The ability to see details on an object ultimately depends on how far away it is and how big it is. Stars are so very far away that they will never show a real disk or ball shape in a telescope.
Planets, the Moon, and the Sun are much closer and will show discernible disks and details even at low or medium powers in most telescopes. Nebulae and galaxies are also very far away, but are so enormous they will also show details in many telescopes.
What you will see looking at stars at high magnifications (assuming a steady atmosphere that will allow this) is an optical pattern known as the diffraction pattern. It’s a bull’s-eye with a bright central area or disk surrounded by one or more concentric rings. It is not the actual disk of the star you are seeing. The diffraction pattern is due to the way the telescope’s circular lens or mirror acts on light from a pinpoint source like a star.
Showing posts with label magnification. Show all posts
Showing posts with label magnification. Show all posts
Saturday, 15 February 2014
How much magnification can I use, and how much is too much?
Telescope magnification is actually a relationship between two independent optical systems – the telescope itself and the eyepiece you are using. To determine power, divide the focal length of the telescope (in mm) by the focal length of the eyepiece (in mm). By exchanging an eyepiece of one focal length for another, you can increase or decrease the power of the telescope. For example, a 25 mm eyepiece used on a 1250 mm focal-length telescope would yield a power of 50x (1250/25 = 50) and a 10 mm eyepiece used on the same instrument would yield a power of 125x (1250/10 = 125). Since eyepieces are interchangeable, a telescope can be used at a variety of powers.
There are practical limits of magnification for telescopes. These are determined by the laws of optics and the nature of the human eye. As a rule of thumb, the maximum usable power is equal to 50-60 times the aperture of the telescope (in inches) under ideal conditions. Powers higher than this usually shows a dim, lower contrast image. For example, the maximum power on a 127 mm telescope (5” aperture) is in the range 250x - 300x. As power increases, the sharpness and detail seen will be diminished. The higher powers are mainly used for lunar, planetary, and binary star observations.
Most of your observations will be done with lower powers (6 to 25 times the aperture of the telescope in inches). With these powers, the images will be much brighter and crisper, providing more enjoyment and satisfaction with the wider fields of view.
A good way to increase magnification is to use a Barlow lens. A Barlow lens rated at 2x can be used with your existing eyepiece and it will double the magnification of any existing eyepiece.
There are practical limits of magnification for telescopes. These are determined by the laws of optics and the nature of the human eye. As a rule of thumb, the maximum usable power is equal to 50-60 times the aperture of the telescope (in inches) under ideal conditions. Powers higher than this usually shows a dim, lower contrast image. For example, the maximum power on a 127 mm telescope (5” aperture) is in the range 250x - 300x. As power increases, the sharpness and detail seen will be diminished. The higher powers are mainly used for lunar, planetary, and binary star observations.
Most of your observations will be done with lower powers (6 to 25 times the aperture of the telescope in inches). With these powers, the images will be much brighter and crisper, providing more enjoyment and satisfaction with the wider fields of view.
A good way to increase magnification is to use a Barlow lens. A Barlow lens rated at 2x can be used with your existing eyepiece and it will double the magnification of any existing eyepiece.
How do I use eyepieces with my telescope?
Your eyepieces are the first accessories you should have and use with your telescope. Since they are interchangeable, a telescope can be used at a variety of powers.
Look through the eyepiece by placing your eye just behind it to take advantage of its eye relief, the spacing between the lens and your eye. This should be at least 15 mm for the best comfort, maybe more if you wear eyeglasses. You’ll lose field of view if you place your eye farther away and may even move your eye out of the beam of light from the eyepiece. Viewing too close to the eyepiece will prevent you from blinking and may also cause a black ring to appear around the field of view.
Now turn the focus knob of the telescope, first one way then the other until the object is in focus.
Always start with the lowest power eyepiece (i.e., the one with the highest number in millimeters printed on it). It is much easier to focus and has a wider field of view, making it easier to aim the telescope at a distant target.
As power increases, the sharpness and detail seen will be diminished. The higher powers are mainly used for lunar, planetary, and binary star observations.
Most of your observations will be done with lower powers. With these lower powers, the images will be much brighter and crisper, providing more enjoyment and satisfaction with the wider fields of view.
A good way to increase magnification is to use a Barlow lens. A Barlow lens rated at 2x can be used with your existing eyepiece and it will double the magnification of any existing eyepiece. Since longer-focal-length eyepieces generally have longer eye relief, using a Barlow to increase magnification will allow more comfortable high-power viewing.
A basic set of eyepieces would be 32 mm, 20mm (or 26mm), and 10mm (or 16mm), plus a Barlow (2x or 3x power).
Look through the eyepiece by placing your eye just behind it to take advantage of its eye relief, the spacing between the lens and your eye. This should be at least 15 mm for the best comfort, maybe more if you wear eyeglasses. You’ll lose field of view if you place your eye farther away and may even move your eye out of the beam of light from the eyepiece. Viewing too close to the eyepiece will prevent you from blinking and may also cause a black ring to appear around the field of view.
Now turn the focus knob of the telescope, first one way then the other until the object is in focus.
Always start with the lowest power eyepiece (i.e., the one with the highest number in millimeters printed on it). It is much easier to focus and has a wider field of view, making it easier to aim the telescope at a distant target.
As power increases, the sharpness and detail seen will be diminished. The higher powers are mainly used for lunar, planetary, and binary star observations.
Most of your observations will be done with lower powers. With these lower powers, the images will be much brighter and crisper, providing more enjoyment and satisfaction with the wider fields of view.
A good way to increase magnification is to use a Barlow lens. A Barlow lens rated at 2x can be used with your existing eyepiece and it will double the magnification of any existing eyepiece. Since longer-focal-length eyepieces generally have longer eye relief, using a Barlow to increase magnification will allow more comfortable high-power viewing.
A basic set of eyepieces would be 32 mm, 20mm (or 26mm), and 10mm (or 16mm), plus a Barlow (2x or 3x power).
I have a refractor telescope with 900mm focal length and 60mm objective lens. Why can't I get a clear image when I try to use a 2x Barlow lens with a 6mm eyepiece?
This is because you’ve exceeded the practical limits of magnification for your telescope. The practical limits of magnification of a telescope are determined by the laws of optics and the nature of the human eye.
As a rule of thumb, the maximum usable power is equal to 50x-60x the aperture of the telescope (in inches) under ideal conditions. Powers higher than this usually give you a dim, lower contrast image. For example, the maximum power on a 60 mm telescope (2.4 in aperture) is in the range 120x-142x. The higher powers are mainly used for lunar, planetary, and binary star observations. As power increases, the sharpness and detail seen will be diminished.
The combination of the 6 mm eyepiece, 2x Barlow, and 900 mm focal length gives 300x. This is well beyond the maximum usable power of the scope.
The Earth's atmosphere also plays an important part in limiting the maximum magnification you can use. Instabilities in the atmosphere such as heat radiating from the ground and surrounding buildings, high altitude winds, and other weather conditions can cause the image to blur. This "bad seeing" can drastically distort your image. This also explains why bright stars appear to twinkle. The best time to use high magnification is on nights when the stars do not appear to twinkle very much.
As a rule of thumb, the maximum usable power is equal to 50x-60x the aperture of the telescope (in inches) under ideal conditions. Powers higher than this usually give you a dim, lower contrast image. For example, the maximum power on a 60 mm telescope (2.4 in aperture) is in the range 120x-142x. The higher powers are mainly used for lunar, planetary, and binary star observations. As power increases, the sharpness and detail seen will be diminished.
The combination of the 6 mm eyepiece, 2x Barlow, and 900 mm focal length gives 300x. This is well beyond the maximum usable power of the scope.
The Earth's atmosphere also plays an important part in limiting the maximum magnification you can use. Instabilities in the atmosphere such as heat radiating from the ground and surrounding buildings, high altitude winds, and other weather conditions can cause the image to blur. This "bad seeing" can drastically distort your image. This also explains why bright stars appear to twinkle. The best time to use high magnification is on nights when the stars do not appear to twinkle very much.
How to determine a telescope's magnification?
The magnification of a telescope changes as the eyepiece is changed. Magnification can be calculated by dividing the focal length of your telescope by the focal length of the eyepiece.
Calculating magnification = Telescope's focal length / Eyepiece focal length
Always start with your lowest magnification eyepiece (longest focal length) and work upwards from there.
A 2x Barlow lens will double the magnification of whatever eyepiece you use with it while preserving its eye relief. For example: using a telescope with a 800 mm focal length with a 16 mm eyepiece will give you 50x magnification. Using the same telescope and eyepiece with a 2x Barlow lens will give 100x magnification. This would be the same magnification as a 800 mm telescope with a 8 mm eyepiece.
Calculating magnification = Telescope's focal length / Eyepiece focal length
Always start with your lowest magnification eyepiece (longest focal length) and work upwards from there.
A 2x Barlow lens will double the magnification of whatever eyepiece you use with it while preserving its eye relief. For example: using a telescope with a 800 mm focal length with a 16 mm eyepiece will give you 50x magnification. Using the same telescope and eyepiece with a 2x Barlow lens will give 100x magnification. This would be the same magnification as a 800 mm telescope with a 8 mm eyepiece.
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