![]() These short optical tubes may not provide enough light-gathering capability for widefield observation due to their lower f/ratios. This means that it dictates whether or not an object should have a high, medium, or low power magnification.Ī narrow field view would require higher powered optics with shorter focal lengths to achieve sufficient magnification for small deep space objects like galaxies and planetary nebulae, which are often faint and diffusely extended over large areas of sky (a requirement for most amateur astronomers). The focal ratio is a major factor in determining what type of objects you can observe. Why Does the Focal Ratio Matter?įocal ratios dictate how many times larger the image will be than the object. But stars, which are point sources of light, are not influenced by aperture size, only the focal ratio. A telescope with a focal ratio of f/5 will show an image four times brighter than a telescope with a focal ratio of f/10, all other things being equal.Ī telescope’s focal ratio dictates how many times larger the image will be than the object. The focal ratio of your telescope can also affect how bright extended objects like nebulae or galaxies are. Widefield telescopes have a narrower focal ratio of less than f/7. If you are interested in capturing wide views of galaxies, star clusters, and the Milky Way, then a low focal ratio is preferred.Ī telescope with a low focal ratio provides more of the sky but less magnification. This is great for observing the moon, planets, double stars, etc.Īn ideal focal ratio for this type of object is f/10 or more. Choosing the Best Focal Ratio for AstrophotographyĪ long focal ratio gives a narrower field of view and higher magnification with a given eyepiece. Point sources are recorded based on the aperture, rather than focal ratio, so that the larger the aperture, the fainter star you can see or photograph. Medium focal ratios of f/6 to f/10 work well with both.Īn f/5 system can photograph faint extended deep-space objects in a fourth of the time it would take an f/10 system, but the image will only be half as large. These numbers also offer an increased depth of field with high-power photography equipment. Slow f-ratios in the range of 11 to 15 make it easier for people to observe lunar, planetary, and binary star phenomena. ![]() The lower the f/number is on an eyepiece or camera, the wider your field of view and brighter your images will be with that device.įor lower power widefield observing and deep space photography, a focal ratio of f/4 to f/5 is generally best. The larger the focal ratio, the greater detail, and slower an object can be moved across its image surface per degree of arc. The focal ratio of a telescope is determined by dividing the focal length by aperture size. Conclusion The Right Focal Ratio for Astrophotography: A Comprehensive Guide What is the Focal Ratio?.Should I or Should I Not Use a Focal Reducer?.Choosing the Best Focal Ratio for Astrophotography.The Right Focal Ratio for Astrophotography: A Comprehensive Guide.
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