What is the magnification of my new telescope?

What is the magnification of my new telescope?

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I just bought a Skywatcher telescope with a diameter of 60 mm and a focal length of 900 mm. I have a 2x Barlow lens and two eyepieces with focal lengths of 10 mm and 20 mm.

How can I calculate the magnification of my telescope, for example using the 2x Barlow and the 20mm eyepiece?

I'm fascinated by watching the sky and I can't wait to try it… Unfortunately, it's currently cloudy.

One more question; what I can see to be fascinated with this telescope?

Magnification of your telescope depends on the ratio of the focal length of your primary optics and the focal length of your objective. We can represent this with this simple formula: $$P=frac{f_{objective}}{f_{eyepiece}}$$ where $P$ is magnification (power) and $f$ is focal length.

Your telescope has 900 mm focal length, so the magnification is $P_1=frac{900 ext{ } mm}{10 ext{ }mm}= 90 ext{x}$ (10 mm ocular) and $P_2=frac{900 ext{ }mm}{20 ext{ }mm}= 45 ext{x}$ (20 mm ocular).

A Barlow lens is used to increase the focal length by the given factor (thus 2 in this case), so directly impacts the magnification in the same way, increasing it to 180x and 90x - but gives you in return an equally smaller field-of-view and makes area-objects fainter as the same light is projected to a larger area. Note that there is a maximum useful magnification which is roughly twice the aperture (measured in mm), which is 120x in your case, so the Barlow lens will not be very helpful as it magnifies beyond reasonable due to inevitable diffraction on the clear aperture.

One usually compares them by their focal length and by their aperture - the ratio defines how bright the objects appear and thus how faint objects you can see. With an aperture of 60 mm, it has an opening ratio of f/15 which is only a moderate one.

Such telescope is particularly useful for observations of the planets in our solar system as well as star clusters and the brighter deep-sky objects. Now in summer would be an excellent time to hunt for the globular clusters like the one in Hercules (M13). Also, you can download a useful program Stellarium to find more interesting features.

EAA magnification or power question

Here is an EAA question that I don't know how to answer. I keep getting asked a question from people that look at my IPAD screen during an EAA session, they want to know what power they are seeing. Power or magnification when using a telescope with eyepieces is figured by dividing the focal length of the telescope (8" = 2032) by the focal length of the eyepiece, so with a 10mm eyepiece you would have 203.2 power. What power/magnification would you tell someone they are looking at with EAA setup since there is no eyepiece? My camera ASI294 attached to an 8' telescope with a HyperStar 8 v4 and no eyepiece would equal what? The camera is set to 391mm in the ASIAir Pro. So my question is what power are they seeing without any pinching the IPAD to increase it from the basic photo?

#2 bips3453

I will try to share an answer that sounds reasonable in my opinion.

There is no direct equivalence of magnification between an eye-piece and the iPad screen.

Here are some calculations we can use to come up with a reasonable number that can be thrown out.

8" SCT with HyperStar (f/1.9) would effectively yield a focal length of about 386 mm.

With ASI294, that would yield an FoV of 2.84° x 1.94°. That's the size of the image you would be seeing on the iPad screen.

How is that projected into the eye? That's a little more complicated.

Consider that the iPad is held close to ideal reading distance from the eye, 15". The horizontal FoV of iPad screen would be about 32.5 degrees. (http://www.calculato. t=0&idealfov=36)

Hence, the magnification on the horizontal axis would be 32.5/2.84 = 11.44x.

I might be totally wrong about this and experts can help correct the explanation.

#3 barbarosa

Note: Posted before seeing the post above. But I think we are in agreement.

Strictly speaking magnification is a property of a lens and not a sensor. However most of us use the term not so strictly.

There are a couple of ways to answer this question. One is a rough rule of thumb is to use a Plossl of the same focal length as the sensor diagonal. substituting the sensor diagonal measurement for the focal length of an eyepiece. For the 294 2032mm/19.1mm = 106.

The other and more correct method compares the angle subtended by the target with the angle subtended by the target as viewed on the display by the observer.

Let's use an image of the full Moon as an example. When you look at the Moon in the sky, we don't say it looks half an inch across. We say how wide an angle the object subtends. The full Moon is about 0.5° across.

So, how magnified is this image of the Moon to the right, then? In the end, the telescope focal length and camera chip size determine the field of view captured (bear in mind, you could also crop the image down a bit as I've done here). All we need to know is the ratio of the actual naked-eye visual angle (0.5°), compared to the visual angle the image on the screen represents.

The visual angle of any object, be it the Moon in the sky, or a photograph on a computer monitor, at a given distance is given by this formula:

Size and distance must be the same units. They can be inches or miles because there is a nice linear relationship between size and distance. Twice as close is always twice as large—doesn't matter if it's inches, miles, or parsecs.

On my computer monitor as I type this, the image is 2 inches across and is 15 inches away. Plugging those values in, I get a visual angle of the Moon image as 7.6° (put your calculator in degrees mode for this unless you want to use radians for your image field-of-view computation).

The image on my screen of the Moon is 7.6°, and I divide that by the .5° the Moon appears to me in the sky, and voila, the image on my screen is about 15× normal size . . . or . . . magnified. Oh yeah! Try this with some of your favorite astro-images to amaze your friends and family!

However, let’s say we want to up our magnification and push the power of our telescope. We need to find the highest useful magnification this is easily found on the manufactories site.

Let’s keep on using the Nextsar 4se as our example.

Highest useful magnification 241x

At this point it’s worth pointing that you’re Focal Length of the eyepiece gets smaller for more magnification.

For example a 25mm eyepiece on a 1500mm focal-length telescope would yield a power rating of 75x (1500/25 = 60) While a using a 10mm eyepiece on the same telescope would give 150x (1500/10 = 150)

This is something that drips up a lot of new people into astronomy so the smaller the mm with eyepiece the more the magnification.

Let’s get back to our example we know we have a 241x Highest useful magnification.

Right we can take a big jump as we know we have a lot of space to work with let’s go with a 5mm eyepiece and apply our calculation.

Focal Length of the telescope 1325mm
Focal Length of Eyepiece 5mm

1325MM / 5MM = 265x Magnification

Over our 241x highest useful magnification so we need to come up a little lets go with a 5.5mm eyepiece.

Focal Length of the telescope 1325mm
Focal Length of Eyepiece 5.5mm

1325MM / 5.5MM = 240x Magnification

This is on the edge of the highest useful magnification so we know that the smallest eyepiece with the Celestron nexstar 4se (our example) is a 5.5mm eyepiece. You can apply this example with your own telescope.

Again it’s worth pointing out that there is a lowest useful magnification, it’s a similar process but one for another post this post is all about the power.

What is magnification/power as it pertains to telescopes?

Magnification of a telescope is actually a relationship between two independent optical systems: the telescope itself and the eyepiece you are using. To determine power, divide the focal length of the telescope (in mm) by the focal length of the eyepiece (in mm). By exchanging an eyepiece of one focal length for another, you can increase or decrease the power of the telescope. For example, a 20 mm eyepiece used on a 1000 mm focal-length telescope would yield a power of 50x (1000/20 = 50). While a 10mm eyepiece used on the same instrument would yield a power of 100x (1000/10 = 100). Since eyepieces are interchangeable, a telescope can be used at a variety of powers.

There are practical limits of magnification for telescopes. These are determined by the laws of optics and the nature of the human eye. As a rule of thumb, the maximum usable power is equal to 50-60 times the aperture of the telescope (in inches) under ideal conditions. Powers higher than this usually give you a dim, lower-contrast image. For example, the maximum power range on a 90mm telescope (3.6 in aperture) is 180x-216x. As power increases, the sharpness and detail seen will be diminished. Higher powers are mainly used for lunar, planetary, and binary star observations.

Most of your observing will be done with lower powers (6 to 25 times the aperture of the telescope in inches). With these lower powers, the images will be much brighter and crisper, providing more enjoyment and satisfaction with the wider fields of view.

Always start with your lowest magnification (longest focal length) eyepiece and work upwards from there.

A 2x Barlow lens will double the magnification of whatever eyepiece you use with it while preserving its eye relief. For example: using a telescope with a 900 mm focal length with a 20 mm eyepiece will give you 45x magnification. Using the same telescope and eyepiece with a 2x Barlow lens will give 90x magnification. This would be the same magnification as a 900mm telescope with a 10mm eyepiece.

The Solar System

The Moon

25-30x – still looks small in the FOV, but you can still start making out large features on the surface depending on the phase.
40-80x – During crescent and first/third quarter phases, the view of all the larger craters is spectacular! For eclipses, up to 60-80x is the most you should zoom so the entire disc is still visible in your FOV.
100x – The disc of the moon now fills up your entire FOV inside the eyepiece, unless you are viewing a thin crescent in the middle. The closer views of the larger and smaller surface features offer a good variety.
200x – Your entire FOV covers about half the surface of the moon. You start seeing smaller features you didn’t know were there, such as small peaks inside craters!
300x and above – You start feeling like you’re flying above the surface of the moon. Larger craters that looked tiny at lower magnifications now take up the entire view! You need excellent seeing conditions for these tiny features to look still enough to see, otherwise the Moon will look way too soupy.


Below 50x – The planetary disc is small and white, but you can still see up to four star-like points in a line, which are in fact its own moons orbiting the planet. This is what Galileo saw in 1611, hence why they’re nicknamed “the Galilean Moons.”
50-100x – The Galilean Moons appear more spread out, and you can barely start to see the striped pattern of brown and white cloud bands
100x – This is a great all around view of Jupiter, as you can see cloud detail on the planet, and see all four moons all in the same FOV. The Great Red Spot can also start being seen as well as a tiny orange colored dot on the planet (if it’s on the side facing Earth).
200x – Details on Jupiter are a lot more visible, and the Great Red Spot looks like a small circle. If one of the Galilean Moons is transiting, if you look at just the right time, you can see them gradually stick out of the side of Jupiter like a pimple. If any of the Galilean Moons are passing directly in front of Jupiter, it is possible to see their shadows being projected on the face of Jupiter – a solar eclipse is occurring on the planet. Io’s shadow will move the fastest over time, while Ganymede’s shadow will be the largest.
400x and above – If the seeing conditions are great, then it is possible for you to begin seeing very tiny “swirls” in the lines between the light and dark colored cloud bands. The Galilean moons no longer look like little points, and start being resolved as tiny dots! While Io and Europa will consistently remain in the FOV (if not directly in front of or behind the planet), Calysto and/or Ganymede can get cropped out due to their wider orbits around Jupiter.


Below 50x – Looks like a tiny eye in space, or an oblong shaped planet depending on the tilt of the rings. If Saturn’s rings are tilted exactly towards earth, they are almost invisible. Saturn’s biggest and brightest moon Titan can be spotted close to the planet.

100x – The rings are now easily visible, though to some they still make Saturn look like an eye. A sharp eyed viewer can see the yellow color of the planet and the white color of the icy rings. Titan, being the biggest and brightest Cronian (Saturnian) moon will be the easiest to spot, while Rhea and Iapetus will be the next two brightest. Rhea is usually closer to Saturn while Iapetus is further out, and they are about 2.5x and 16x dimmer than Titan respectively. Other moons are dimmer than Iapetus, may be too dim for smaller telescopes – they are also much closer to the planet itself.

200x – Detail on the rings are now visible, especially if they are tilted at just the right angle. One such detail is the Cassini Division, which looks like a black stripe on the rings. Another detail is the outline of the planet as the rings go behind it. If you look closely, you can see different shades of yellow on the planet as well! Iapetus may be cropped out of your FOV due to its wider orbit around Saturn.

400x and above – Assuming the seeing conditions allow, Saturn will look as good as any picture you may have seen it. The Cassini division and different shades of yellow should be obvious at this point! The smaller and dimmer moons such as Dione, Tethys, Enceladus should have enough angular separation from the planet to be easily detectable, though their brightness may make it difficult under light pollution or with smaller telescopes.

It should be noted that during years where Saturn’s Rings are tilted directly towards Earth, the Cronian moons will appear to transit in front of or behind the planet similar to Jupiter’s moons. But in years when the rings are not pointed at Earth, then we can observe the moons to orbit around Saturn from above or below. This is because the moons and rings share the same orbital plane around Saturn!


Venus is bright enough to be observed with telescopes during the day if the planet is far enough away from the Sun’s glare. When it is further away from the sun, you can see it in the night sky for up to a few hours before it either sets or gets lost in twilight.

Below 40x – This is about the same level of magnification that Galileo had. As he could see the phases of Venus with his small telescope, so will you! When Venus is in a crescent phase, it’s very obvious, and it’s fun to see inexperienced users confuse it with the Moon. As Venus gets further away, the crescent gradually switches back to a full circle, yet the angular size shrinks.

40x and above – Since Venus is covered with white carbon dioxide clouds, you will only see a white planet whether it is day or night. Increasing the zoom will make the phases much easier to see, especially the phases for when Venus is further away from us than our Sun.

During Solar Transits – With a proper Solar Filter , Venus is large enough to be seen as a black dot moving across the Sun even with the naked eye! Through a telescope, the disc is obvious even at low power, and the sight is a super rare treat. Missed the last one in 2012? Sorry, chances are you won’t be alive for the next one in 2117.


Because of Mars’ small size and the distance between Earth and Mars varies a lot, a good view of Mars is surprisingly not as common as you think. At its furthest, it barely looks like a red dot, but when it has close approaches near oppositions, then its angular size allows some details to be seen.

Below 100x – Mars appears like a bright rusty salmon colored orb. Depending on its distance, it’s either a really bright star, or a tiny disc in your FOV.

100x – IF the planet is close enough and the air is steady, the apparent size is almost as big as Jupiter, and you can start making out darker albedo features and even an ice cap if you know where to look. But if the planet is far away, good luck seeing any features!

200x and above – If Mars is close enough under good seeing conditions, then it definitely appears impressive. Usually, if you can see some darker albedo features and the ice caps, then you have a good view. If you cannot, even zooming in higher will not fix the problems. Colored filters may help bring out some of the details.

Mars is so dusty and full of haze that it will take a night of good seeing, plus a skilled photographer that can process stacked images to make the planet look like it does in pictures.


Mercury is much harder to spot because of its close proximity to the sun in the sky. But ever so often, it gets far enough to be visible when the sun is below the horizon. Still, it’s a small planet, therefore it won’t appear that big.

below 100x – Mercury still appears star-like in your telescope.

100x and above – Like Venus, Mercury also exhibits phases, but you don’t glimpse them until you reach 100x. The higher you go, the phase shape appears bigger in your FOV and is much easier to see, but the planet will still appear white due to its distance.

During Solar Transits – In rare events when Mercury is directly crossing the face of the sun, you can see a tiny black dot – the disc of Mercury – gradually move across the sun’s disc. Since Mercury is so small, it’s almost impossible to see with the naked eye using eclipse glasses, but through projection methods, or through telescopes with proper filters, the transit is easily seen even at lower magnifications.

Uranus and Neptune

Notice how much dimmer Neptune (magnitude 8) appears next to Phi Aquarii, a magnitude 4 star. Triton at magnitude 14 is barely at the limit of detection through an 8″ Telescope, and this picture was taken through a light polluted sky!

There is not much difference between these two planets when it comes to night sky observing. While both planets are similar in size, Uranus will have the advantage of being closer and brighter. If you can find Uranus, then you can find Neptune as well, but don’t expect them to be spectacular like Jupiter and Saturn. No matter how much magnification you use, they will just appear as tiny discs with their own distinct colors.

Below 100x – Both Uranus and Neptune appear star-like and can be indistinguishable from background stars. Use a good star chart to figure out which one is in fact the planet you’re looking for. You should notice that the planets’ lights are steadier than the surrounding twinkling stars. Uranus will have more pale cyan color, while Neptune will have a distinct blue azure color.

100x – Uranus starts showing a tiny pale cyan colored disc. It appears about as big as Mars appears when Mars at its farthest distance from Earth. Neptune still appears like a pale blue star.

200x and above – Though still small, Uranus’ disc is much more obvious and it starts looking like a planet. Neptune starts showing a blue tiny disc if the air is steady. The more you increase in magnification, the dimmer the planets will appear, hence the larger your telescope, the easier it’ll be to observe!

Apart from Titania and Triton, the largest moons of Uranus and Neptune respectively, none of the numerous moons between the two ice giants will be bright enough to detect through most backyard telescopes. You’ll need at least an 8″ telescope to be able to spot either one, and if your eyes can’t see them, then a few seconds of exposure can bring them out.

“Super Nanotechnology Zoom Telephoto Telescopes” – Don’t Drink Their Kool-Aid!

These advertisements on social media are doing nothing but preying on the gullible and the layperson who doesn’t understand how telescope optics are supposed to work. It’s very common for people to assume that magnification is what a telescope is all about, and think that because the magnification is being advertised with a big number, then that means the telescope is “super!”

I am NOT kidding when I say that I’ve seen these same ads pop up every few postings… you click on one to investigate and all of the sudden more of these pop up in your feed! It gets more repetitive than a broken record!

If you skip to the sections that talk more about popular “zoom cameras” like the P900, and learn about how magnification works on telescopes and cameras, then you’ll get a better idea!

How to Tell If These Product Ads Are Too Good To Be True – Red Flags To Look For…

Red Flag 1 – Advertisements from different “companies” always seem to be word for word and play the same attraction video, or contain the same images/clips.

Keep scrolling down on Facebook and you’re bound to see a different “company” trying to sell you the same product under a different listing. This is more likely a black market company creating different “clones.”

Look at the images above, it’s almost word for word the same listing! These were all found within a few minutes of scrolling through Facebook!

Even if it’s not exactly the same attraction video on every listing, if you look carefully, you’ll see that they have the same clips or slideshow images in a different order.

Red Flag 2 – The actual website selling the item is selling other random merchandise.

Are there legitimate sellers based in China? Absolutely. For example, if they are selling a powerful laser, and their website is specific in making and selling lasers of different models, watts, colors, etc., then that’s much more legitimate and trustworthy. I’ve bought my astronomy presentation lasers from such vendors.

But if the actual sellers website (where social media often takes you to buy the product) is not specializing in similar items, and instead you just see a bunch of random items all on sale with a “marked down” price, then that should be a red flag if you’re interested in buying whatever you saw on social media.

Red Flag 3 – Check the Comment Section

Do you notice that there are no comments with pictures shared by people who actually got the item?

Legitimate comments may be lost in swaths of inquirers, and these sellers have also been known to delete and/or censor negative comments, and prevent accounts from further trying to warn. But I have caught the pissed off people who got fooled before their comments got removed, and as you can see, I’ve preserved some of them!

Positive comments? They’re likely bots or paid comments!

Red Flag 4 – How does the customer care contact information look, if there is any at all?

So the attraction video in the advertisement often gives you the impression that you’re buying something large enough that it needs a protective case, and requires two hands to hold carefully, and you instead end up getting something that fits entirely in the palm of your hand.

So it turns out this product you waited 2-4 weeks to arrive from China is a low quality knockoff? And there is next to ZERO customer service?

Hate to say I told you so!

“But they say on the posting that they ship from (a USA location)” If that were the case, then if you decide to return the item back to the seller, you’ll find that instead of going back to a USA address, it will have to go back to China, and the cost to return it will be worth more than the product’s value!


Something being advertised as a “replacement” for a telescope has a lot of balls to make that claim, especially when going against people who actually know how telescopes and optics are supposed to work!

These listings boast about how you can achieve 300x magnification with it… again, another ballsy but easily check-able claim!

Well, first off, let’s talk about the popular Nikon P900… a camera often referenced for high zoom capabilities. By default it has focal lens that goes all the way up to 2000 mm and achieves 83x… a Nikon P1000 can get to 3000, or 125x zoom, and I’m not including what you can get with their “Dynamic Fine Zoom” functions.

Both of these products start selling for as little as $500 – $800 respectively… they’re obviously not cheap.

What about telephoto lenses?

The Nikkor 70-300 mm F/4.5 to F/6.3 lens by itself without the camera body it can cost as low as a couple hundred bucks new… and as you can see in the pictures below, they definitely do the job for faraway objects in landscape and astrophotography.

This long exposure shot of the Andromeda Galaxy above was shot with a Nikon D5300 using the same Nikkor 70-300 mm focal lens mentioned prior on a tracking mount – not through a telescope.

In prime focus astrophotography where the camera body becomes the eyepiece, the telescope itself becomes the focal lens depending on the focal length involved – thus a 1000 mm focal length telescope becomes the equivalent of a 1000 mm focal lens! Oh, and let’s not forget about Barlow Lenses, which multiply the focal length depending on the specific lens. Hence, a 1000 mm scope using a 2x Barlow is now virtually 2000 mm.

To achieve 300x magnification in the telephoto lens sense, that means the focal length has to go all the way to 7200 mm! Try looking for DSLR telephoto lenses that exceed above 500 mm… they’re not that widely available, and IF they are, they’re NOT cheap ! The price tags on these lenses can often cost more than the camera body itself!

Okay… What About Telescope Magnification

When it comes to telescopes, technically, you can magnify any telescope as much as you want, but if you go beyond what the scope itself can resolve, then your blurry images that become super noisy and filled with compression pixels from overdoing the Sharpen function in Photoshop will be good enough to convince people there are cities on Jupiter.

Telescopes must have an aperture of 6″ (150 mm) or wider to be theoretically capable of resolving details at 300x magnification. Magnification being used is calculated by the focal length of the telescope divided by the eyepiece being used, hence a 1000 mm focal length scope using a 25 mm eyepiece is magnifying at 40x. How would I get to 300x on a 1000 mm focal length telescope? The easiest way would be to insert a 10 mm eyepiece (100x) into 3x Barlow, which will triple the magnification into 300x.

If telescopes need to be as wide as 150 mm in diameter to even think about resolving at 300x , then how can something 35-40 mm in diameter resolve anything at that amount of zoom?!

So you’re telling me these “super nanotechnology zoom monocular telescopes” that fit in the palm of your hand and sell for cheap are going to perform like the examples mentioned prior?!

will have the same optical quality and zoom capabilities as THIS?!

Then if you buy one of those so called “super nanotechnology zoom telescopes,” I just have two words…

With Telescopes – IT AINT ABOUT THE ZOOM!

Perhaps those things will be great for terrestrial viewing, and anything bright enough that you can see. I’m sure one would have a lot of fun zooming in on faraway landscape features or into your neighbor’s… – So yes, there is some practical use for them.

But there is absolutely no way it will replace a telescope when it comes to views of the celestial sights in the night sky! You may get the Moon, but that’s because it’s big enough and bright enough!

Here is what they don’t tell you!

  1. Telescopes are not glorified magnifiers , they are what we call collectors of light, or “light buckets.” The wider the scope, the more light it collects, hence the more into deep space you can see!
  2. Most “beginner telescopes” are between 60 – 114 mm (2.3 – 4.5 inches) in aperture. That’s anywhere between 73x to 265x light gathering power . These “nanotechnology 35mm telephoto zoom monoculars” are only collecting 25x more light than the unaided eye sees under the best conditions.
  3. the more you zoom in, the more that tiny movements in your hand affect how unsteady the view is… hence your FOV is prone to a lot more shaking and blurring from your hands trying to hold the thing steady!
  4. The more you magnify, the less light you are allowing into your eye, hence your trek into deep space and quest to see distant galaxies will most likely be nothing but black through such a small aperture device!

So much for all that zoom power, right?

Most of the time when looking at the deep sky, we observe objects at low magnification so we can allow the most light possible to see these deep sky objects. Zooming in can make closer details impossible to see because the incoming light is too dim, especially through smaller telescopes! Trying to search for anything at super high magnification is like searching for a needle in a haystack while looking through a narrow straw!

Even if you could get the thing steady enough to look at the Rings of Saturn at 300x magnfication, and somehow track it with Earth’s rotation, then your 35-40 mm aperture will be simply too small resolution and make things way too mushy to have a pleasant viewing experience!

Oh, but don’t listen to me, I’ve only been observing the sky with telescopes for the past 20 years… 3 of them professionally…

If you made it all the way to the end of this article and feel this opened your eyes, please share this to anyone who you think might be getting fooled into a newbie trap!

Once again… Things that Telescope Users Say for $200 – What is, “Don’t Drink the Magnification Kool-Aid,” Alex…

I sincerely hope that if you read this and already have purchased these items from these sellers, that the product does exactly what the advertisements said they do, and if anything, you get something that looks like the pictures! Have fun checking out what you can see over land or sea… but don’t think you’ve found anything revolutionary that puts astronomy telescopes out of commission!

If you have not bought one of these yet, save the $40 and just look for a pair of binoculars instead!

Support Your Neighborhood Astronomers!

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Common Telescope Magnification Myths

By: Al Nagler July 17, 2006 0

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Through the years, many myths (or, if you prefer, misconceptions) have become woven into the fabric of amateur astronomy. The following is a selection that involves telescope magnification.

See "How to Choose Your Telescope's Magnification" for a full explanation of the answers given below.

A 7-mm exit pupil gives the lowest useful magnification

Not so! With a refractor there is no limit on the size of the useful exit pupil. Use whatever is necessary to get the field you need to frame the subject. A reflector's low-power limit is reached when the black spot in the exit pupil (caused by the secondary obstruction) becomes obtrusive.

While a 7-mm exit pupil, by matching that of the eye, does give the brightest views of deep-sky objects, it does not necessarily give the best ones. Higher magnifications, despite their smaller exit pupils, will reveal more details, maintain contrast, show fainter stars, and help by-pass defects in the eye itself.

Exit pupils larger than 7 mm waste light and resolution

With refractors, larger pupils do waste aperture. But the magnification is so low that the wasted aperture is of little concern: both image brightness and resolution are as great as possible at that magnification. With reflectors, however, larger pupils do waste light, but primarily because the black spot in the pupil caused by the secondary obstruction becomes larger. Both light loss and field shadowing occur with reflectors, but as with refractors there is no resolution loss because of the low power.

Faster telescopes show brighter images.

This is a misconception carried over from photographic use, where the fast f/ratios do mean brighter images and shorter exposures for extended objects. Telescopes with equal apertures and equal magnifications have the same visual image brightness, regardless of the objective's f/number.

Long-focal-ratio telescopes give higher-contrast images.

In general, refractors offer the potential for higher contrast because mirror coatings, by their nature, tend to scatter more light. But when comparing well-made, highly corrected refractors, there is no gain in contrast with instruments of long focal ratio.

Reflectors too, if well made and having the same size of secondary obstruction, will have the same contrast at the same telescope magnification regardless of the f/ratio.

The highest useful telescope magnification is 50× per inch of aperture.

What is "useful"? Although small telescopes little affected by the atmosphere may give pleasing images even up to 100x per inch of aperture, no more detail is seen than at 50x per inch. On the other hand, large instruments, more affected by atmospheric seeing, may top out at 20x or 30x per inch. In practice, a 3- or 4-inch refractor may work well at 200x, but it is rare indeed that any size instrument benefits from more than two or three times that telescope magnification.

Barlow lenses degrade image quality.

There may have been some truth to this when Barlows were made with low-index glasses and not specifically designed for use with modern eyepieces. Modern, high-index Barlows actually improve eyepiece performance by reducing astigmatism at the edge of the field. Furthermore, using a Barlow reduces the effective f/number of the objective and permits using longer-focal-length eyepieces (with their longer eye relief) for high-magnification viewing.

Telescope Magnification Guide Where Does It Get Blurry?

Much depends on the condition of the atmosphere on how well you can see as you start to magnify. Check out our quick chart on some typical name brand telescopes and how based on their specs, you can determine the theoretical max magnification will be.

Quick Table:
Popular Brandmodelfocal lengthf/ ratioAperture in mmMax MagSmallest Useful Eyepiece In mmEyepiece to see Saturn RingsResearch Specs on Amazon
CelestronPower Seeker 127EQ1000f/812740820Learn More
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This chart, as mentioned, was figuring an average fair weather night. As an example, if you could take your telescope to space, you would be able to get a much higher, probably more than twice the magnification ability before the blurry comes into the picture. Of course, it is dependent on lens quality as well.

Download Our Free Telescope Calculator and Calculate Your Scopes Capability

Download the calculator here. You can calculate all the numbers you ever may need to know for your telescope. Use it for the scope you are thinking of buying as a comparative analysis. It is simple to use. Click Here

What All Does Come In To Play with Magnification and Blurriness?

Whether you are a newbie or professional in Astronomy, you are still going to experience blurry images at some point while viewing through your telescope.

This is because several things are affecting the clearness of the image you are seeing in your scope. These factors may either be controllable or not. Controllable variables influencing magnification includes the focal length of your scope and the eyepiece size, or aperture.

Meanwhile, the uncontrollable factors can include the atmosphere, the observer’s eye condition, and slight deviations of the objective lens’ specified focal length.

Here now, we can start to dig deeper on the essential things regarding scope magnification and how it works.

How Does Telescope Magnification Work?

Our naked eye does not have enough ability to view objects located from far distances. This happens because our eye can no longer accept enough light coming from the object. Thus, one tends to see a smaller and blurry image of the object from afar.

Because of a hankering curiosity for what was out there, the telescope and other optic systems were invented to help us see objects that we could not otherwise see. The telescope collects light from very far distances like the galaxies, planets, and nebula.

The key to seeing things much clearer is having an objective lens which can capture huge amounts of light from the object. So, bigger is better, at least in this case. This is the main concept driving the mechanism of a telescope.

A telescope is composed of two lenses or mirrors: the objective end and the eyepiece lens. Both lenses are located at opposite terminal ends.

The wide objective lens or mirror is responsible for gathering the light from stars and other objects which travels down the scope’s focal length and converges at the focal point. After meeting at the focal point, it then diverges and emerges out from the eyepiece lens where the observer views the object. The eyepiece lens is responsible for magnifying the light going towards the observer’s eye.

Telescope Magnification Formula

Magnification or power of a telescope is its ability to enlarge small objects from far distances. This feature can be manipulated using different combinations of objective and eyepiece lens.

In general, when the magnification of scope increases, the image brightness, and field of view (FOV) decreases. When comparing magnification versus field of view, the second feature has a more significant impact on the performance of any scope.

The formula for computing the magnification or power of a telescope:

Sample Computation:

You bought a telescope with a focal length of 600 mm, and the eyepiece’s focal length is 30 mm. What is the magnification of your scope?

Aside from that, you can also compute for the theoretical maximum magnification of your scope. You can do this in two ways: (1) scope’s aperture in inches multiplied by 50 or (2) focal length of aperture in millimeters times two.

In contrast, the minimum magnification of any scope is equivalent to its exit pupil diameter. This refers to the cone size of light emerging out from the eyepiece lens.

You also have two means of computing for the minimum magnification: (1) aperture focal length expressed in inches multiplied by 3.6 or (2) aperture expressed in millimeters divided by 7.

See our full article on telescope equations for more detailed information, or download our free Telescope Equations calculator.

What Magnification Do I Need To See Planets

Before going to the answer to this question, let us first cite a few points to bear in mind. We already know what magnification is, so let us try to know the reason why you need a bigger aperture to ensure higher magnification.

  • As magnification increase, the image gets dimmer, which means you need a wider aperture to capture more light to make the resulting image brighter.
  • If you want to have an excellent resolution, then you need to have a wider aperture.

In general, it is pointless to have a magnification higher than 2x the aperture (in millimeters) of your scope, because the image clarity will still be dependent on the time, season, and atmospheric condition of viewing.

Consequently, the ideal scope for your planet viewing must have an aperture bigger than 100 mm and scope focal length longer than 1000 mm.

A good rule of thumb for the magnification of viewing planets is the aperture diameter in millimeters multiplied by two or the objective lens in inches multiplied by 50.

Consequently, if you have a scope with a focal length of 1000 mm and an aperture diameter of 100 mm, then its maximum magnification is around 200x. You need to use an eyepiece with a focal length of 5 millimeters to acheive the 200X magnification.

Likewise, this scope with a 20mm eyepiece would bring Saturn and its rings into view.

Meanwhile, if you plan to view planets Neptune and Uranus, you need to use a scope with higher magnification. Telescopes must have a higher focal ratio, say an f/10 or so focal length and preferably an aperture over 100mm to achieve useful magnification.

A good example telescope for planet viewing is the Meade LX65, it has a focal length of 1800mm and an aperture of 150mm. So, that means the rule of thumb is 300X useful magnification. It comes with a 26mm Plossl eyepiece that allows you to see Saturn and discern its rings creating 69X Mag.

Check out more information on Amazon for the Meade Lx65 6″ Maksutov-Cassegrain

An Example Chart

Recall the formula for computing magnification or power of a scope. That is the focal length of the telescope divided by the eyepiece’s focal length.

Below is a sample table containing easy math for details on focal lengths of telescopes, eyepieces, and the resulting magnifying power.

Focal Length of Telescope Focal length of Eyepiece Magnification
1000 mm 30 mm 33.33x
1000 mm 20 mm 50x
2000 mm 30 mm 66.67x
2000 mm 20 mm 100x

Why Not Use the Magnification Number as a Buying Decision?

It is a wrong concept to have magnification as your determining factor in choosing for the best telescope. Why? Because magnification does not reflect the overall performance of a telescope. This feature can be mainly manipulated through various combinations of objective and eyepiece lens.

Consequently, this makes magnification a very weak variable in choosing the right scope for your requirements. There are other important things worth noting when looking for the proper telescope.

For instance, the aperture is an important variable to consider. It is essential because the amount of light entering the scope is dependent on the diameter of the aperture. The wider the aperture, the more light that comes in, and the brighter the image that is perceived by the observer.

Other notable features worth noting include resolving power and the field of view (FOV). The resolving power refers to the scope’s ability to clearly see the separation between two bodies closely-spaced with each other.

Whereas, the field of view pertains to the amount of space in the sky which you can view from the eyepiece. A clear understanding of this will help you interpret what you see through the telescope and a chart generated by computer programs like Stellarium.

Need More?

The universe and everything outside the Earth’s atmosphere is fascinating. Before the telescope was invented, people always looked up and wondered what things were above their night sky.

After the invention of the telescope, constant study about celestial bodies, planets, and stars happened in various societies and academic institutions. This instrument greatly helped in the advancement of Astronomy and studying the universe.

Speaking about telescopes, Check out our articles on the James Webb Space Telescope like this one: What Kind of Scope is NASA’s James Webb Space Telescope?

If you have any questions or need clarifications, please feel free to leave a message.

Also, Don’t hesitate to drop by my Recommended Gear Page where we have telescopes or accessories picked out based on your viewing.

Pros and Cons

  • Generally light and small, easy to carry around.
  • Great pricing
  • Plenty of options. All the good manufacturers make at least one product in this range.
  • The small lenses lead to a low-resolution image.
  • Few options when it comes to magnification. One of the biggest drawbacks of small aperture lenses is they don’t allow you to use high magnification eyepieces. It doesn’t matter if you try to use 300x magnification. It’s going to look the same as 100x magnification because your telescope is gathering a limited amount of light. The highest useful magnification per inch of aperture is 50x, meaning a 70mm aperture can only go as high as 135x magnification.
  • They can be outgrown really fast. Someone who enjoys looking at the stars will want to move up really fast.

I hope you love the products I recommend! Just so you know, I may collect a share of sales or other compensation from the links on this page. is independently owned and the views expressed here are my own. is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Astronomy Scope is compensated for referring traffic and business to these companies.


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Watch the video: Madness Magnification (June 2022).


  1. Wanrrick

    What entertaining question

  2. Corbett

    strongly disagree with the previous sentence

  3. Gyuszi

    Rustic and, most likely, not in the top.

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