4
Filter Descriptions
Numbering/Naming of Filters
Each lter is referred to by its color and Wratten number (based upon the historical
standard numbering system developed by Kodak/Wratten).
Figure 4. The Orion 1.25" 8-slot lter slider provides an affordable method to use and
exchange lters quickly.
Figure 5. Filter showing name and Wratten number (No 15. Yellow).
Luminance transmittance %
Associated with each lter is a luminance transmittance value (LT %).
All lters will reduce the total amount of light reaching the eye based upon this
value.
For small aperture telescopes best results will be found by using lters that have a
higher luminance transmittance value.
Larger aperture telescopes are required for low transmission lters such as the W46
Deep Blue (2% LT), the W47 Violet (3% LT) and the W29 Deep Red (6% LT).
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