why is this thing not showing chromatic aberration

I recently purchased a 60mm "Circle T" 700mm (f11.7) refractor off Craig's list. I have been giving it a workout and am somewhat startled by it.

The main thing I'm pondering is why it is so hard to see chromatic aberration in this thing. I had thought that you needed to be at f/15 to f/18 to control CA in an achromat. I have *seen* chromatic aberration in other achromats including an f/7 80mm Antares that I used to own. I have seen purple hazes around bright objects and odd colors round the moon.

Well right now I only have the comes-with 4mm and 23mm so I am limited to 30x and 175x. But I have done all the standard tests on this thing. I have looked at branches against bright blue sky at 175x and I have looked at the moon at 175x.

The bottom line is that it is *very hard* to make this scope show chromatic aberration. One must go considerably out of focus: many more cranks than, for example, my F/6.5 102mm ED Vixen.

Bottom line is that at 73x per inch this scope is much closer to the color control of the FS128 and the Vixen ED then it is to any of the more recent achromats I've used.

Obviously the focal length has a lot to do with it, but I'm wondering whether the small objective is also a factor. I guess I'm picturing in my head that a larger objective might put more rays in front of and behind the focus plane because the larger objective would have more of a curve.

Anyhow I'd appreciate some edification.....

thanks
Greg N

Greg:

Chromatic aberration is a function of both the focal ratio and the aperture. The ratio of the Focal Ratio to the Aperture measured in inches is sometimes called the Chromatic Ratio (CR). There are two rules of thumb regarding the false color in an achromat, a CR of 3x will have acceptable color correction, it can be seen but it doesn't get in the way too badly, that's the 3x rule.

The 5x rule is that an achromat with a Chromatic ratio of 5x will show minimal false color. Your 60mm F/11.7 has a chromatic ratio of F/4.93, for all practical purposes it meets the 5X criteria. In my experience a scope with a CR of 5 or more requires high powers on a bright, bright target to see the fringing.

If you want to see the color fringing, take a look at Venus at relatively high magnifications, it should be there. If it is not, it is possible that the scope is operating at a reduced aperture, somehow masked down.

There are a variety of 60mm scopes that meet the 5x criteria, the 60mm/700mm, the 60mm/800mm and the 60mm/900mm as well as the 76mm /1200mm. But it gets harder and harder to meet, a 4 inch has to be F/20, a 5 inch F/25.

That's pretty much how it works.

Jon