APO v. Achro

Since many of the active writers are Refractor users I know I can get good feedback here.

The definitions I have of an Achromoatic lens include statements about how the "lens does not exhibit chromatic aberration" and "A lens system that refracts light without spectral color separation". Yet when I read the threads on this list or reviews of refractors on other sites I see statements like "very little extra color" or 'with x EP and this telescope there was discoloration at the edges', and most of you recommend APO's over achros if the writer can afford it etc. Do achros really show that much color? If so why is the dictionary definition telling me that its a system designed to eliminate color?

I've been contemplating an AT1010 as a finder for a big dob, as quick look scope, and potentially as an astrophoto lens on a GEM. But some of the "go APO" cheerleading has me a little nervous about it. It's just that APO's cost BUCKS, that I'd rather throw into the primary mirror for my Dob.

Appreciate your insite

Color can become quite apparent in most achromats at high magnifications and on bright objects and is more a problem in achromats with short focal lengths. An achromat with a very long focal length won't show color, or at least very little.

I doubt that the Stellarvue will show objectionable amounts of color even when used for planetary viewing.

Why not go look through both types yourself, Leif? You may find that the achromat won't bother you at all or you may find you don't want an achromat. Why not let your eyes guide you (and spend some time - not just a quick look).

Ron B[ee]

Technically (per Zeiss, but I'm sure there's slightly other definitions out there), an achrochromat corrects chromatic aberration such that two colors concide in the focal plane with the effect that the other colors aren't that far off any more. How far depends on the scope; a good achromat can show very little color already in the main part of the spectrum, but at the extremes it's very hard to get close - thus the violet hues in achromats.
The apochromat corrects the chromatic deviations from the ideal image plane (the secondary spectrum) to the extent that "overall image quality is no longer limited by the secondary spectrum," i.e., the lens is nearly ideally color corrected (whatever that means). *In practice* that usually means that the lens is corrected for three colors, although that doesn't necessarily have to be true - an excellent achromat could, by that technical definition, be called an apochromat, and could possibly require only two lenses (I guess that's what they sometimes call semi-apo?). Then again, some apos are four-lensers.
Using excellent glass (like ED) or multiple lenses doesn't necessarily qualify a lens as apo, and possibly not even as achromat if badly enough designed .. And, technically no refractor can ever be completely free of chromatic aberrations (unless it had an infinite number of lenses), although obviously the definition of an aop means that they are negligible as they should be ..

That all said, there's hardly a way to spend more $$ per aperture than on an apochromatic refractor - the reason being that them chromatic aberrations are just a bummer to get a handle on especially once you get past a few inches aperture and to make an apo, you need to get (maybe) four, (usually) six, or (sometimes) eight generally full-aperture diameter glass surfaces near-perfectly right.

Anyways, the APO crowd I'm sure will agree (even with me ) that the intended primary application of an apo is high magnification viewing of brighter solar system objects, which is where you really can reall tell the difference between apo and achromat. Maybe double stars, too, but I'd argue it doesn't matter that much there. For what you want to do - quick grab and go, widefield viewing, prime focus photography - I would say a good achromat like the AT1010 will do practically as nice a job, for a lot less money. And if you want really good planetary views - well, you already have a big dob

I use a ST80 (much cheaper than a 1010 in all aspects) for just that purpose, and if I stay away from the Moon, Jupiter, Saturn, Venus, and a couple of bright stars it delivers wonderful images up to about 80x-100x magnification, at which point you run out of light on most objects anyways. I just use my 11"er for the high mag stuff. I think you'll love the AT1010, but if you want to make yourself feel good, look through a good achromat first at a star party.

Leif,
I owned an AT1010 at one time and there will be color (a fair amount on bright objects). For wide field use it is a pretty decent performer. As a finder and quick look scope it would do well but if you plan on spending a lot of time with it especially on brighter objects like planets and the moon you may want to consider something else. There are some good ED scopes that come in somewhere between your basic achro and true APOs that will show considerably less color than an achro at less cost than a full-fledged APO.
Of course, I think the best advice is for you to find a club or star party and take a look through an AT1010 as well as other achros and then through an APO and decide for yourself whether the difference is worth it. I did exactly that and after looking through an APO decided the AT had to go.

Thank you all for your contributions, ideas and suggestions.
I joined the Boise Astronomy Society this year and I'm still looking for opportunities to get some views through different refractors. I'll check out the club's televue pronto. Hopefully I'll run into an AT1010 at a local event. The Craters of the Moon star party is coming up and so is the Idaho Star Party. I don't think I'm going to make it to Table Mountain this year, but maybe next.
The last contribution to this thread, by Barry Simon realy helped as it showed an image with "aberation" in it. I also never realized how much of a difference the filters made.

Sounds like a fast achro will make a great finder on my bigger dob and perform well as a quick look scope. If I was choosing a refractor as my main scope I'd be looking at the 4" to 6" f/9 and up designs but that is not the main purpose of this scope.