Frequently asked questions about telescopes

Our answers to frequently asked questions about telescopes will help you to develop a better understanding of how telescopes work. If you can't find an answer to your riflescope question, please don't hesitate to contact us - we're always here to help.

What are the differences between different types of telescopes?

There are three basic types of telescopes, refractors, reflectors, and catadioptrics. All three types have the same overall purpose; to collect light and bring it to a point of focus so it can be magnified with an eyepiece for you to examine it. However, each type of telescope does this differently, and each has its advantages, as well as disadvantages. Why not take a look at our buyers guide for telescopes to find out more about the advantages and disadvantages of reflector, refractor and catadioptric telescopes.

What is the focal length of a telescope?

The focal length of a telescope is the distance (mm) inside the optical system between the lens or primary mirror to the point where the telescope is in focus. This point is called the focal point. Generally, the longer the focal length of a telescope, the more powerful it will be, the more magnified the image will be, and the smaller the field of view will be. For example, a telescope with a focal length of 2000mm has twice the power and half the field of view of a 1000mm telescope.

Can I alter the magnification of my telescope?

The magnification of your telescope is dependent on the focal length of your telescope and the focal length of the eyepiece that you are using. To find out the magnification, you simply divide the focal length of the telescope by the focal length of the eyepiece.

The fantastic thing about telescopes, is that you can increase or decrease their magnification/power by changing the eyepiece that you are using. This makes them very versatile and allows them to be used for a variety of different viewing applications.

As an example, if you use a 25mm eyepiece on your telescope that has a 1000mm focal length, then your telescope would have a magnification power of 40x (1000 / 25 = 40). Whereas if you use a 10mm eyepiece on the same telescope, then it would have a magnification power of 100x that of your naked eye. (1000 / 10 = 100).

What is a telescope eyepiece? Will my telescope work without an eyepiece?

A telescopes eyepiece is the part of the telescope that magnifies the ‘real image’ that is produced by the telescopes objective lens, and projects a ‘virtual image’ into your eye so that you can see the target. Your eye cannot process the ‘real image’ produced by the telescopes objective lens so it is vital to use an eye piece with your telescope to convert the image into one that your eye can understand. Your telescope may be used without an eyepiece if you are connecting a camera or other instrument to it.

What’s the best eyepiece and magnification to use on my telescope?

When buying a new eye piece for your telescope it is very important to consider that each telescope has a maximum and minimum useful magnification power. This ultimately means that any magnifications above or below a set level will not aid in your viewing and may intact inhibit how much you can see.

As a rule of thumb, the maximum usable power of a telescope is equal to 60 times the aperture of the telescope (in inches) under ideal conditions. Powers higher than this will usually produce a dim, lower contrast image lacking in sharpness and detail.

For example, the maximum power on a 60mm telescope (2.4" aperture) is 142x.

Much of the observing that you will do through your telescope will be done at lower powers somewhere between 6x and 25x, but higher power magnifications can be used or lunar, planetary, and binary star observations.

Using a telescope with a lower power will produce images that are much brighter and crisper, and have a larger field of view. These images are ultimately more enjoyable to observe than low quality, dim and grainy images that would be produced at a higher power.

It is important to remember that their is also a lower limit of magnification power which is between 3 to 4 times the aperture of the telescope at night. During the day the lower limit is about 8 to 10 times the aperture. Powers lower than this are not useful with most telescopes, and a dark spot may appear in the centre of the eyepiece in a Catadioptric or Newtonian Reflector telescope due to the secondary or diagonal mirrors shadow.

What is a Barlow Lens?

Barlow lenses are used to increase the magnification of your telescopes pre-existing eye pieces. Barlow lenses come in different magnifications, but the most common magnification is 2x.

Barlow lenses are placed between your telescopes objective lens or mirror, and your eye piece. When in place, a Barlow lens with 2x magnification will double (2x) the amount of magnification offered by your eye piece. So, if for example, you use a 2x Barlow lens with your 18x magnification eye pieces, your telescope will now have a magnification of 36x (2x18 = 36).

Barlow lenses really are a fantastic addition to your telescope kit. Owning a Barlow lens can basically double the amount of magnifications that your eye piece collection can produce, and you should consider buying one early on in your astronomy hobby as it will help to limit the number of eye pieces that you need to purchase in the future.

Barlow lenses come in two sizes; short and long, and you should choose carefully between the two depending on the type of telescope that you own. If you have a refractor telescope then a long Barlow lens is most appropriate, whilst shorter Barlow lenses are used with reflector telescopes.

 

What does field of view mean?

The field of view of a telescope refers to the width of the area that you can see when looking through the telescope, quite simply, how wide an area of the night sky or landscape that you can see through your optics.

What is the advantage of a large aperture telescope?

When it comes to telescopes, aperture is the most important factor to consider. Telescopes with a large aperture, or objective lens diameter are able to collect more light than those with smaller apertures. The more light a telescope collects and brings to focus, the more clearly fainter objects can be seen. Telescopes with larger apertures also have a high practical magnification limit, allowing you to see the benefits of higher magnification eye pieces. Regardless of an eye pieces magnification, if you have a telescope with a small aperture, there will be a limit to how much it magnifies an object. Under good seeing conditions when air is not turbulent, a larger aperture objective gives higher resolution, letting you see the finer details of a celestial object.

What's the difference between altazimuth or alt-azimuth (AZ) mounts, equatorial (EQ) mounts and Dobsonian mounts?

AZ mounts allow for manual adjustments to the telescope, and you basically use the mount adjustments to pull the telescope left and right and up and down to view and track moving celestial objects such as stars and planets. These are the simplest telescope mounts available, they're easy to set up and use and a great for beginners.

EQ mounts have a slow motion control that moves the telescope in an arc across the night sky making it easier to keep track of objects you're following. Using an EQ mount can be a bit of a learning curve and they do take a bit more practice and getting used to, however once you've mastered them, they do offer a more enjoyable viewing experience.

To get started with an EQ mount you need to align it with the North Star and adjust it to match your latitude. Once you've got this sorted, your telescope will be able to move on the same axis as the objects in the sky allowing you to follow the movement of celestial objects through the night sky with the simple twist of a knob on your mount. 

The inclusion or exclusion of an EQ mount can significantly effect the price of a telescope regardless of the aperture and focal length that the telescope may have.

Dobsonian mounts are found on Dobsonian telescopes, the largest reflector telescopes available. Due to their large size, Dobsonian telescopes sit on the ground and need a large mount to support them. Dobsonian mounts are very similar to AZ mounts and are adjusted by moving them left, right, up, or down to look at objects in the sky. They also have tension adjustment and brake systems to make finding and tracking objects easier and smoother.

Do I need a motor drive with my equatorial mount?

If you’re planning on using your telescope for astrophotography then a motor drive is a must. Motor drives are also very useful and a real benefit for normal astronomical viewing. As the world is continually moving on it’s axis, objects such as the moon and stars gradually move out of a telescopes view. When viewing these objects at high magnifications, e.g. 200x magnification, these objects can move out of your telescopes field of view within as little as twenty seconds. When you’re trying to get a good look at Jupiter or Saturn this isn’t ideal, but a motor driven equatorial mount can help. A Right Ascension motor drive will keep an object in the centre of the field where the image is the best, meaning that you can get a much better look at it. You can of course manually track an object in the night sky, but this can cause vibrations in the image which make it difficult to focus on.

What does OTA stand for?

OTA stands for Optical Tube Assembly. This is usually something that is stated in a telescopes name or description if the product only includes the telescopes optical tube and does not come with a tripod. 

What is a GoTo telescope and what's the difference between that and a manual telescope? 

GoTo telescopes are built with a motor and are essentially computerised telescopes that have the ability to take you as the user directly to any object in the sky with just a push of a button. Once they're set up, and aligned to certain points in the sky, GoTo telescopes create a model of the sky, and uses that to locate any future objects that have known coordinates. Moreover, GoTo telescopes can also track moving objects in the sky as the earth rotates on it's axis, making them ideal for deep space photography. 

Manual telescopes do not have a motor, and adjustments to them and the direction that they are viewing in, are made by hand or by levers. Manual telescopes usually have a finder scope to help you find celestial objects. 

When’s the best time to observe the moon?

Contrary to popular belief, full moon is actually one of the hardest times to observe the moon. At this time in the moon calendar, the moon is so bright that it can often just end up looking like a big fuzzy bright light through your telescope, and observing its craters and such like can be pretty tricky.

The optimal time to view the moon is either side of the full moon when an adequate percentage of the moon is visible but it isn’t as bright.

Why can’t I find the moon, planets and stars when I look through my telescope?

If you can see the celestial feature of interest through your finderscope but simply can’t find it when looking through your telescope eyepiece then it’s likely that your finderscope and telescope aren’t lined up properly.

Many telescopes come with instructions on how to line up your finderscope, so if you’re experiencing this problem then we’d advising having another go at following them.

In general, lining up your telescope eyepiece and finderscope easiest and most effectively done during daylight hours. Simply look through your eyepiece and find a set target in the distance (we’d suggest an electricity pole, arial or top of a tree), then look through your finder scope and adjust it’s target until you can see exactly the same target through both. Remember never to look at the sun through your telescope as this can be extremely dangerous.

Why is the moon a fuzzy bright light when I look at it?

The most common cause of this in first time telescope users is that your eyepiece isn’t focused properly. Getting your eyepiece in focus can take time and patience, but once you’ve got it sorted it will make for a much more pleasant and far less frustrating viewing experience. Plus, you’ll actually be able to see the moon and stars that you bought the telescope to view in the first place.

If you think that your eyepiece could be out of focus, then we’d recommend removing it from the eyepiece holder, reinserting it, aiming your telescope at a large and obvious point of reference and then using the adjustment knobs to move the eyepiece into focus. Be sure to use the lowest magnification eyepiece to start with (the one with the highest number written on it). The initial view through the scope will probably just be of the same, bright, fuzzy light of the moon that you were seeing before, so we’d suggest moving the telescope until you find the edge of the moon and then beginning your focusing. You’ll soon establish which way to move the adjustment knobs to take the eyepiece in or out of focus and be able to fine tune your scope until the image you’re seeing is much more crisp and showing greater detail of your subject.

To make things easier, you may even want to try focusing your telescope in the day. Simply point it at a recognisable object in the distance and follow the procedures above. Make sure to never point your telescope anywhere near the sun as this can be very dangerous.