On 12 Oct 2004 16:49:36 -0700, (Bratislav)
wrote:

"Thierry" - wrote in message ...

Indeed, using ED lens FLP-53 this is the cheapest "semiapo-like" than I
know.

I think you need to clarify your definition of "semi-apo".
FPL53 is capable of full apochromatic correction even as a doublet
(paired with
BK7 variant) as partial dispersion is so similar that residual
secondary sepctrum can be pushed well into 1/20,000+ (an APO by all
definitions).
Semi APOs have residual spectrum in 1/4000-1/10000 range (C to f),
some 2 to 5 times better than achromat. Anything above that is an APO.

This ST-80 gives a better image and cripser than the first generation of
Megrez but remains an apochromat; chromatic aberration stay visible at 200x
or so, and it cannot fight against a true apochromat like Stowaway" 92
f/4.9, Borg 76 ED, Brandon 80 f/5.6, William Optics Megrez 80 APO, Pentax 75
EDHF, TMB 80 mm f/7.5 and other Tak FS 78.

Most of the scopes listed are triplets and as such unfair to compare
with a doublet. But doublets like Borg and Tak 78 CAN be compared, and
I do not think they are better in any way (chromatic correction wise)
than ED80. In fact Borg will be inferior as it is faster (f/6.6),
while Tak could be marginally better, depending on what they have used
to pair with fluorite.
You will need to provide longitudinal aberration measurements to
convince anyone otherwise.

Bratislav

The Tak's i've owned seemed to correct for colour fringing well
enough, but compared to some triplets (AP for eg) they display
an overall colour cast that looks yellowish.
Because they use fluorite, do the Taks correct to 1/16,000?
I think I remember that from one source.