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Transmission Calculator
Transmission Calculator allows you to compute the percentage of light that passes through an instrument's optical system. System transmission is used by the visibility model.
Instrument Type allows you to select one of:
▪Reflector - a typical Newtonian design with a primary mirror and a secondary mirror. Corrector Plate and Star Diagonal Transmission are disabled for this selection.
▪Refractor/Binoculars - a typical refractor or binocular design has an unobstructed primary lens or lens group. Secondary Aperture, Secondary Transmission and Corrector Plate Transmission are disabled for this selection.
▪Catadioptric - a typical Maksutov or Schmidt-Cassegrain design has a primary mirror, secondary mirror, front corrector plate and star diagonal.
The calculator supports the computation of transmission for only the above instrument designs. If your instrument is of a different design, you can calculate the system transmission by calculating the following:
total system transmission = ((primary area - secondary area) / primary area) * transmission of each element in the system. Note that the unit of measure of the primary and secondary apertures should be the same.
Primary Aperture displays the diameter of the primary element of the instrument. It is set by the Instrument Browser and is specified in the units (either millimeters or inches) selected in the Instrument Browser.
Primary Transmission allows you to enter the percentage of light transmitted by the primary element in the system.
Secondary Aperture allows you to enter the diameter of the secondary element of the instrument. It should be entered in the same unit of measure as the primary element (either millimeters or inches)
Secondary Transmission allows you to enter the percentage of light transmitted by the secondary element in the system.
Corrector Plate Transmission allows you to enter the percentage of light transmitted by the secondary element in the system.
Star Diagonal Transmission allows you to enter the percentage of light transmitted by the star diagonal in the system. If there is no star diagonal, set this to 100%. A typical star diagonal containing a mirror with dielectric coating has a transmission rate of 99%.
The total system transmission is calculated and shown on the dialog box. If Secondary Aperture is enabled for Reflector or Catadioptric types and set to zero, total system transmission is not displayed.
Some typical instrument transmission factors
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Type of instrument
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Primary Diameter
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Primary Transmission
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Secondary Diameter
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Secondary Transmission
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Corrector Transmission
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Star Diagonal
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Total Transmission
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8" Schmidt Cassegrain (premium coatings)
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8"
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95%
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2.63"
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95%
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97%
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99%
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77%
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8" Schmidt Cassegrain (standard coatings)
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8"
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89%
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2.63"
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89%
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92%
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99%
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64%
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8" Dobsonian (premium coatings)
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8"
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97%
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1.3"
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97%
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n/a
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n/a
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91%
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8" Dobsonian (standard coatings)
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8"
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91%
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1.3"
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91%
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n/a
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n/a
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80%
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4" Apochromatic Refractor (premium coatings)
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4"
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97%
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n/a
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n/a
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n/a
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99%
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96%
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Telephoto Lens
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any
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95%
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n/a
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n/a
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n/a
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n/a
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95%
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Further Considerations
Reflective coatings on mirrors degrade over time due to oxidization. A typical degradation rate is 1% to 2% per year. The rate may increase in harsh environmental conditions such as those near the sea coast. You should deduct this from the transmission rate of primary and secondary for the period of time since the coatings were new.
Press OK to accept the changes and return the total system transmission value to Transmission in the Instrument Browser, or Cancel to leave Transmission in the Instrument Browser unchanged.
Help file version 9.2.3.0 ▪ Copyright © 2024-2026 Knightware, LLC
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