then what are we missing?

roland says red ,blue violet are over 1wave out of focus, then how much resolution are we losing?

thomas zukouski said:

roland says red ,blue violet are over 1wave out of focus, then how much resolution are we losing?

I think if you look through a large fast Achromat and compare the view to a similar APO that someone like Roland has produced you will see that you are missing a great deal of fine detail.

Color correction gets tougher as the aperture increases, it scales with with the focal ratio rather than the focal length. A 3 inch F/16 has very little chromatic aberration in the visual spectrum. To maintain this level of color correction in a 6 inch Achromat, the scope would have to be F/32 and about 16 feet long. Even if one accepted a lesser level and kpet the focal ratio constant, it would need to be 8 feet long.

The mount required for an 8 foot long scope, that is awkward, cumbersome and expensive.

Regarding the upgrade of the 110mm F/7 to a 6 inch... Things are going to get worse and it would depend on the availability of the glass in blanks that large. But given that the 110mm F/7 costs about $1000, one could have to expect the 150mm (150mm F/9.45) version to cost something around $2000 or more. I am not sure anyone would pay that much...

But I am not quite sure why one would want to go to all the trouble and expense and still end up with a scope with significant limitations.

For planetary and double star viewing, a high quality 8 inch Newtonian would be a significantly superior performer and more manageable as well.

$2000 buys a lot in the Newtonian world.

If you want the good views, the time comes when it is necessary to recognize the limitations of the refractor. Roland's comments on "The best Planetary Telescope" is worth reading:

http://www.cloudynights.com/item.php?item_id=410

Jon Isaacs