This page is meant to serve as a general guide for what you can expect at certain magnifications while using your telescope!
Know your Focal Length and Theoretical Limit
Some objects can look spectacular in high magnification, while other’s won’t; only the the moon looks good either way.
To determine magnification, you divide the focal length of the telescope by the focal length of the eyepiece. For example, if your telescope has 1000mm in focal length and you’re using a 25mm eyepiece, then you’re using 40x magnification.
It is also important to know your theoretical magnification limit on your telescope, because if you go above the theoretical limit, then everything just looks mushy and impossible to focus. To figure this out, the rule of thumb is 50 times the aperture of your telescope in inches, or two times the aperture in millimeters. The theoretical limit on my 8″ Skyview is 406x, and I only try going that high if the seeing conditions are good enough.
The resolving power is calculated by dividing 4.56 by the aperture in inches (or 116 by the aperture in mm). As such, my 8″ (203 mm) telescope can resolve as small as 0.57 arc seconds across. a 4.5″ (114 mm)’ scope can resolve as small as ~1 arc-second across. Do you know how tiny Pluto appears from Earth? 0.1 arc-seconds!
Seeing Is Everything!
More often times than not, the local conditions in the atmosphere determine how much you can zoom before it gets too blurry and distorted. The post, What Makes A Good Sky to Observe In? further elaborates on choosing your ideal sky and talks more about seeing conditions – which is essentially how steady the air above is. Under excellent seeing conditions, you can easily still resolve details past 200-300x magnification, and on rare occasions even push beyond your telescope’s limit. However, under poor conditions where the air above is very turbulent, then objects will appear like they’re under water even at low magnification.
Remember how we talked about resolving power with aperture? Surely the 200 inch Hale Telescope at Mt. Palomar can resolve some details on Pluto because its resolution power is 0.022″ and should handle Pluto’s angular size of 0.1″ right? Nope! Most nights our atmospheric seeing limits even the best telescopes to resolving 1″ across.
Here is the General FOV Width in Degrees and Arc Minutes per Magnification!
25x ~ 2° FOV
50x ~ 60′ or 1° FOV
100x ~ 30′ or ½° FOV
150x ~ 20′ or 1/3° FOV
200x ~ 15′ or ¼° FOV
300x ~ 10′ or 1/6° FOV
There will be some variances depending on the the apparent field of view (essentially meaning how wide the barrel and your vision is), and of course the arc-minutes and degrees listed above are not exact figures, they’re rounded to give a general idea how much of the sky you’re covering through a telescope.
Hypothetically, if you want to get a planet like Jupiter or Saturn to take up a large portion of your FOV, you need to go higher than 3,000x. Good luck resolving anything with that!
With all of this out of the way, here is what you can expect at certain magnifications!
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
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.
It depends on the angular separation between the two companions. If they’re separated enough, they are resolved well at low power, but others will require a minimum amount of magnification to resolve the angular separation. While you could divide 300 by the A.S. to see where you can start resolving the double into individual stars, you should divide 750/A.S. to have the best minimal magnification.
Name – 750/A.S. = best minimal magnification
Alberio – 35″ = 21x
61 Cygni – 25″ = 30x
“The Double Double” – 2″ = 375x
Castor – 6″ = 125x
Algieba – 4″ = 187.5x
Almac – 9.6 = 78.125x
Keep in mind that seeing conditions may make it difficult to resolve double stars tighter than 2″ in separation, plus you risk magnifying more than what the telescope can theoretically resolve when using smaller telescopes. If your telescope can resolve much more than the best minimal magnification, then have at it and see how high you can go!
Deep Sky Objects
Open Star Clusters – Some open clusters such as the Pleiades are visible with the naked eye, and through a telescope should only be viewed at LOW power to see the entire cluster. Others such as The Double Cluster (Caldwell 14), Ptolemy’s Cluster (M7), and Beehive Cluster (M44) are visible as fuzzy smudges to the naked eye, and through telescopes at low power reveal a large group of stars. Some of them form shapes after their namesakes, such as Caldwell 13 (the Owl Cluster), M6 (Butterfly Cluster) and M11 (Wild Duck Cluster).
At this point, it matters how much light your telescope collects, hence the phrase “the bigger the better!” While I have certainly observed the brightest nebulae, globular clusters, and galaxies with a small telescope, they do appear dim and don’t show much structure. The larger the aperture, the brighter these objects will be!
Nebulae – Through telescopes, you can see some structure and glow from the gas clouds. While some nebulae have hints of green and blue tints, they will appear mainly gray due to your own eyes not accumulating enough light in the dark to see color. Therefore, if you want to see obvious color, then you’ll need long exposure.
Globular Clusters – Through telescopes, globular clusters appear as dim gray “snowballs” in space. Since you need to gather as much light as possible, it’s best to view at a low power with a wide eyepiece. If your telescope has enough aperture, and the globular cluster is big and bright enough, then you can definitely resolve individual stars when using higher magnification – just don’t expect them to be as bright as their pictures! M13 and M22 are the two largest, and an 8″ telescope can resolve them pretty well under good viewing conditions. Omega Centauri also looks like its pictures under dark skies.
Galaxies – Galaxies are notoriously disappointing through a telescope because of their dim appearance. Because you’ll mainly see their cores and not their spiral arms, do not expect to see any form of structure and detail unless you have a large enough telescope! M81 and M82 can be seen as two gray smudges in the same view even with small telescopes. M51 you can definitely tell there’s two “core” structures interacting with each other. The spiral arms of M101 can be glimpsed with fairly large telescopes (11″ or bigger) under dark clear skies. Long exposure photography is the reason why we know what these objects actually look like.