Meade's new Series 6000 astrograph is a 70-mm apochromatic refractor with a focal length of just 350 mm and an f/5
focal ratio. That's fast, short, and wide.
The optical bench measures
focal ratio degradation (FRD) in specific optical fiber samples to determine if the fibers are appropriate for astronomy applications.
Another consideration is the
focal ratio or 'speed' of the lens: f/1.2 or better will give good results.
The fibre uses a large core-to-clad ratio to reduce stress transfer to its core, maintaining laser beam quality and improving
focal ratio degradation in lasers used for cutting, welding, drilling, soldering and marking.
Many spectroscopic applications require high performance optical fibres that transmit light over a broad spectrum and demonstrate minimal
focal ratio degradation.
The focal length divided by the aperture is the
focal ratio (the f/ number).
The parameters are element spacing (d),
focal ratio (g), lens width (G), and scan angle ([alpha]) (Figure 1).
Table 1 - The correction required for different mirror diameters and
focal ratios.
Focal ratio (F/D] 3 4 4.5 5 6 Diameter D (ins) 6 10.06 4.24 2.37 2.17 1.25 8 13.41 5.65 3.97 2.39 1.67 10 16.76 7.06 4.96 3.61 2.09 12 20.11 8.47 5.96 4.33 2.51 16 26.32 11.3O 7.93 5 79 3.34 20 33.52 14.12 9.91 7.22 4.1S 25 41.90 17.65 12.39 9.03 5.22
Focal ratio (F/D] 8 10 12 15 Diameter D (ins) 6 0.53 0.27 0.16 0.08 8 0.71 0.36 0.21 0.11 10 0.88 0.45 0.26 0.13 12 1.06 0.54 0.31 0.16 16 1.41 0.72 0.42 0.21 20 1.76 0.90 0.52 0.27 25 2.20 1.13 0.65 0.33 Table 2 - Image quality for different correction errors.
"A two-stage concentrator works best if you have a moderately long
focal ratio design, that is, where the focal length is substantially greater than the diameter of the aperture of the primary of the focusing mirror," said Joe O'Gallagher, a senior research associate and senior lecturer in the Physics Department at the University of Chicago and a colleague of Winston's since 1976.
One other thing I appreciated was the Meade's f/7
focal ratio. While f/6 APOs are popular, I found the scope's slightly longer-than-average focal length allowed me to use somewhat longer (and more comfortable) eyepieces to reach high powers.
reflector telescope newtonian aperture: 114mm (4.49 "), focal length: 1000mm (39.37"),
focal ratio: 8.77, 1:25 mm eyepiece (40x), eyes 2 : 9mm (111x), seeker: starpointer, mounts: altazimuth motorized tripod: aluminum.
The right exposure will depend on the
focal ratio of your telescope, the clarity of the sky, and the phase of the Moon.
