Coatings
So what does the calculator tell you? Getting either your primary or your secondary coated with high-reflectivity coatings produces essentially the same overall system improvement. Coating a secondary with a high-reflectivity coating is a lot less expensive than coating your primary. So, unless money is no object, or you plan on coating both primary and secondary, get just the secondary coated with the better coating. Save some money and go for standard coatings on your primary.
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How's it calculated?
I have tried to add clear comments to the programming code embedded within the calculator page. But, if you don't know JavaScript, it's still going to be pretty cryptic. Comments are the lines that begin with // or <!--.
The full formula for calculating the reflectivity efficiency involves converting the aperture to area, multiplying by the efficiency of the coating, then re-converting to aperture. I used a shortcut provided by Larry Manuel. The formula I use is:
Equivalent Aperture = Square Root (primary peflectivity * secondary reflectivity) * primary aperture
You enter data into the form. That data is sent automatically to the formula through a programming trick called onChange. When your browser detects a change in the text box, it does the programming command defined by the onChange(do_this) statement. In this case, it makes sure your input is reasonable and then calculates the equivalent aperture.
To compare with standard coatings, I calculate the equivalent aperture of standard coatings (assuming 88% reflectivity) and then figure the percent improvement through a simple comparison.
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This document, its contents, and its Web representation are Copyright
©1996, Tim Poulsen. For complete copyright information, including allowed uses of this FAQ, please see Section 8. Initially created on Thursday, September 12, 1996 by Tim Poulsen.