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Telescope Magnification Charts
Marc Stowbridge

I do not have a very good memory, and math-on-the-fly was never my strong suit. Consequently, I needed a crib sheet when asked by eager young astronomers “What Power is your telescope?” The WORKS template below produces charts that suitable for laminating to the telescope, and providing a quick answer. It’s also useful when choosing what eyepiece to use given the size of various deep space objects.

   

 

Eyepiece Chart for LX-90

 

 

Aperture=

203.2

f/=

10

@50/in.:

Max Power =

415

focal length

2000.0

True Field

Mag.

True Field

Eyepiece

Mag.

Deg - min

Barlow X2

Deg - min

4

500

0.06

1000

0.03

6.4

313

0.10

625

0.05

9.7

206

0.15

412

0.08

12.4

161

0.19

323

0.10

15

133

0.23

267

0.12

20

100

0.31

200

0.16

26

77

0.41

154

0.20

32

63

0.50

125

0.25

f/=

6.3

(using a Reducer-Corrector)

 

 

focal length

1280.16

True Field

Mag.

True Field

Eyepiece

Mag.

Deg - min

Barlow X2

Deg - min

4

320

0.10

640

0.05

6.4

200

0.16

400

0.08

9.7

132

0.24

264

0.12

12.4

103

0.30

206

0.15

15

85

0.37

171

0.18

20

64

0.49

128

0.24

26

49

1.03

98

0.32

32

40

1.18

80

0.39

 

The template gives instructions about where to enter data, and provides 3 models of complete charts. These are for the ETX-125, LX90 and GT 114. The template can be modified for most any telescope and eyepiece combination, as long as you know: 1) the focal length (in mm); 2) the f/ ratio; 3) the aperture (diameter in mm) of the telescope; and 4) the eyepiece focal length and the apparent field of view. Most of these figures will be on the telescope or eyepiece, except for the AFV, which is often available on the manufacture’s web site.

The math and design are not elegant, but the product is, I think, very useful. One can produce charts with almost any combination of eyepieces and use it to decide what eye pieces to buy or which combinations of EP, Barlow and reducer are redundant.

Please feel free to use this template. Attribution is good, as someone once said. If you can figure out better math or better WORKS methods, please let me know. Collaboration is good, too.