Other types of objects are rated with the same magnitude scale, based on the total amount of their light that reaches us; if the object is large, though, that light is spread out over an area. 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. A Barlow lens is a concave lens that when placed in a telescopes before the eyepiece, it will increase the focal length of the telescope by 2x, 3x, 4x and so on, depending on the size you use. Notes: 1: Atmospheric seeing conditions (the sky) often limits the maximum usable magnification to 250-350x. 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. var sc_project=1047655; 2: An exit pupil size (diameter of light beam as it exits eyepiece) over 7.5mm might be too large for telescope designs with central obstructions (i.e. Telescope Calculator Results: This value is complicated by the fact that magnification also dims the brightness of the sky background, increasing the contrast of stellar objects to the background. If that is not possible, Apparent Field can be approximately computed by measuring the diameter of the Field Stop in the eyepiece; it is the aperture (opening) usually located up inside the eyepiece barrel on the telescope side of the eyepiece (but may be internal and not accessible in some wide-field eyepieces). In general, when the magnification of scope increases, the image brightness, and field of view (FOV) decreases. Click on Calculate for the scope's magnification … Be sure your telescope & eyepiece information is filled in correctly above; you can go back and alter it anytime you want, too. Enjoy! See the section below for more information on your pupil size! This gives you some idea of how fast something will drift to the edge from the center of the field (half of the full Field Transit Time). Your browser does not support JavaScript, or has "active content" blocked; Menu system will not function. The eyepiece’s focal length (for example, 25mm) To find the magnification, we’ll simply divide the numbers: 1200mm / 25mm = 48x magnification That’s it! If you actually know what your maximum pupil dilation is, you can correct the figure (your actual measurement might vary as much as a millimeter from the estimated value). Beyond a certain point (usually accepted to be 0.5 arc seconds for locations at our altitude), the atmosphere always prohibits seeing any smaller details, even if the telescope's optics could deliver them. Field Transit Time: A product of the True Field, this is how long it will take a star, located near the celestial equator, to drift across the center of the field from one edge to the other (when the telescope is not clock driven to follow the sky). The eye relief a given eyepiece provides changes some depending on the telescope it is used on. Enter your age below, and this program will show an average value for your age group. For brighter objects, such as the planets, higher magnifications may be desirable, but the sharpest appearing views will still probably be found within this range. Find this out from the manufacturer or seller of the eyepiece. The magnification factor M B of a Barlow lens depends on its position relative to the focal plane of the telescope’s objective lens and the focal plane of the eyepiece. The telescope’s focal length (for example, 1200mm) 2. For larger objects, which need a larger True Field for you to see their full extent, lower magnifications can certainly be used (watch out for oversized Exit Pupils, though! Usually noted as a number on the eyepiece itself, normally measured in millimeters. This calculator is designed to give the magnification characteristics for a given telescope, based on the data entered for the scope's operating specifications. Exit pupil: The diameter of the beam of focused light shining out of the eyepiece. Again, the figure shown is based on the telescope information entered above, and actual performance will vary with other factors as noted earlier. var sc_partition=6; I am looking for the calculator that tells how much the power of a Barlow with a given focal length varies with spacing. Now you have 4 eyepieces and 4 magnifications using one Barlow lens. var sc_security="794b1315"; This feature can be manipulated using different combinations of objective and eyepiece lens. If the value is over 1.0, then the exit pupil for your selected telescope/eyepiece combination is larger than your eye's pupil, and this would cause light loss, bringing the achieved Brightness Factor back down to 1 (or potentially less, especially in reflecting telescopes with central obstructions). To best match your telescope/eyepiece combination for your use, you have to know how wide the pupil of your eye can open in the dark. To go beyond the basics, and explore what you should be able to see through your telescope, continue on below. This calculator is designed to give the magnification characteristics for a given telescope, based on the data entered for the scope's operating specifications. It starts off with some basic information every telescope user should know for their equipment, and continues into some more esoteric calculations. True Field: The angular diameter of the part of the sky you can observe through the telescope. It is equal to the Focal Ratio (f number) times the Objective Diameter. Approximate Eyepiece Field Stop diameter (mm): Magnification (power): How many times larger (in angular size) an object looks through the telescope than it would look to the unaided eye. Magnitude is the measurement of the brightness of an astronomical object; the lower the number, the brighter the object is (the faintest stars you can see in from in town may be 3rd magnitude, while out in the country you might see 6th magnitude ones; the brightest stars actually have negative magnitudes-- Sirius' is -1.4). To get started, we just need two numbers: 1. The Resolving Power for a telescope tells what the size of the smallest details which can be seen through it, atmospheric conditions allowing. A Barlow Lens, placed between the eyepiece and the telescope, will increase the Resulting Magnification by the power of the Barlow; i.e. Objective Diameter: the size of the main light-gathering ("primary") lens or mirror of your telescope (also called the telescope's "aperture"). Here is the approximate Field Stop diameter for your eyepiece, based on the Apparent Field entered above; if you replace this number with an actual measurement and click the Enter Measured FS button, we'll recompute your Apparent Field based on this measurement. As you change the eyepiece values, note that the brightness factor will go down at higher powers, and up at lower ones. This distance varies from eyepiece to eyepiece. I understand that the calculation for a single barlow is simply: Magnification = - d / f + 1where d is the distance to the barlow lens (back, center?) Barlow Lens: A lens that's placed into the focusing tube to effectively increase a telescope's focal length and, in turn, the magnification of any eyepiece used with it. Can be measured in Degrees or Minutes of Arc (arcminutes). 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