How much better would be a 5" fluorite doublet if the mating element would be optimally chosen for optical quality of the objective instead of putting priority on the ease of production (Tak example). Would there be e.g. noticeably less "warmth' in the picture ?
Optimization of a fluorite doublet
Started by progenitor, 01/19/2005 11:58PM
Posted 01/19/2005 11:58PM
Opening Post
Posted 01/22/2005 07:50PM
#2
>>How much better would be a 5" fluorite doublet if the mating element would be optimally chosen for optical quality of the objective instead of putting priority on the ease of production (Tak example). Would there be e.g. noticeably less "warmth' in the picture >>
If you are asking the general amateur public, you will get all kinds of answers that are uninformed at best.
The reason any apo doublet has a "warm" color is quite simple. The design allows some blue color error in order to achieve high production capabilities. In order to make the production straight forward with modern machine polished optics, the two elements have to be chosen so that their natural melt variations do not cause large differences in the color correction. This naturally limits the range of full color correction to a smaller value. Blue-violet being the hardest to bring in, it is allowed to fall outside of focus and thus the image is deprived of this cool color, resulting in a "warm" appearance. Since there is little information in the bluest wavelengths on planet surfaces, most people don't see the difference (except for the purists, of course).
Yes, one can make a doublet with no visual color error, but it requires having to match each set of lens elements individually with slightly different curves. Beyond a certain point in the wavelength range, a doublet of any kind, whether fully corrected for color or not, will have enough sphero-chromatic error that it is limited by that aberration. In other words, even if the ends of the spectrum have zero color error, the spherical error will begin to dominate.
With three lens elements, one can extend the range of perfect focus far beyond the visual into the region where CCDs still have quite high sensitivity. So for imaging, a triplet will have certain advantages. They stay sharp over a much wider wavelength range than any comparable doublet. For visual, it is not readily apparent.
Roland Christen
If you are asking the general amateur public, you will get all kinds of answers that are uninformed at best.
The reason any apo doublet has a "warm" color is quite simple. The design allows some blue color error in order to achieve high production capabilities. In order to make the production straight forward with modern machine polished optics, the two elements have to be chosen so that their natural melt variations do not cause large differences in the color correction. This naturally limits the range of full color correction to a smaller value. Blue-violet being the hardest to bring in, it is allowed to fall outside of focus and thus the image is deprived of this cool color, resulting in a "warm" appearance. Since there is little information in the bluest wavelengths on planet surfaces, most people don't see the difference (except for the purists, of course).
Yes, one can make a doublet with no visual color error, but it requires having to match each set of lens elements individually with slightly different curves. Beyond a certain point in the wavelength range, a doublet of any kind, whether fully corrected for color or not, will have enough sphero-chromatic error that it is limited by that aberration. In other words, even if the ends of the spectrum have zero color error, the spherical error will begin to dominate.
With three lens elements, one can extend the range of perfect focus far beyond the visual into the region where CCDs still have quite high sensitivity. So for imaging, a triplet will have certain advantages. They stay sharp over a much wider wavelength range than any comparable doublet. For visual, it is not readily apparent.
Roland Christen
