But beware: these high magnifications may not be what we want! A subject's contrast is sometimes as important as its brightness. Likewise, this scope with a 20mm eyepiece would bring Saturn and its rings into view. How sharp can you get your telescope magnification? It is promoted by the 7-mm exit pupil of so-called night binoculars and corresponds to the exit pupil of a telescope used at a magnification of 3.5× per inch of aperture. Barlows can improve image quality and provide more eye relief for comfortable, relaxed high-power viewing. So, that means the rule of thumb is 300X useful magnification. Large secondaries also limit your visual performance by blacking out the center of your eye's pupil, which is the sharpest part. Notes: 1: Atmospheric seeing conditions (the sky) often limits the maximum usable magnification to 250-350x. When we look at an object which is an infinite distance away (e.g. Increasing the telescope magnification will reduce the size of the exit pupil and darken the background sky. Floaters, those bits of debris in our eyes, are mainly a problem when we use magnifications that produce very small exit pupils that accentuate their visibility. The true field of a telescope is the amount of actual sky we see in the eyepiece. With very high magnifications and small exit pupils, images (except for stars) grow dim, atmospheric turbulence and shakiness in the telescope's mounting are much more noticeable, and "floaters" (particles inside the eyeball) can be annoying. With 13- to 25-inch Dobsonians, you can use all the power the atmosphere and optical quality will permit. The formula for computing the magnification or power of a telescope: You bought a telescope with a focal length of 600 mm, and the eyepiece’s focal length is 30 mm. Regardless, you can find the true field of any eyepiece-telescope combination by the star-drift method. Often small refractors outperform larger reflectors because of superior contrast. Once you have enough magnification to see the diffraction pattern clearly, further telescope magnification is "empty.". Dobsonians are inherently stable, but they must be moved frequently at a high telescope magnification. There are other important things worth noting when looking for the proper telescope. To calculate the magnifi… This appears to contradict the old adage about using big exit pupils when viewing nebulae. On such nights the seeing is often poor. To see an example of this, make a diamond-shaped aperture by pressing your thumbs and forefingers together. This is a figure that you'll usually see printed or engraved near the eyepiece focuser and usually lies in the range of 400- to 3000-mm, depending on the aperture and type of telescope. 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? You can calculate all the numbers you ever may need to know for your telescope. While this is an extreme case, it points out the value of reflectors with small secondary obstructions and keeping the exit pupil to about 7 or 8 mm. The first ring has about 7 percent, and the rest is distributed in successively fainter rings. For example, a 20 mm eyepiece used on a 1000 mm focal-length telescope would yield a power of 50x (1000/20 = 50). So, bigger is better, at least in this case. The harder you push them together, the smaller the aperture will become. When telescope magnification gets too high; subjects become dim and lose contrast. What Kind of Scope is NASA’s James Webb Space Telescope? Does the image appear dimmer than it does in 7×25 binoculars, which have a pupil that matches your eye's? I'm not arguing that it is particularly wonderful to have an 8× scope, but the concept is valid. Leave plenty of breathing room around the subject so it appears in context with its surroundings. The bottom line for low power is to frame the subject. Sure. The key to seeing things much clearer is having an objective lens which can capture huge amounts of light from the object. August 1, 2006, By: Richard Tresch Fienberg Planetary observers using Newtonian reflectors want the smallest possible secondary mirrors for exactly this reason. This is the main concept driving the mechanism of a telescope. 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. Meanwhile, if you plan to view planets Neptune and Uranus, you need to use a scope with higher magnification. Many eyepieces have field stops that are accessible for measurement by calipers. Sky & Telescope maintains a strict policy of editorial independence from the AAS and its research publications in reporting developments in astronomy to readers. The first of these is telescope magnification, and by this I mean angular magnification. It is essential because the amount of light entering the scope is dependent on the diameter of the aperture. October 31, 2006, By: Alan MacRobert In practice, two or three times that magnification is more comfortable. You can calculate the exit pupil by dividing the diameter of the objective mirror or lens by the eyepiece magnification. It also ignores the fact that telescopes with apertures larger than about 9 inches can seldom achieve their theoretical ½-second or better resolution because of bad atmospheric seeing. If you want to have an excellent resolution, then you need to have a wider aperture. Owners of large Dobsonians find that, for the finest resolution and contrast, an off-axis aperture mask (best placed near the mirror to minimize tube currents) gives the best of all possible worlds — unobstructed, color-free images. 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. In his view, two stars are just resolvable if the center of one star's Airy disk lies in the first dark ring of the other's diffraction pattern. July 8, 2016, By: The Editors of Sky & Telescope Furthermore, resolution is poorer when double stars have components of differing magnitudes. While our eyes are not perfectly color corrected, our brain processes out the errors. Let's take up the segments separately. The shift in light from the Airy disk to the diffraction rings also reduces contrast, rendering planetary details less sharp. Controllable variables influencing magnification includes the focal length of your scope and the eyepiece size, or aperture. Others have field stops between the lens elements; such a stop's size cannot easily be used to determine the true field. Where can I find double star SAO numbers for my Go To telescope. The universe and everything outside the Earth’s atmosphere is fascinating. A steady atmosphere is a prerequisite for effective high-power observing. The central obstruction that exists with conventional reflectors places a much stricter limit on the situation. we have a larger fiel… Use it for the scope you are thinking of buying as a comparative analysis. You need to use an eyepiece with a focal length of 5 millimeters to acheive the 200X magnification. A small telescope shows twinkling stars jumping about playfully, but large apertures average out the motions, giving us steady blobs. Newtonian telescopes). Other defects vary from individual to individual. If a 14-mm exit pupil at 8× doesn't cost you anything in brightness or resolution, does it have any benefits? The 7-mm diameter of the dark-adapted eye's pupil seems to be a popularly enshrined value among astronomers. Another factor we must all confront is encroaching light pollution combined with atmospheric haze from industrial pollution. The Beehive Cluster is 1° across, the Pleiades about 2°, and the Hyades 5°.