Introduction to basic optical properties of astronomical telescope, only for reference.
1. Aperture: The effective aperture of the objective lens determines the performance of the telescope in theory. The larger the caliber is, the greater the concentration ability will be, the higher the resolution ratio will be, the larger the available magnification can be.
2. Light harvesting: It is the ratio of telescope receiving light and eye receiving light. When the pupil of the eye is completely open, its diameter is about 7mm. The light harvesting ability of telescope with caliber of 70mm is 702/72=100 times.
3. Resolution ratio: the ability to distinguish details of the images. It’s mainly determined by aperture.
4. Magnification: This is the ratio of focal lengths of objective lens and ocular lens. Such as the pioneering 60/700 telescope, which use ocular lens H10mm. Magnification=focal length of objective lens 700mm/focal length of ocular lens 10mm= 70 times. As the signification becomes bigger, the image looks bigger as well.
Magnification is not the bigger the better, the maximum available magnification is generally not greater than 1.5 times the number of millimeter of the aperture. If it’s more than the maximum effective magnification, the image still gets larger, but the clarity will not increase.
5. Focal ratio: Ratio of focal length of objective lens and caliber, which is equal to the aperture on the camera lens. When the caliber is fixed, the longer the focal length of the objective lens is, the bigger the ratio will be, the easier to get higher rate. The shorter the ratio is, the harder it will be to get high rate. But the image and view will get bigger.
*Lens of short focal length(small focal ratio<=6): ideal for observing nebula and discovering comets.
*Lens of long focal length(big focal ratio>15): ideal for observing the moon and planets
*Lens of medium focal length(medium focal ratio, 6<ratio<=15): ideal for observing double stars, multiple stars, variable stars and star clusters. It can observing two or more things at the same time, which is suitable for beginners.
6. Field of view: The angle of sky in the eyes of the observers which the telescope forms is also known as the field angle. The larger the magnification is, the smaller the field of view will be.
7. Limiting magnitude: It is the darkest magnitude which telescope can find. The magnitude is related to the caliber and focal ratio. People with normal vision can see six magnitude stars in dark place. However, the light harvesting ability of telescope with 70mm caliber is 100 times the ability of eyes. It can see 11 magnitude stars which is 5 magnitudes darker than 6 magnitude stars.
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