A cheap alternative with excellent views for birders who do not mind carrying an extra few hundred grams of equipment with them. I have used the C90 for birding in Afria, Asia, Europe and the Americas with success. Some technical information:
Aperture = 3.5 inches (90mm); Focal length = 1200mm; Focal ratio = f/13.3
Angle of view = 1.4 degrees; Field of view = 24m at 1000m (73 yards)
Relative brightness = 5.9; Twilight factor = 58.1; Light gathering power = 165.2
Close focus = 6.09m (20 feet); Weight = 1.6 kg (3.1 lbs); Physical length = 279mm (11 inches)
The three 1.25 inch eyepieces (ocular) I am using at present:
1. X-Cel Series 1.25 in - 21mm (item no. 93392) = 1200 divided by 21 = 57x magnification; FOCAL LENGTH = 21 mm (0.83 in); EYE RELIEF = 20 mm (0.79 in)
ANGULAR FIELD OF VIEW = 55 ; SIZE = 1.25 in (31.75 mm)
2. 32mm Plossl = 1200 divided by 32 = 37.5x magnification; Eye relief = 22mm
3. 2x Barlow Ultima fully multicoated
Maximum usable power: As a rule of thumb, the maximum usable power is equal to 60 times the aperture of the telescope (in inches) under ideal conditions. 60 x 3.5 = 210x for the C90 Mak = 5.7mm ocular or 11.42mm ocular with 2x Barlow.
Lower limit of usable power: Nighttime: 3 to 4 times the aperture in inches = 3 or 4 x 3.5 = 10.5-14x magnification. Daytime: 6 to 10 times the aperture = 6 or 10 x 3.5 = 21-35x. With these lower powers, the images will be much brighter and crisper, providing more enjoyment and satisfaction with the wider fields of view. Powers lower than this are not useful with most telescopes and a dark spot may appear in the center of the eyepiece in a Catadioptric or Newtonian telescope due to the secondary or diagonal mirror's shadow.
Focal ratio: This is the ratio of the focal length of the telescope to its aperture. To calculate, divide the focal length (in mm) by the aperture (in mm). For the C90 it is 13.3
Exit pupil diameter: The exit pupil of a telescope is the circular beam of light that leaves the eyepiece being used and is measured in mm. To calculate exit pupil, divide the aperture (in mm) by the power of the eyepiece being used.
For 21mm lens (57x): 90 divided by 57 = 1.58;
For 32mm lens (38x): 90 divided by 38 = 2.36
Or, you can calculate the exit pupil by dividing the focal length of the eyepiece (in mm) by the focal ratio of the telescope.
LIGHT GATHERING POWER (LIGHT GRASP): This is the telescope's theoretical ability to collect light compared to your fully dilated eye. It is directly proportional to the square of the aperture. You can calculate this by first dividing the aperture of the telescope (in mm) by 7mm (dilated eye for a young person) and then squaring this result. In theory, when you double the aperture of a telescope, you increase its resolving power by a factor of two and boost its light gathering ability by a factor of four. But more importantly, you also reduce the area of the airy disk by a factor of four resulting in a sixteen-fold gain in stellar image brilliance.
90 divided by 7 = 12.85; squared, this is 165.2
Aperture = 3.5 inches (90mm); Focal length = 1200mm; Focal ratio = f/13.3
Angle of view = 1.4 degrees; Field of view = 24m at 1000m (73 yards)
Relative brightness = 5.9; Twilight factor = 58.1; Light gathering power = 165.2
Close focus = 6.09m (20 feet); Weight = 1.6 kg (3.1 lbs); Physical length = 279mm (11 inches)
The three 1.25 inch eyepieces (ocular) I am using at present:
1. X-Cel Series 1.25 in - 21mm (item no. 93392) = 1200 divided by 21 = 57x magnification; FOCAL LENGTH = 21 mm (0.83 in); EYE RELIEF = 20 mm (0.79 in)
ANGULAR FIELD OF VIEW = 55 ; SIZE = 1.25 in (31.75 mm)
2. 32mm Plossl = 1200 divided by 32 = 37.5x magnification; Eye relief = 22mm
3. 2x Barlow Ultima fully multicoated
Maximum usable power: As a rule of thumb, the maximum usable power is equal to 60 times the aperture of the telescope (in inches) under ideal conditions. 60 x 3.5 = 210x for the C90 Mak = 5.7mm ocular or 11.42mm ocular with 2x Barlow.
Lower limit of usable power: Nighttime: 3 to 4 times the aperture in inches = 3 or 4 x 3.5 = 10.5-14x magnification. Daytime: 6 to 10 times the aperture = 6 or 10 x 3.5 = 21-35x. With these lower powers, the images will be much brighter and crisper, providing more enjoyment and satisfaction with the wider fields of view. Powers lower than this are not useful with most telescopes and a dark spot may appear in the center of the eyepiece in a Catadioptric or Newtonian telescope due to the secondary or diagonal mirror's shadow.
Focal ratio: This is the ratio of the focal length of the telescope to its aperture. To calculate, divide the focal length (in mm) by the aperture (in mm). For the C90 it is 13.3
Exit pupil diameter: The exit pupil of a telescope is the circular beam of light that leaves the eyepiece being used and is measured in mm. To calculate exit pupil, divide the aperture (in mm) by the power of the eyepiece being used.
For 21mm lens (57x): 90 divided by 57 = 1.58;
For 32mm lens (38x): 90 divided by 38 = 2.36
Or, you can calculate the exit pupil by dividing the focal length of the eyepiece (in mm) by the focal ratio of the telescope.
LIGHT GATHERING POWER (LIGHT GRASP): This is the telescope's theoretical ability to collect light compared to your fully dilated eye. It is directly proportional to the square of the aperture. You can calculate this by first dividing the aperture of the telescope (in mm) by 7mm (dilated eye for a young person) and then squaring this result. In theory, when you double the aperture of a telescope, you increase its resolving power by a factor of two and boost its light gathering ability by a factor of four. But more importantly, you also reduce the area of the airy disk by a factor of four resulting in a sixteen-fold gain in stellar image brilliance.
90 divided by 7 = 12.85; squared, this is 165.2
