Just as in camera lenses, telescopes with lower "f numbers" are "faster" they form brighter (but smaller) images than telescopes of the same diameter with with higher f numbers.Įyepiece Focal Length: This determines the "power" an eyepiece delivers on a given telescope the shorter the focal length, the higher the magnification. It is equal to the Focal Ratio (f number) times the Objective Diameter.į Ratio: The Focal Ratio (f number) is the Focal Length divided by the Objective Diameter. Objective Diameter: the size of the main light-gathering ("primary") lens or mirror of your telescope (also called the telescope's "aperture").įocal Length: The Effective Focal Length of your telescope is important to know it is often listed on a label right on the telescope, sometimes as Fl, and usually in millimeters. in mm)įield transit time at 0°declination (sec.): (or use Change f number to compute Focal Length from f Ratio and Obj. Enter Objective Diameter in inches -or- millimeters To try different eyepieces, enter their info in the Eyepiece Specifications field(s), and use the adjacent buttons to re-compute the outputs. Enjoy! (And for further information, also see our How Telescopes Work and Choosing a Telescope pages.)įill in the numbers in the green boxes below as instructed, then press the "Enter Diameter" button for whichever measurement system you used the computed values will appear in the other boxes. It starts off with some basic information every telescope user should know for their equipment, and continues into some more esoteric calculations. Object and sky surface brightnesses are the same as in the small 'scope, but magnification is higherĪs a handy tool for the telescope user, we present this JavaScript-driven calculator, which will compute the values of a number of paramaters for any telescope & eyepiece combinations you enter. Same object thru a larger telescope at same "Brightness Factor": View thru same telescope at higher power:īoth the object and the sky background are reduced in brightness by the same amount it may be easier to spot the object since it is larger and the sky is blacker, though Stars thru larger telescope at same magnification: View thru an eyepiece of the same Focal Length with a wider Apparent Field: View thru a narrow Apparent Field eyepiece The larger the f Ratio of the telescope, the further the Exit Pupil moves out from the eyepiece this holds true if the telescope's focal length is "artificially" extended by using a Barlow lens.ĬOMPARING FACTORS USING SIMULATED TELESCOPIC VIEWS The eye relief a given eyepiece provides changes some depending on the telescope it is used on. Eyepieces with very long Eye Relief may be confusing for some people to use, as the image may disappear if the eye comes in too close to the eyepiece. Eyepieces with longer Eye Relief are easier for eyeglass wearers to use. This distance varies from eyepiece to eyepiece. While part of the image may be seen with the eye closer in or farther out, the best image will always be obtained if the eye is at the correct distance. The other results will also be changed by the same Barlow factor: The Power Per Inch will be doubled (2x), while the the Exit Pupil, True Field, and Field Transit Time will each be halved (divided by 2).Įye Relief refers to the distance from the outer lens of an eyepiece to the prime spot to place the pupil of your eye (the eyepiece's Exit Pupil). if the calculator shows that a certain eyepiece gives 100x in your telescope, and you add a 2x Barlow, the resulting magnification will be 200x (100 x 2). A Barlow Lens, placed between the eyepiece and the telescope, will increase the Resulting Magnification by the power of the Barlow i.e.
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