spherochromatism

Roland mentioned in "equipment" that my C14 has spherochromatic error in the violet range at the edges and center of the mirror. I have three follow-up questions.

1. The "sphero" sounds like the violet is due to the shape of the optics, but I'm curious to know whether, at least on the drawing board where cost is no object, the use of a fancy FL glass in the corrector would diminish or control the violet error.

2. Is the violet error in my FS128 also spherochromatic? Or is that a different cause?

3. I'm having a hard time imagining how "adding violet back in" would make planet images whiter. To my untutored way of thinking, adding a tad of violet to a very slightly creamy colored palette would make for a rosy hue. So clearly the physics at work here are not the same as mixing colors on a palette, but I'm hoping to get a better grasp....

thanks
Greg N

Greg Nowell said:
3. I'm having a hard time imagining how "adding violet back in" would make planet images whiter. To my untutored way of thinking, adding a tad of violet to a very slightly creamy colored palette would make for a rosy hue. So clearly the physics at work here are not the same as mixing colors on a palette, but I'm hoping to get a better grasp....

Yes, mixing light is different from mixing paints. The example most folks should be familiar with is if you
mix red, green, and blue light beams together you get white. Obviously, if you did that with paints you'd
get some sort of brown. Another example with light is mixing red and green light to get yellow. Google
"mixing light beams" to find examples. There's interactive java applets which let you move the "spotlights"
around.

And, if you've never seen some of the 2-color color perception experiments which Dr. Land did in the 50's,
then those are interesting to read about as well. They have less to do with the physics of light and more to
do with the human physiology of color perception.

So, back to "adding violet back in", with light you want to add back in the missing blue to get to white,
not take it away.

Dan

2. Is the violet error in my FS128 also spherochromatic? Or is that a different cause?

Greg: My guess: Fluorite or no Fluorite, a 128mm F/8 doublet is not going to be able to completely control the chromatic aberrations even in the visual range.

Jon

The "sphero" sounds like the violet is due to the shape of the optics, but I'm curious to know whether, at least on the drawing board where cost is no object, the use of a fancy FL glass in the corrector would diminish or control the violet error.

Not a single glass; corrector can be achromatized if made of two different glasses. But the wavelength specific error for a single glass corrector depends on
how accurately it's been made. Deviations in corrector depth will result in the shift to some other wavelength being optimized (i.e. w/minimum spherical aberration). Since in the denominator for needed curve profile there is (n-1), "n" being the glass refractive, a deeper corrector will shift optimum correction toward blue/violet, with the correction worsening in green and red, while a shallower one will shift best correction toward red.

This, of course, assumes that mirror contribution is exactly what it is supposed to be (i.e., that they are perfect spheres of specified radii). The final specific wavelength with the minimum spherical aberration in either case (accurate vs. deeper or shallower corrector) will depend on the amount and sign of mirror aberration contribution. It can be near the green e-line, but it could as well be in blue, violet or red. Your SCT could have, for instance, something like 1/20 wave PV in violet, 1/4 PV in green and 1/3 wave PV in red - or the other way around.

Vla