Apochromatic Triplet Specifications

Apochromatic (ED APO) lenses are designed to bring all the colors into focus in the same plane where as Achromatic lenses are corrected to bring two wavelengths into focus in the same plane. 

Chromatic aberration is the phenomenon of different colors focusing at different distances from a lens. Telescope objective lenses for wide-band digital imaging in astronomy must have apochromatic correction, as the optical sensitivity of typical CCD imaging arrays can extend from the ultraviolet through the visible spectrum and into the near infrared wavelength range. Apochromatic designs require optical glasses with special dispersive properties to achieve three color crossings. Explore Scientific achieves this using high-quality ED (Extra-Low Dispersion) glass from Hoya and Ohara.

Glass Type Code Refractive Index nd Abbe-number vd Dispersion nF-nC Refractive Index ne Abbe-number ve Dispersion nF'-nC'
Hoya FCD1 497-816 1.497 81.61 0.00609 1.49845 81.19 0.006139
Hoya FCD100 437-951 1.437 95.1 0.004595 1.4381 94.66 0.004628
Ohara FPL-53 1.43875 94.93 0.004622 1.43985 94.49 0.004655

Explore Scientific Telescope Image Circle Sizes
Telescope Objective Diameter mm Focuser Size Adapter Size Focal Length mm Focal Ratio A B C FULL IN Unvignetted Image Circle Diameter mm FULL OUT Unvignetted Image Circle Diameter mm
80 FCD1 metal tube 80 2” 2" 480 6.0 46 51.0 292.0 26.1 31.0
80 FCD1 CF tube 80 2” 2" 480 6.0 46 51.0 311.0 22.4 33.0
80FCD100 metal tube 80 2.5” 2" 480 6.0 46 63.5 285.9 21.3 37.8
102FCD1 metal tube 102 2” 2" 714 6.9 46 51.0 507.0 29.6 36.9
102FCD1 CF tube 102 2” 2" 714 6.9 46 51.0 520.0 30.6 37.9
102FCD100 metal tube 102 2.5” 2" 714 6.9 46 63.5 504.9 34.5 45.8
102FCD100 CF tube 102 2.5” 2" 714 6.9 46 63.5 507.9 34.7 46.1
115FPL53 CF tube 115 3.3” 3" 632 5.5 76 84.0 419.0 30.4 55.6
127FCD1 metal tube 127 2” 2" 952 7.5 46 51.0 757.0 35.2 40.6
127FCD1 CF tube 127 2” 2" 952 7.5 46 51.0 764.0 35.6 41.0
127FCD100 metal tube 127 2.5” 2" 952 7.5 46 63.5 749.2 41.7 50.1
127FCD100 CF tube 127 2.5” 2" 952 7.5 46 63.5 753.5 42.0 50.4
140FPL53 CF tube 140 3.3” 3" 910 6.5 76 84.0 704.0 47.4 65.0
152FCD1 CF tube 152 3” 3" 1216 8.0 76 76.2 988.6 51.3 62.0
165FPL53 CF tube 165 3.3” 3" 1155 7.0 76 84.0 945.9 55.0 68.8

Note: The values listed are for the image circles with the focuser drawtube inserted all the way (worst case) and pulled all the way out (best case)

User Focus Position Circle Calculation

Using value A, B, C, and the Focal Length from the table, calculate your specific focus position image circle using the position as read off your drawtube:
A = Focus Travel Length (mm)
B = Drawtube Inside Diameter (mm)
C = Length between main objective and end of drawtube (fully inserted)

 

Procedure

1. Using your image configuration, focus your camera. 
2. Read the focus position (mm) on the drawtube.
3. Calculate your image circle = B* ((C-A) + step 2 value)/Focal Length
Example:
For the 80 FCD1 with a reading of 35 mm from the drawtube:
Image Circle = 51.0 * (((292.0 - 46) + 35)/480)
                      = 51.0 * (281/480)
                      = 29.9 mm

 

 

Camera Sensor Size Comparison 

Camera Sensor Size Comparison