Question on Achromats for the Optical Experts

I frequently hear various amateur astronomers referring to certain telescopes as being, "well corrected achromats." That always made me curious...

I know that the designer of an achromatic doublet can decide where the color error is going to be--split equally between red and blue or less on the red side at the cost of larger errors in the blue, etc., but I had always thought that for a given focal length and aperture virtually all achromats had fundamentally the same color error. Am I wrong? Do differences in quality affect the amount of chromatic error? Are there variations in commonly used glass that would matter? Without resorting to special dispersion glasses?

Just as an example, I heard someone claiming that one model of 6" f/8 achromatic refractor had substantially less CA than another model of the same size and focal ratio. Is this likely to be true? Or is it just that the color error may be in a different part of the spectrum?

Thanks - Jared

>>I heard someone claiming that one model of 6" f/8 achromatic refractor had substantially less CA than another model of the same size and focal ratio. Is this likely to be true?>>

No, not true. A manufacturer can shift the correction around, but the amount of aberration cannot be lowered this way. For instance, he can reduce the blue halo but this increases the red halo. For dep sky, this may work fine. Since eyes are less sensitive to red at night, it may look like the CA is better because you see less blue on the brighter stars. However, such a lens would show poor planetary images because their surfaces are bright enough to trigger your daytime vision. You would then see very strong red halo and red defocus on the planet's disc, which tends to wipe out subtle detail, especially on the red planet Mars.

In fact, one can make a very good Mars scope by shifting the correction toward the red and letting the blue go out. Since Mars has very little blue, then one will see it sharply defined in red, yellow and green. However, since Jupiter and Saturn are much more white and have lots of blue light, these planets would now have a huge blue halo and blue defocus, and would look like they are veiled in a light fog.

No free lunch, I'm afraid

Rolando

Jared Willson said:

I frequently hear various amateur astronomers referring to certain telescopes as being, "well corrected achromats." That always made me curious...
Are there variations in commonly used glass that would matter? Without resorting to special dispersion glasses?
Thanks - Jared

I always been curious about the use of different (non special dispersion) glasses for doublet achromats, and claimed reduction in secondary color. One manufacturer/source says "The BaK1 and F2 oiled design....which uses a barium crown glass (BaK1) as opposed to a borosilicate crown glass (BK7) reduces the effects of the secondary spectrum by nearly 20 percent, resulting in a noticeable reduction in secondary color...and at only slightly increased cost." If this is the case, why don't more makers opt for this design for such a drastic reduction in CA at minimal cost? Or is this maybe NOT the case?