However, a 70 mm refractor (which collects 36% more light than a 60mm telescope) is considered by many amateur astronomers to be the minimum size for a good quality beginner refractor telescope. It is acceptable for observing bright objects like lunar details, planets, star clusters, and bright double stars.
The refractory telescope with a size of 700 x 70 mm is ideal for beginners to explore the wide sky such as moons, planets and clusters and enjoy distant landscapes such as mountains, flowers, birds and wild animals.
The focal length of the telescope is 900mm, so to achieve the maximum useful magnification, then a 4.5mm eyepiece would be ideal. One of the best parts about planetary viewing or imaging is that since the objects are so bright, you can do it just about anywhere regardless of light pollution.
The above formula dictates that a telescope eyepiece with a shorter focal length yields a higher magnification than an eyepiece with a longer focal length. For example, a 10mm eyepiece will always provide a higher magnification than a 25mm eyepiece.
A 20 mm is useful just as a 13 mm is useful in most any telescope but I'm not sure "all purpose" is a good description. My telescopes range in focal length from under 400 mm to about 2800mm, a 20mm is not a planetary or high power or even mid-power eyepiece in any of them, it's a low to mid power deep sky eyepiece.
Magnification is determined simply by dividing the focal length of the telescope by the focal length of the eyepiece. This means that a smaller number on an eyepiece gives a higher magnification. A 10mm eyepiece would provide twice as much magnification as a 20mm eyepiece.
50mm (2 in) telescopes are the most basic, entry-level, budget telescopes on the market. They are mostly targeted at kids and some of them could even fall into the toy category. We generally don't recommend 50mm telescopes unless you are on a very tight budget or you are looking for a gift for a 5-year-old.
25mm – 30.9mm Telescope Eyepieces: These are extended field eyepieces for longer focal length – good for large nebula and open clusters. For shorter focal length, they are fantastic for large objects such as the Orion nebula, views of the full lunar disc, large open clusters and more.
At 1000x magnification you will be able to see 0.180mm, or 180 microns.
Even with a small aperture you can see quite a lot at moderate power. In fact, a 70mm telescope will put you in touch with a surprising number of celestial objects, including dozens of the Messier's, all the planets out to Jupiter and Saturn, and some very pleasing images of the moon.
One can study the moon nightly for years and still not see everything a 60mm telescope is capable of revealing! The planets are excellent targets as well. You'll be able to see Saturn's ring, Jupiter's moons, and the phases of Venus. Be sure to check out Mars as well.
Overview. The Orion Astroview 90mm refractor is an ideal telescope for novice astronomers ready to invest in their first model. There are some shortcomings, but this affordable telescope offers the laser-sharp optics that refractors are known for and is ideal for your first views of the Moon, planets, and stars.
But it's enough, that things that are difficult in a 70mm are considerably easier in an 80mm. And size wise, the 70mm is smaller but not a lot smaller and I use them on the same mounts so the difference in portability is essentially zero. If one has a good 80mm ed/apo, a good 70mm doesn't make much sense.
114mm aperture has impressive light-gathering capability to provide excellent views of planets and bright deep sky objects. Fast focal ratio (f/5.2) shortens exposure times for deep sky photography . Pan and tilt control on alt-azimuth mount allows for smooth movements when navigating the night sky.
Aperture: A Telescope's Most Important Feature
The aperture's diameter (D) will be expressed either in millimeters or, less commonly, in inches (1 inch equals 25.4 mm). As a rule of thumb, your telescope should have at least 2.8 inches (70 mm) aperture — and preferably more.
Bring land and sky objects 30 times closer; and view large astronomical features like bright lunar craters. Simple, yet robust, this compact telescope is the ideal introduction to the wonderous world of astronomy. Can be used as a hand-held 'spy glass' as well as supported in the small, metal tripod.
The Apertura 4mm Plossl eyepiece produces sharp, high magnification views of the Moon and planets, and, depending on the telescope, will also work great for double stars, planetary nebulae, and other deep sky objects. Like most Plossl eyepieces, the High Point 4mm has a 52º apparent field of view.
With a 50mm telescope, the Moon will look great and you will be able to see al the major craters and geological features. Mars, Jupiter, and Saturn will be visible as long as the orbits allow it.
The Fusion Science 360mm Refractor Telescope has been designed to show you the moon's craters, Jupiter's major moons, and even glimpses of Saturn's rings. It also delivers great views of mountains and animals near and far in the world around us.
10mm Eyepiece - for seeing more detail
Or in a telescope with a 1000mm focal length, 100X magnification.
This is marked somewhere on the eyepiece body and appears as a dimension in millimetres. The larger one is normally between 20mm and 25mm and is the lower power (lowest magnification). The smaller (higher magnification) is normally around 10mm.
In such a case, you might consider an eyepiece that produces a lower magnification, such as one with a focal length of 9mm or 10mm to produce a magnification of 222x or 200x, respectively.