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A SIMPLE CALCULATOR FOR MAGNIFICATION IN YOUR TELESCOPE

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A SIMPLE CALCULATOR FOR MAGNIFICATION IN YOUR TELESCOPE

All telescopes produce an image at the final focal plane of the telescope. An eyepiece is a simple magnifier of that image. Like a magnifying glass, it increases the size of the telescope's image so that we can see details in that image. The shorter the focal length of the eyepiece, the higher is its power. The simple formula is:

M (magnification) = TFL (telescope focal length) / EFL (eyepiece focal length).

Here is a simple calculator so you can plug in your focal lengths and get the magnification produced: EYEPIECE CALCULATOR.

HOW FAINT CAN I SEE? A CALCULATOR

You don't have to look in many books until you start seeing magnitude figures for the objects in the sky. The idea of magnitude dates back to the Egyptian astronomer, Ptolemy. He called the brightest stars in the sky 1st magnitude, because they were first to be noticed as it got dark. The faintest stars he could see, he called 6th magnitude. And all the other visible stars fit in the magnitudes between. When instruments were developed that could measure the brightness of stars, it was discovered that the 5 magnitude spread (1 to 6) of Ptolemy represented a difference of 100:1. Soon after, the brightness of all stars started being measured using this scale.

Because some objects were brighter than the first magnitude stars, these had negative numbers assigned to them, so the Sun became magnitude -26.8, the full Moon -12, the brightest star (Sirius) -1.42, the bright star Vega 0.0, etc. Of course, with telescopes, we could see much fainter than Ptolemy's 6th magnitude stars. Today, our instruments have seen all the way to magnitude 31, or 25 magnitudes fainter than can be seen with the naked eye!that's 10,000,000,000 times fainter!

This scale is backwards to common sense, because a larger figure means less bright.

Extended objects are given magnitudes that would be their equivalent magnitudes if they were shrunk to the size of a star. Because the brightness is spread out, they will be harder to see than a star of the same magnitude.

In our galaxy the number of stars and objects that you can see goes up significantly as your ability to see fainter improves. In essence, you are seeing farther into the Universe as the telescope's size increases. The number of objects goes up very rapidly as the telescope's size increases because the sphere of viewable Universe is increasing so dramatically in volume.

How faint can I see with my telescope? Here's a LIMITING MAGNITUDE CALCULATOR for stars visible. And here's another for deep-sky objects CALCULATOR.

Whatever result you get, you will have to adjust the figure according to where you live, in the Urban/Rural sense. Here are correction factors for different locations:

  • The center of a big city!subtract 5 magnitudes (14 becomes 9, because smaller numbers are brighter)
  • The edge of a big city!subtract 4 magnitudes
  • The suburbs of a big city!subtract 3 magnitudes
  • The Exurbs of a big city!subtract 2 magnitudes
  • A rural site 50 miles or more from a city!subtract 1 magnitude
  • A pristine, rural site!make no adjustment
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Bushnell 1250 x 90mm Motorized GoTo Maksutov-Cassegrain Telescope
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Meade 16 inch LX200ACF Advanced Coma Free Telescope
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Meade ETX-90 AT Catadioptric Telescope 90mm f/13.8 Maksutov-Cassegrain w/26mm & 9.7mm Plossl, tripod, UHTC coatings, case
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Meade ETX-90 AT Catadioptric Telescope 90mm f/13.8 Maksutov-Cassegrain w/26mm Plossl, tripod, UHTC coatings
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Meade Schmidt Cassegrain 8 inch SC Telescope with LX80 Multi Mount
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Carson RedPlanet 50-100 x 90 mm Refractor Telescope
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Carson RedPlanet 25-56x80 mm Refractor Telescope
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Takahashi Mewlon-210UH Telescope, 8.3 inch f/11.5 Dall-Kirkham, with EM-200 Temma-2 go-to mount, FC-L tripod
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