Reflecting vs Refracting Telescopes

Astronomy Frequently Asked Questions

Welcome to our resource page dedicated to answering amateur astronomy frequently asked questions. Here you will find all sort of useful information relating to amateur astronomy.

Whether you are new to amateur astronomy or a seasoned astronomer we hope this page has something for you.

Also, be sure to check out some of the links below that will direct you to more in-depth articles for your topic of interest.

How Do Telescopes Work?

The telescope, in a nutshell, is an optical device that has the ability to magnify far away objects so they appear bigger and much closer.

To really understand how a telescope works, we have to understand how our eyes work, especially why the further an object is located from our eyes, the more difficult it would be to clearly see them, and at a certain distance, we won’t be able to see the object at all.

The explanation for this phenomenon is relatively simple: the object must take enough space on your eye’s retina for our brain to be able to perceive it clearly.

This is why, for example, we can’t read very small writings on the wall that is 200 feet away from us. The writings don’t cover enough “pixels” on our retina for us to read the writing.

The telescope essentially collects more light from the object to create a brighter image and then magnifies this image so it covers more ‘pixels’ on your retina.

The telescope can use various methods to achieve this, but mainly there are two main types of telescopes to consider:

  1. Reflector telescopes, which use mirrors to magnify the object
  2. Refractor telescopes, which use lenses instead of mirrors

To magnify the object and make it appear brighter, there are two parts of the telescope that are crucial:

  1. The objective lens (in refractor telescopes) or primary mirror (in reflector telescopes) collects and focuses light from the object to the telescope’s focal point.
  2. The eyepiece lens (located on the eyepiece) or secondary mirror in reflector telescopes, takes this focused object from the focal point and spreads it out (magnifying it) so it will take a larger portion of the user’s retina. This allows the object to look bigger in our eyes.

This is the basic principle of how a telescope works, and so we can determine the quality of a telescope by measuring two key properties:

  • How well the primary mirror/objective lens can collect the light. This is directly related to the diameter of the objective lens or the primary mirror, which is called the aperture. The larger the aperture, the more light the telescope can collect and bring to focus, resulting in a brighter image.
  • How much the eyepiece lens/secondary mirror can magnify the image. This is determined by the combination of lenses/mirrors used by the telescope, which will dictate the telescope’s

In choosing between different telescopes, it’s important to understand that we can get a bigger magnification by using different eyepieces (which are much more affordable than getting a telescope), while we can’t manipulate the telescope’s aperture.

Thus, in most cases aperture is a more important feature than magnification.

What Is the Resolution/Resolving Power of a Telescope?

In telescope terms, the resolution is the smallest angle between two close objects of which the two objects can be distinguished clearly.

Resolution is also called angular resolving power, or just resolving power.

Technically, the resolution is the telescope’s ability to separate two objects or sources into separate images, and can be calculated by the following formula:

Resolution/resolving power= 11.25 seconds of arc divided by d.

 d refers to the diameter of the objective lens in centimeters (the aperture). So, a telescope with a 50 cm aperture has a theoretical resolution of 0.225 seconds of arc.

The resolution, however, is limited by the wave nature of light, and so if the telescope operates at a certain wavelength (λ), then the angular resolution of the telescope is the ratio λ/d with d being the aperture.

In practice, however, telescopes located on Earth’s surface are limited by atmospheric effects, allowing a practical resolution of only about one arc second.

We can use adaptive optics or computing software to enhance this resolution, where adaptive optics continuously and rapidly adjust the surface of the mirror/lens to compensate for atmospheric turbulence that would distort the image and lower the resolution.

A possible solution is to merge image data from several telescopes focused on the same object via computer software to produce an end image with much higher angular resolutions than any single image produced by a single telescope.

What Are The Most Important Properties of a Telescope?

A common misconception is to think that magnification is the most important factor determining a telescope’s quality.

However, as discussed, we can quite easily get a bigger magnification on a telescope by using different eyepieces.

Yet, a magnified object won’t really be valuable unless it’s also sharp and clear, and so there are two most important properties in a telescope:

  1. How big its light-collecting object (lens or mirror), which determines how much light it can effectively gather, and thus how bright the produced image will be.
  2. Angular resolution or resolving power, which determines how much detail we can see in the produced images.

A telescope with a 50-inch objective lens diameter (aperture), for example, can collect 5 times more light than a telescope with a 10-inch aperture, which in theory can produce images that are 5 times brighter.

The angular resolution, as discussed above, is the smallest angle over which we can tell that two objects (i.e. two starts) are separated from each other. Our eyes can only distinguish two objects that are separated by more than 1 arc minute.

What Is The Primary Purpose of an Astronomical Telescope?

The main purpose of an astronomical telescope is, as the name suggests, to help users study distant astronomical objects such as our Moon, Sun, planets, and stars.

We can not only use astronomical telescopes to observe these celestial bodies but also to measure vantage points and how near or far those objects are from Earth.

What Are The Different Types of Telescopes?

Today’s telescopes come in a wide variety of shapes and sizes, but we can differentiate them into three major categories: refracting, reflection, and Schmidt-Cassegrain telescopes.

Refracting Telescopes

Refracting telescopes use lenses to gather, refract, and focus light to a point. This is the first type of telescope to be invented in the 17th century. It produces an upright image (not flipped), so refractor telescopes are also often used for terrestrial observation.

Refractor telescopes are the easiest to use, but they suffer from chromatic aberration, and it’s much more expensive to manufacture a large-diameter lens than a mirror, and so large aperture refractor telescopes are expensive.

Reflector Telescopes

Reflector telescopes use mirrors rather than lenses to reflect and focus light from distant objects. There are various designs for telescopes, but the most popular one is Isaac Newton’s design, which is called the Newtonian telescope.

Isaac Newton initially developed this design to avoid the chromatic aberration problem in refractor telescopes, using a curved primary mirror to reflect and focus light onto a secondary flat mirror, and then the light is directed to an eyepiece.

The downside of the reflector design is that the mirrors must be perfectly aligned, and we must calibrate the mirrors’ positions regularly in a process called collimation, which can be a hassle although modern reflectors have accommodated a much simpler collimation process.

Another advantage of the reflector design is that a large aperture reflector is much cheaper than any other design.

Schmidt-Cassegrain Telescopes

Schmidt-Cassegrain is another variation of the reflector design, allowing it to be more compact by using a series of mirrors to fold the light path.

Most professional telescopes nowadays use the Schmidt-Cassegrain (SCT) design. Check out our comprehensive article here.

Schmidt-Cassegrain telescopes use a spherical primary mirror and a convex secondary mirror, and then the secondary mirror reflects the light back through a hole in the primary mirror to the eyepiece/detector.

The only downside of the SCT design is that the spherical primary mirror causes spherical aberration which can blur the end image. This issue is typically addressed by introducing a corrector plate or using a parabolic instead of a spherical primary mirror.

How To Aim a Telescope

How we aim a telescope would depend on the type of mount it is on. There are two types of mount for your telescope:

Alt-Azimuth Mount

The most common type of mount that is also often used in camera tripod, the alt-azimuth mount allows us to move the telescope in straight lines in up, down, left, or right direction.

It is completely manual with basic movement controls, but should be easy enough to operate even for beginners.

Despite its simplicity, alt-azimuth mounts are still used in some of the best telescopes today, and can be fully computerized (more on this later).

Equatorial Mount

 If your telescope mount has one or more counterweights, then most likely you have an equatorial mount.

The equatorial mount does not move in straight lines, but rather in an arc to simulate the stars’ movements across the sky. The shape of this arc would depend on the latitude you are observing from, and you can adjust the equatorial mount to compensate for your location in latitude.

Can be fully computerized and motorized on one or both axes, but there are also pretty basic equatorial mounts that are pretty easy to use.

Computerized vs non-computerized mounts

 If you are working with non-computerized mounts, then you can start by loosening the lock knobs of the mount (both alt-azimuth and equatorial mounts should have at least one).

Then, you can grasp the optical tube and aim it in the direction you wish to go before re-locking the lock knobs. If you only want to make smaller, more precise movements, typically the mount will have cables of knobs designed for slow-motion control.

For computerized telescopes,  you can use the included hand controller or app to move the computerized telescope. Typically the control software should provide the ability to control speed, slew rate, and other factors.

Aligning your finder

Another important consideration when aiming your telescope is aligning your telescope’s finder.

A telescope’s finder is designed to help you find objects and is a core part of aiming your telescope. There are two main types of finders in most telescopes: optical and red dot.

  • Optical finder, is a mini-telescope that is mounted onto the top of the main telescope with a bracket. Basically the optical finder offers a low magnification view of the sky (around 6x to 10x), and there’s a crosshair in the middle of the eyepiece to help you center the object you’d like to observe in the finder’s field of view.
  • Red dot finder, displays a wide field, zero magnification view of the sky on a glass or plastic screen with a red dot or bull’s eye in the middle. We can adjust the red dot both in brightness and placement. Red dot finders are typically battery-powered.

You should align the finder (both types) to your telescope properly before you start observing by following these steps:

  1. Install the finder bracket to your telescope by following the finder’s instruction manual
  2. Use the lowest magnification eyepiece you have and put it in your diagonal or focuser.
  3. During the day, put the telescope in a location that gives you a view of a stationary object that is located sufficiently far away (a tree, stop sign, a faraway building, etc. )
  4. Manually aim your telescope to the target, and look through the eyepiece. Use the slow-motion control dials on your mount if necessary until the target is dead center on the eyepiece. Tighten the lock knobs on the telescope so it won’t move anymore.
  5. Look through the finder scope, and use the adjustment knobs on the finder (or finder bracket) to center the target object as precisely as possible.
  6. Look through the main eyepiece again and check if the object is still properly centered in the field.
  7. If you have another eyepiece or a Barlow lens, then change the eyepiece to the next highest magnification (or add a Barlow to the current eyepiece). Check whether the object is still centered.

When the target is properly centered in both your eyepiece with the highest magnification and your eyepiece, your finder scope is now properly aligned and you’ve finished aiming your telescope.

What Can You See Through a Telescope?

You can check our guide on astronomy basics for a more in-depth answer to this question, but you can see the following celestial objects through a telescope:

The Sun

 We can observe the star of our Solar System through virtually any telescope, but, you’ll need to make sure you have a reliable solar filter to protect both your eyes and your telescope from the sun’s intense light.

The Moon

 You can view the Earth’s satellite even with a relatively small telescope, and as long as the weather condition is good, you can see the Moon virtually every night.


We can observe our Solar System’s planets with a telescope. We can only view Mars and the further planets (Saturn, Jupiter) with larger telescopes to observe them clearly, but they will be worth it.

You can even observe four of Jupiter’s satellites with larger telescopes, but Uranus and Neptune will be only visible as glowing spots even with the largest telescopes.


One of the closest galaxies we can observe with our telescope is the Andromeda Nebula, and with larger telescopes, we can observe various galaxies’ spirals and white spots.


If you want to observe a nebula in detail, you’ll need to observe it in a very dark condition with a telescope of at least 200mm aperture. You can still observe Orion Nebula and the Dumbbell Nebula in the Vulpecula constellation, among others with fairly small telescopes.

Star clusters

 Pleiades, or star clusters can be observed with the larger telescopes, and the closer to the center, the more densely they are located to one another.

Terrestrial objects

 Especially with refractor telescopes where the images are displayed upright, you can use your telescope to observe earth-based objects like a faraway building or an outgoing vehicle.

Also be sure to check out our article on the Top 10 Small Telescope and Binocular Objects.

Why are Most Telescopes Reflectors and Not Refractors

There are actually a lot of smaller refractor telescopes comparable in quantity to the reflector counterpart.

However, it’s true that most large telescopes are reflectors because it’s much cheaper and easier to manufacture a large mirror than a large lens.

Mirrors are easier to construct compared to lenses and are also more durable.

Thus, a reflector telescope is typically more affordable than a refractor one with a comparable aperture. Check out our comprehensive guide comparing reflectors to refractors.

In What Sense are Telescope Like Time Machines

Astronomy, in a sense, is history because light takes time to travel from an object to another object, so when we are observing a celestial object that is millions of years away, we are technically looking through time.

If, for example, we observe a supernova through our telescope from a star that is 100 light-years away from Earth, the supernova actually happened 100 years ago.

What are the Primary Reasons for Making Telescope Larger?

The larger the telescope, the larger the diameter of the objective lens or primary mirror (the telescope’s aperture), and the more light it can capture.

The larger the aperture, the brighter the object will appear to the human eyes, allowing us to use higher magnifications without losing clarity.

However, keep in mind that a telescope with a larger aperture will also capture more light pollution.

How to Clean Telescope Mirror

To clean your reflector telescope’s mirrors, you’ll need the following:

  • A screwdriver (depending on your telescope’s screw sizes)
  • Distilled water
  • Dish soap
  • A towel (lint-free)
  • Cotton balls

You should first remove the secondary and primary mirror before you can clean them.

Removing the primary mirror 

While this might vary depending on your telescope model, typically you can remove your telescope’s primary mirror by taking off the small screws all around the tube near the primary mirror. Carefully remove the holding cell and the clamps, be careful not to scratch or damage the mirror.

Removing the telescopes secondary mirror

To remove the secondary mirror, carefully loosen the collimation screws near the secondary mirror. Hold the mirror with one hand, and remove the middle screw until it’s loose enough for you to remove the mirror.

Cleaning the telescope mirrors 

  1. Put enough water (regular water) in your sink so that you can fully submerge the mirror (don’t do it yet). Mix a tiny drop of dish soap with the water.
  2. Carefully submerge the primary mirror in the soap water solution, and carefully move it around the water.
  3. Do the same with your secondary mirror.
  4. Dry them first and drain the water out
  5. Refill the water and add soap
  6. Submerge the primary mirror again, and take off the remaining dust with cotton balls starting from the center of the mirror and moving outwards. Repeat as needed.
  7. Do the same with your secondary mirror.
  8. Drain all water, and pour the distilled water on both mirrors.
  9. Use the lint-free towel to absorb the remaining water, then leave the mirrors to dry.

How To Clean Telescope Lenses

As a general rule of thumb, you should only clean the telescope lenses when necessary. We have developed a comprehensive lens cleaning guide to assist.

It’s not necessary to clean the lens or mirror of your telescope when there’s just a few visible specks of dust.

In fact, overly frequent cleaning can damage the coating on your lenses (and mirrors), permanently damaging their performance.

For basic cleaning, you can use compressed air (canned) to blow off loose dusts with a very small chance of scratching the lens or mirror. Use the can in an upright position (never shake the can first) and make sure it’s filtered.

You can use alcohol or other cleaning solutions to gently clean any remaining dirt or smudges. 50-70% alcohol works best.

Use cotton balls to apply the solution, but don’t use too much solution and never use the same side of a cotton ball twice, not to scratch the lens.

Gently drag the wet cotton ball across the surface in straight strokes with the lowest force possible.

Great care should be taken when attempting to clean your telescope lenses. In fact, less is usually more when it comes to telescope lens cleaning.

How To Collimate a Reflector Telescope

You can check our previous guide on collimators here for a more in-depth answer, and the collimation process might vary depending on the telescope’s model.

However, the general steps are as follow:

Step 1: mark the center of your primary mirror, you can use a white adhesive for this purpose, and you’ll need a hole in the middle of this marker if you are using a laser collimator.

Step 2: Ensure the secondary mirror is centered under the focuser. You can use a Cheshire collimator to check the position of the secondary mirror. Move the secondary mirror accordingly. The secondary mirror should appear circular and center.

Step 3: Insert the collimator (using a laser collimator is much simpler) into the focuser, and adjust the secondary mirror with the adjustment screws. Check until the laser falls on the marker on the primary mirror.

Step 4: Adjust the primary mirror until the laser beam returns to the laser collimator.

If the telescope is too far out of alignment, point the telescope to a non-reflective surface (nearest wall) and change the mirror’s tilt as needed.

How To Polar Align a Telescope?

Step 1: Align to Polaris

The first step is to roughly align your telescope mount in the direction of the North Pole. You can do this by physically moving your mount at dusk, and use a compass to aim the polar axis in the general North direction.

In a clear night sky, you should be able to spot Polaris quite easily, the brightest star than any surrounding stars.

Step 2: leveling the tripod 

Make sure the tripod of your telescope mount is as level as possible. It’s very important to keep the tripod even, which will affect your mount’s altitude.

Again, make sure the tripod is already pointing North before levelling, or you might need to rotate the tripod and re-level.

Step 3: Adjusting altitude 

Find the latitude and longitude coordinates of your location, especially your latitude. Adjust the telescope’s mount to your current altitude with the available altitude adjustment mounts.

Your telescope mount should be roughly aligned with the height of Polaris.

Step 4: Perfecting your alignment

 You can use various apps like PolarFinder to tell you the exact position of the Polaris. It will display the correct position of Polaris compared to the North celestial pole.

Simply use this information to further adjust the telescope mount.

If you are using an equatorial mount, you should have the azimuth adjustment knobs for moving the mount left to right.

Loosen one knob and tighten the other while looking trough the polar finder app or polar scope.

Step 5: Finishing up 

Go back to the mount’s altitude adjustment bolt and raise or lower the mount further, at this point, you should be very close to a perfect polar alignment. Tighten back the adjustment knobs and bolts once you’re done.

What Are The Components of a Typical Refracting Telescope

There are only two core components of a refractor telescope: the convex objective lens and a convex eyepiece.

However, there may be slight variations to this design, but the objective lens is typically larger than the eyepiece lens.

Refracting Telescope Design 1

The light rays from the target object cross at the focal point of the objective lens.

Refracting Telescope Design 2

The eyepiece is positioned so that the focal plane meets the focal plane of the objective lens. Typically, the objective lens has a longer focal length than the eyepiece.

Which Type of Telescope Uses a Concave Mirror

The Newtonian reflector telescope uses a concave primary mirror (the objective mirror) and secondary mirror. The concave mirrors allow the light rays to reflect off the mirrors, producing an upside-down image.

Orientation of the image is not important in observing celestial objects, so the image is typically left uncorrected.