Image of the day

Captured by

M97 Owl nebula

My Account

New to Astromart?

Register an account...

Need Help?


Started by AstroMart, 04/07/2018 10:51PM

Poll Results:

0 Total Votes
Posted 10/11/2004 12:18PM #1
"Achromat" means without color, and, as I've read one quip, APOchromat means without color "but we really mean it this time". So does Semi-APO mean between Achro and APO (implied) or semi-color free which might put it short of an achromat? smile I think semi-apo exists but is a moving target that is mostly marketing hype. The real measurements of a lens's color correction is buried in the specs deep enough that only experts can decipher it. The name is irrelevant since the meaning is neither clearly defined nor rigidly enforced.

Each manufacturer has their own standard definition, some may change over time though, but agreement across the industry is lacking. Too bad resellers don't even the field when they sell competing products by certifying or grading the instruments performance.

I know, buyer beware... and I try, like most of you. But it seems that experience and a deep pocket is the only way to really understand what is right for an individual.


"I know engineers. They love to change things."

- Leonard McCoy (Star Trek: The Motion Picture)
Posted 10/11/2004 11:41PM #2
Televue didn't seem to mind using the term for the Ranger and Pronto. Companies like APM/TMB use it to refer to some scopes they sell from other makers. In fact, APM had a Zeiss Scope listed as a "semi-apo" in the auction section just recently. So there must be some validity to the term if the manufacturers use it also.
Posted 10/13/2004 01:16PM #3
Some vendors referred to the Pronto/Ranger as a "Semi-APO" but I believe TeleVue never did.

And with good reason because as the owner of a Pronto and following the issue of the "Semi-APO" it seems quite clear to me that the Pronto is really just an Achromat, a well made achromat, but an achromat none the less.

The fact that vendors call various scopes "semi-APO's" does not make them "semi-APO's." I think the Zeiss scopes are a particular case and of rather long focal length apparently.

But I think in general the term is used for scopes that are nothing more than achromats.

Posted 10/14/2004 12:32PM | Edited 10/14/2004 12:34PM #4
"Achromat" also means without color.

Within the family of triplet APOchromats there exists a range of color correction but I expect none are truly without color. In the same way, doublet achromats also have a range possible corrections but a triplet has more potential available. Specific correction depends on design and execution.

The semi-apo label still seems to be more marketing hype to me than substance but ED glass in a doublet does seem to improve on the traditional achromats correction. No clear distinctions, just opinions.


"I know engineers. They love to change things."

- Leonard McCoy (Star Trek: The Motion Picture)
Posted 10/14/2004 04:59PM #5
Check out

Three crossing are possible with a doublet and if you look around you will find that Roland and Valery D. have both indicated they believe the ED-80 to be an APO..

Posted 10/31/2004 09:20AM #6
Greetings most exalted ones...

Who cares? Who really cares what a particular mfg calls a particular scope? I know of very few people who would lay out some serious $$ without looking through a scope. Seeing is believing. If you don't believe me try looking down any Borg ED doublet 8)

BTW, the Borg "achromats" look better than any semi-apo's I have looked through lately...