Thread: 102 f9.8 CA ??
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Old 20-12-2013, 01:03 AM
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johnt (John)
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Quote:
Originally Posted by Regulus View Post
Can anyone give me an indication of the extent of CA in the Black Diamond 102/1000 refractor (non-ED)??
And while on the subject, is there some simple math to calculate an ideal tube length for a give lens aperture using basic Crown and Flint. I understand that multi-coating might effect this equation.

Trevor
Hi Trevor,

Try this chart, (I think it gives what you are asking).

http://www.cityastronomy.com/CA-ratio-chart-achro.jpg

The "CA ratio" is "Focal length" divided by "Lens Diameter squared", all in INCHES. Or, just "F ratio" divided by "Lens Diameter in Inches". As they say, an Achromat with a CA ratio greater than 3 is getting quite good.

The 102 1000 Achromat gives CA ratio of 2.45, so although its not 3, its still quite good. (ie. F ratio=9.8, D=4", so 9.8/4 = 2.45)

Also, some further explanation is given in:

http://www.cityastronomy.com/color-c...-refractor.htm

So, that's why for Achromats to maintain a good CA ratio with increasing lens diameter, they have to increase the "F ratio" in proportion with the increased lens diameter. As the table indicates, a 4" F10 is about equivalent in CA to a 5" F12, and a 6" F15 etc...


John

Last edited by johnt; 20-12-2013 at 01:25 AM.
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