On Mon, 30 Aug 2004 15:38:22 +0000 (UTC), (Brian Tung)
wrote:

XxXxXxX wrote:
I'm wonder if anyone has tried this or thought to try this... I have an
ETX-60 (lots of fun so far) which is a 60mm achromatic refractor. Now with
the ETX-60 and 70 the front objective lens can be removed. Has anyone
thought to remove that lens and replace it with an apochromatic lens? The
Backyard Astronomy guide shows that the difference between the two are the
number and shape of the lens.

...and the cost. One can save a substantial money by scrimping on the
mechanicals, as the recent low-cost small apos (or near-apos) have
demonstrated. But manufacturing the lenses to the point where the
smoothness is on par with the color correction has at some point an
irreducible cost. It just isn't a matter of $100 or so.

I would think this is more than a garage project as someone would have to
figure out the optics to fit the tube and mirror design of the ETX-60/70.
Since the ETX-60 and 70 both share the same OTA only one kit would be needed
for both scopes. Given the fact that there are thousands (if not 10's of
thousands) of ETX-60/70's on the market a $100-150 dollar acro to apo
conversion kit might be a sweet little cottage business.

The tough part isn't fitting the lens to the scope (although it certainly
is something you'd need to deal with). It's manufacturing an apo lens
(of any appreciable size) for only $100. If you can do that, screw the
tube, people will snap the lens up.

I won't say never, because seeming optical miracles have happened. But
it's definitely not a weekend project. If anyone were to do this on the
cheap like you describe it, it would have to be someone who knows quite
a bit about optics, taking advantage of something quite unusual. It's
not a matter of tinkering with glasses until something "clicks."

One problem is that you're still stuck with the reflex mirror in the
ETX which from what I've seen, isn't as good as a good diagonal.
But, cannibalizing a Tele-Vue 60mm Apo might be possible, since
it's not a Petzval. You could also do the same with the ED lens
from a spotting scope, but the least expensive way would be using
the new Orion or Celestron 80mm apos and putting an extension/cell
adapter on the front of the ETX. Least cost will probably be around
$100 for some kind of cell adapter and the two scopes of course.
-Rich