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Teo

27 Feb 2013 23:23



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Ceva util si instructiv :

http://www.rfroyce.com/refractor%20spots.htm

si aici :

http://www.rfroyce.com/refobs.htm

" Without getting into details, secondary spectrum increases both as the focal ratio decreases and aperture increases. An 8" f/15 will make more secondary spectrum than a 6" f/15. In fact, an 8" refractor will have to work at f/18 to be as achromatic as the 6" f/15. "

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" We do not recommend the construction of refractor objectives for serious planetary or lunar astronomical work in ratios less than f/15. This has been considered the nominal standard for  acceptable performance since the mid 1800s when the Clarks began making virtually all of their refractors in focal length to aperture ratios of 15 to 1. However, the use of the BaK1 glass does allow for f/12 focal ratios in 6" sizes if customers insist - though f/15 is still preferred. I am occasionally asked to make refractors of 6" and 8" at f/10. I avoid the production of these refractors inasmuch as they will invariably produce a rather strong secondary color spectrum. The ability to recognize secondary color is interesting in that while some people do not see it at first, progressive use of the instrument and an increasing skill and awareness will eventually result in dissatisfaction. The existence of secondary color is nothing more than a defocused blur in the red and blue and a loss of contrast and information. "

Se pare ca la lunete acromate cu apertura de 6 inch, cel putin un raport f/15 ar fi de dorit  pentru observatiile planetare si selenare, iar la cele cu apertura de 8 inch, un minim de f/18 ar fi ideal .

Desigur, pana la urma pentru corectarea cromatismului mai intervine si tipul de sticla utilizat.

Totusi, asta nu inseamna ca valorile de f/10- f/12 nu ar fi bune pentru lunetele acromate cu aperturi mai mici de 6 inch.

Totul consta pana la urma in a construi, a experimenta si a observa. 
Satisfactia vine la final cand indrepti telescopul spre cer.

