We see jaw-dropping impressive images on social media and the Internet of the planets, galaxies, and nebula. Most of these are taken with highly sophisticated equipment. These are the Hubble type images, and not quite what you will see through a home telescope. What can you expect to see with a home telescope? If you are looking to buy a telescope as a beginner or one for a child, here are some basics.
Distances in the universe are in light years. And typically, you’ll be looking at between a few hundred to up to many millions of light years. That in itself is mindblowing. To think that what you are looking at, may no longer exist.
Apart from the quality of your telescope, other factors that affect what you will see include the atmospheric conditions and your level of experience. A dark location is important.
The quality of your eyepieces will play an important role in what you will see. So, don’t forget that when buying that telescope, think about getting extra eyepieces so you can reap the most from your purchase.
The main function of a telescope is to concentrate light into your eye. The telescope increases its power based on the amount of light it can gather. So, the larger the aperture the more light it is capable of gathering.
Magnification = Telescope focal length (TFL) ÷ Eyepiece focal length (EFL).
With higher magnification, you will have a smaller field of view (VOF) of the night sky, but, theoretically, the celestial object being viewed will be brighter and more easily seen.
Limit of Magnification
Here’s how you work out maximum magnification of a telescope…
Maximum magnification = Aperture (in inches) x 50
The Earth’s atmosphere can be a main limiting factor in magnification, creating image motion and distortion. Under average sky conditions the limit of magnification is considered about 250×. This limitation (due to Earth’s atmosphere) affects even the high magnification telescopes.
Apart from this and the telescope itself, another limiting factor is the viewer’s eye. The viewer’s eye must align with and be of similar size to the instrument’s exit pupil.
The exit pupil is the diameter of the light beam that exits the telescope eyepiece and enters the eye.
In terms of the exit pupil, too large (that is larger than the viewer’s pupil size) and light will be lost, too small and the view will be vignetted.
The average pupil size for dark-adapted eyes in viewers under 30 years is 7 mm and this reduces with age (see the table).
There are a few ways of measuring pupil diameter in your dark-adapted eyes. One way is to use something that has a known diameter as a gauge. The following is an example…
You can measure pupil diameter using a set of drill bits. With one eye closed, hold a bit at arm’s length in front of a star. Pupil diameter lies between the largest bit that doesn’t quite cover the star and the smallest bit that covers it completely.Contributor to Cloudy Nights
To calculate the exit pupil diameter…
Exit Pupil = Eyepiece focal length (mm) ÷ Telescope focal ratio (focal length/aperture)
The exit pupil affects the minimum magnification.
What to Expect as a Beginner
As a beginner, you’re going to be keen to get a handle on everything out there in that night sky.
A bit of everything to start with would be the Moon, some planets, galaxies, nebulae, globulars, and open clusters.
The Moon View
The distance to the Moon is just over one light second and the Moon is second to the Sun as the brightest object in our sky. So, it’s a good place to start and because it is so close, you should easily see its features.
Craters, the Maria, and rilles (long narrow depressions in the surface) are features you’ll see. Your first impression of the Moon will be that craters characterize its surface. Then you’ll see the many other features.
We always see the near side of the Moon. Because the Moon’s rotation is in synchrony with the Earth that’s the side we see. We never see the far side of the Moon.
Tip: In viewing the Moon, you’ll get a brighter image and more detail with a higher aperture size.
The Planets of Our Solar System
There are seven planets in the solar system other than the Earth. Most can be easily spotted by the naked eye.
Mercury and Venus
These two planets are relatively close to Earth. Mercury is the smallest planet in the solar system.
Through binoculars or a travel scope, you’ll likely see these planets as small disks and you’ll be able to observe their phases, but, you won’t necessarily see detail without using a telescope suited to viewing planets.
Mars is the second smallest planet in the system. On average, it is 12.5 light minutes from Earth.
Mars will appear as a small reddish disk. If you want to see more you’ll need to invest in a good quality eyepiece and have decent magnification with clear atmospheric conditions. With this, you may see its tiny white-ish pole and you may see tiny dark markings.
A better view will be seen with an 8″ aperture and taking time to observe.
On first look, you’ll see a whitish disk and possibly some brownish bands. Relaxing your eyes and allowing them to respond to the faint markings can present you with an observation of the different shades of the Jovian disk. Jupiter’s four main moons will appear like very bright stars.
You will see more detail of this planet as you get better at visually observing it and the more you observe, the more you will get better at it.
Take your time and with a 4–8″ telescope you should pick up the rings of Saturn and up to five of its moons. You may even see on a good night, Cassini’s Division, which is a black circle appearing between Saturn’s A and B rings.
Uranus and Neptune
Of our solar system planets, these are farthest from Earth.
They are not visible with the naked eye. They will appear as specks through the telescope.
With decent magnification, they may appear as disks and you may detect some color, such as a pale blue or greenish color.
You can see sunspots and the internal structure with proper white light filters. Warning: Never look at the Sun without proper filtration.
Alpha Centauri is the next closest star system to Earth. It is some 4.4 light-years away.
Through a home telescope, Alpha Centauri A and Alpha Centauri B, the two brightest stars of the Alpha Centauri system, can be seen, but you wouldn’t see Proxima Centauri, which is the third but faint object of this system.
Galaxies and Nebulae
These will appear fuzzy. However, by training your eye with practice and with much patience, you may eventually see more.
Filters can help with viewing nebulae, especially narrowband ones like UHC and OIII filters.
These deep space objects can be classified in three ways:
- Surface brightness
For these guys, as a rule of thumb, the attributes most relevant are surface brightness and size.
A home telescope is worth investing in as it will broaden your world and open you to new perspectives. The benefit of the above knowledge is that in knowing these things you will be less likely to become frustrated and more likely to spend the time in training your eye to see and learn more about the huge expanse above you.