Sorry, as I posted this question again, you replied. So please ignore my
latest post concerning this same subject..

Why aren't the ED telephotos not good objectives for astronomy purposes?
Because they are not designed for it? Any simple optical explanation that
can be easily understood? I am asking in reference specificaly to high end
ED 35mm telephotos Vs APO refractors.

Thank you.

"Chuck Taylor" wrote in message
...
"Birdie" wrote in message
...

Has anyone converted (or attempt to convert) a Nikon or Canon ED super
telephoto camera lens ( eg. 400mm F2.8, 500mm F4, 600mm F4, 800mm F5.6 or
1000mm 5.6) into a refractor telescope and attach an eyepiece for visual
use? If so, how do you increase the back focus so that a star diagonal can
be used? The back focus of these 35mm designed lenses are short. Will a
barlow lens increase this back focus distance and enables the use of a
star
diagonal?
If it can be done, will the images equal or surpass the normal APO
refractors like Takahashi, tele Vue, etc? Any comments? Just thinking
aloud.

Camera lenses do not make good telescope objectives (other than for low
power viewing).

Clear Skies

Chuck Taylor
Do you observe the moon?
Try the Lunar Observing Group
http://groups.yahoo.com/group/lunar-observing/