Some math is required to get the best results, but don't worry; it is fairly easy to grasp. The formulas for afocal photography are listed in Table 2.1. The projection magnification is the camera lens focal length divided by the eyepiece focal length.

Table 2.1. |
Afocal Photography Formulas |

Ft |
Telescope focal length |

D |
Telescope aperture diameter |

ft |
Telescope focal ratio = Ft/D |

Fep |
Eyepiece focal length |

Telescope exit pupil = Fep/ft | |

Fc |
Camera focal length |

fc |
Camera focal ratio |

Camera entrance pupil = Fc/fc | |

Mp |
Projection magnification = Fc/Fep |

Feffective |
Effective focal length = Ft*Mp |

feffective |
Effective focal ratio = ft*Mp |

The projection magnification can act like a Barlow when it is greater than one (shaded cells in Table 2.2) or as a focal reducer when it is less than one (clear cells in Table 2.2). Your telescope effective focal length is the telescope focal length multiplied by the projection magnification (Table 2.3). The telescope focal length and focal ratio are increased in the shaded Table 2.3 cells and decreased in the clear cells.

Table 2.2. Projection Magnification Factor | |||

Nikon |
990 Camera Focal Length | ||

Eyepiece Focal Length |
Minimum Zoom 6 |
.2mm |
Maximum Zoom 23.4mm |

4mm |
2.05 |
5.85 | |

10mm |
0.82 |
2.34 | |

14mm |
0.59 |
1.67 | |

18mm |
0.46 |
1.30 | |

20mm |
0.41 |
1.17 | |

24mm |
0.34 |
0.98 | |

35mm |
0.23 |
0.67 | |

40mm |
0.21 |
0.59 | |

55mm |
0.15 |
0.43 |

Table 2.3. Effective Focal Length and Ratio for a 1800mm |
Focal Length, f/12 Telescope | |||

Nikon 990 Camera Focal Length | ||||

Minimum |
Zoom 8.2 mm |
Maximum Zoom 23.4mm | ||

Eyepiece Focal Length |
Effective |
Effective |
Effective |
Effective |

Focal Length |
Focal Ratio |
Focal Length |
Focal Ratio | |

4mm |
3690mm |
24.6 |
10530mm |
70.2 |

10mm |
1476mm |
9.8 |
4212mm |
28.1 |

14mm |
1054mm |
7.0 |
3009mm |
20.1 |

18mm |
820mm |
5.5 |
2340mm |
15.6 |

20mm |
738mm |
4.9 |
2106mm |
14.0 |

24mm |
615mm |
4.1 |
1755mm |
11.7 |

35mm |
422mm |
2.8 |
1203mm |
8.0 |

40mm |
369mm |
2.5 |
1053mm |
7.0 |

55mm |
268mm |
1.8 |
766mm |
5.1 |

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