I have found the images in the Takahashi 5" Fluorite FCT (just two elements) to be superior to the 5" three element apo's in side by side comparison. Both kinds are virtually color free but perhaps the two elements gives superior detail to three. I have also compared the 5" Fluorite to a good 6" achromat and I think the 5" is superior in both detail and contrast. Clear Skies Don Colton -----Original Message----- From: utah-astronomy-bounces+djcolton=piol.com@mailman.xmission.com [mailto:utah-astronomy-bounces+djcolton=piol.com@mailman.xmission.com]On Behalf Of Brent Watson Sent: Wednesday, March 31, 2004 3:40 PM To: Utah Astronomy Subject: [Utah-astronomy] The Refractor In my experience looking through refractors, I have been struck by the fact that the apos are not as sharp as the achromats. I am not saying the image in an apo is bad - to the contrary. They are great. The image through a good achromat just seems sharper to my eye. Yes, there is the secondary color you have to deal with, but you learn to "see through" that very quickly. The apo is not optimized for a better image, but rather is optimized to reduce the secondary color. I understand that it seems like this should add to the contrast, but the MTF should be no different. The MTF is diameter dependant. Since the secondary color is in the shorter wavelengths, the human eye cannot focus them well anyway, especially when focusing on a low light level image rich in longer wavelengths. One must also consider that there are more surfaces in a true apo than in an achromat. The more surfaces, the more chance for errors. At least this is the current model of the situation in my mind. This is like adding filters - they REDUCE detail. I demonstrated this to a few folks in St. George in Feb 2003. Chime in if you remember the tests we did on Jupiter with my 12.5" dob. Now, I have seen some absolutely STUNNING images throuh an apo, but I think those of an achromat are superior. What has been the experience of this group? BTW, the fact that I own a large achromatic refractor does not and should not have any bearing on this postulate nor remarks. Brent __________________________________ Do you Yahoo!? Yahoo! Finance Tax Center - File online. File on time. http://taxes.yahoo.com/filing.html _______________________________________________ Utah-Astronomy mailing list Utah-Astronomy@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Visit the Photo Gallery: http://www.utahastronomy.com