"Markus Ludes" wrote in message news:fd5bf00f8d41fbccc6b3259b1816faf7.30545@mygat e.mailgate.org...

To find out how the colorcorrection is against a achromat, I took now a
Skywatcher Achromat 102F/1000 and made several masks. Doing a Mask with
60 mm , the Skywatcher offers a bit better colorcorrection, doing a mask
with 80 mm the Skywatcher was a bit worser.
The best matched the Mask of 70 mm , than my eyes saw about same level
of colorcorrection as in the 80 mm ED. This means estimated by huma eyes
the colorcorrection in the orion 80F/7.5 ED Apo is about equal to a 70
mm F/14 achromatic refractor.

This does mean, that Orion 80mm ED has about (80/70)x (14/7.5)^2=4 better
color correction, than equivalent (D=80mm F/D=7.5) achromat.

We should conclude, that relative color correction in these 80mm ED scopes
is the same as in Meade ED refractors, where color correction is about
4.5x better, than in equivalent achromat. If a 80mm ED shows "less" colors
than ,say, Meade 4" F/9 ED, than this is purely threshold effect.

All in all, Orion ED 80mm is an entry level APO - about the same, as Meade ED
APOs. These days such color correction can be considered as semi-apo.

However, I can exclude tha trick with "establish reputation and then lower the
quality". If some scopes from China can be called semi-apo, while they are
plain achromats (Mergrez) or they can use such methods of color correction
"improvement" as internal aperture stop, then it is difficult to exclude
objective design change in worser side.


V.D.

Valery Deryuzhin.