What Can You See With a Telescope

Soprano

Village Elder
#1
What Can You See With Different Telescopes

Introduction
When someone considers buying a new telescope - the first question will usually be "what can I see through it?" In this article I will try to illustrate the views you can expect through different telescopes, and maybe help to decide which one to choose. The subjects covered here are: Moon, Planets, Sun, DSO,Comets, Double stars, Light pollution.
The amount of observable celestial objects and their visible details will depend on several factors: The size and the optical quality of a telescope, the observing location (darkness and atmospheric stability) and even the observer's experience. Let's start with the instrument:
A telescope has two important parameters: "Aperture" and "optical quality". Aperture is the diameter of the objective lens (or a mirror) - it defines how much light the telescope gathers and the maximum resolution (and magnification) it can hope to achieve. Optical quality is a general therm which describes the ability of a telescope to transmit unaltered image. In this article there are 3 telescope categories, which will be referred to using [orange]tags:
  • [1] - Small and cheap (price range 50-150$). For example a cheap 70-80mm Newtonian reflector or a 50-70mm achromatic refractor (usually with inferior optical quality).
  • [2] - Medium (price range 200-500$). For example a 150 mm Newtonian reflector, 80-120mm refractor or a 90-130mm Maksutov Cassegrain of a moderate quality. Note that considering deep sky objects this category refers to an aperture of 140mm or more.
  • [3] - Large (price range 600$ and higher). For example a 200-300 mm reflector, a quality 120-150mm refractor or a 200-250 mm Schmidt Cassegrain. Note that considering deep sky objects this category refers to an aperture of 200mm or more.
This should only give you a general idea, since the price of a telescope is also affected by other parameters (mounting system, brand, accessories and so on). Also for observing different types of objects there are different optical requirements. For Moon, planets, double stars and the Sun it is better to have a smaller but higher quality 80mm refractor rather than a larger 150mm reflector with poor optics. For deep sky objects it is the opposite - a cheaper 150mm reflector will usually outperform a quality 80mm refractor with the same price tag.
The following sections will provide illustrations of how different objects should appear in different telescopes to a visual observer. Based on my observing experience I have used some real photos and processed sketches in order to simulate these views. While these images may illustrate the level of detail you can expect to see, they do not show the whole visible field of view (which is often a large, black circle with a small object inside).
The Moon
Our Moon is the easiest and the most impressive observing target. At low magnifications it will look approximately the same through any telescope. But a larger, more quality instrument will allow you to "zoom in", and reveal countless craters, rills and mountains.


Moon through a small and cheap telescope at low power of (100x) [1]


Moon through a large, quality telescope at high power (350x) [3]
The Planets
There are 8 planets in our solar system you can observe, however only three of them will show notable surface details: Jupiter, Saturn and Mars (mostly during opposition). Also these three planets will display time varying weather related phenomena - such as clouds and dust storms on mars or cloud bands on Jupiter. Jupiter also has easily observable 4 Jovian moons which rotate around it, occasionally transiting and casting a shadow over the planet's disk. There are several visible moons around the Saturn as well. In a moderate telescope Venus and Mercury will reveal their phases (a crescent shape) and Venus can even show hints of cloud details with a right filter. Neptune and Uranus will look like small, featureless, bluish or greenish disks through any telescope. The Pluto is very hard to observe visually, especially now - when it's in the milky way area, and even if you succeed it will look like a featureless faint star.
The following images show comparison of how Jupiter and Saturn should look in different instruments, at their highest useful magnifications:


Jupiter through a small, cheap telescope (100x power) [1]


Jupiter through a medium telescope (180x power)[2]


Jupiter through a large, quality telescope (300x power) [3]


Saturn in a small, cheap telescope (100x power) [1]


Saturn in a medium telescope (180x power) [2]


Saturn in a large, quality telescope (300x power) [3]
In order to see fine planetary details - the telescope, apart from quality optics and sufficient aperture - must be properly collimated and, in case of larger apertures, properly cooled. Apart from the telescope, the factor which greatly affects the observable planetary details is so called "seeing". It is the amount of atmospheric turbulence which causes the image to "dance" and become blurry (it is also the phenomenon which causes stars to twinkle). The images above show how the planets look during a moderate seeing. Under better conditions an experienced observer with a quality, properly collimated and cooled instrument might be able to resolve more surface details. And the opposite is true - the image can be much worse if a telescope isn't properly collimated, cooled or the seeing conditions are poor.
These illustjrations simulate the visible detail rather than the actual field of view. As you can see in the following example - the planetary discs look really small through an eyepiece.


An example of how the jupiter looks in a medium telescope's whole field of view at 180x
Deep Sky Objects
Deep sky objects (DSO) is a general name for galaxies, nebulae and star clusters - objects beyond our solar system. Unlike with planets - observing deep sky objects does not necessarily require using high magnifications. What important is the aperture of your telescope, since you need to gather a lot of light. Another factor affecting our ability to observe DSO is sky darkness, and it is even more important than aperture. If you observe in the middle of a big, light polluted city - even owning a huge, 100000$ telescope won't help.
The following sketches simulate the level of details you can expect to see in some of the brightest deep sky objects, through telescopes with different apertures, under truly dark skies (and by "dark" I mean a place where you can see the Milky Way bright and detailed):


Hercules Cluster in a small telescope (dark skies) [1]


Hercules Cluster in a medium telescope (dark skies) [2]


Hercules Cluster in a large telescope (dark skies) [3]


Pinwheel Galaxy in a small telescope (dark skies) [1]


Pinwheel Galaxy in a medium telescope (dark skies) [2]


Pinwheel Galaxy in a large telescope (dark skies) [3]


Swan Nebula in a small telescope (dark skies) [1]


Swan Nebula in a medium telescope (dark skies) [2]


Swan Nebula in a large telescope (dark skies) [3]
There are only several dozens of deep sky objects which will show this amount of detail. The vast majority of visible DSOs will look much fainter. The following images show some more examples of deep sky objects which you may observe (there are hundreds of such objects) as they would look in a moderate telescope:


NGC 7662 planetary nebula in a moderate telescope [2,3]
 

Soprano

Village Elder
#2
The Sun
The Sun is a special subject - it must be observed with a full aperture filter. There are 2 general types of them. One is the narrowband filter (usually an H-alpha), which is an expensive device (starting from 600-1000) and it is often mounted on a dedicated solar telescope - see Coronado, Lunt or Daystar Quark products for examples. The other type is so called "daylight" filter - it can be mounted on any telescope, and can be quite cheap. The following images represent how our sun (and its sunspots) may look like through a simple daylight filter mounted on a cheap, small telescope (left) and a more quality instrument (right). Note that in this case it is often better to have a smaller aperture but higher optical quality (i.e. a quality 80mm ED doublet refractor). This is because during the daytime - atmospheric turbulence doesn't allow a larger telescope to fully utilize its resolving power. See this article for an overview of solar observing methods.


Sun through a small, cheap telescope [1]

Sun through a higher quality telescope [2]
Other Objects

International Space Station through a quality telescope
There are several more "exotic" categories of objects you can observe as you become more experienced. Variable stars, for example, are stars which change their brightness over a period of months (days for some). Observing them and reporting the data via organization such as the AAVSOallows amateur astronomers to make their small scientific contribution. Novae and supernovae are violent explosions of distant stars, which occur from time to time, and can also be visible with an amateur telescope. Occultations of stars by our Moon (or by asteroids) are very fast events and can only be observed at very specific locations on Earth. Timing and reportingthese events improves scientific data for these asteroids, and allows scientists to pinpoint the exact position of occulted star. Earth orbiting satellites can also be observed using amateur telescopes. The International Space Station, which is the largest of them, may even show glimpses of details (such as solar panels) during its closest approaches.
 

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