also the camera that is made for video astronomy. I’m using a modified CCTV camera with a highly sensitive sensor to do video astronomy. If you are interested, you can learn more in my article What is Video Astronomy And Why I Like It.