Color Correction in Refractors
 
by Roland Christen




Color correction is a very complex thing, and there are many ways a
designer can skew the design of a refractor to make color more or
less perceptible.

Of course, there will always be a bit of color error due to
sphero-chromatism, etc. in apo refractors. The very fact that the
colors in an Apo come so very close to focus (and are not spread out
into a thin imperceptible haze), allows you to see them as slight
fringes on the out of focus patterns. It is ironic then, that when a
lens has very poor color correction, you may not see or notice a lot
more of this when defocusing the image. For instance, if the blue end
of the spectrum goes out of focus ever so slowly with decreasing
wavelength, there is a lot of energy in the slightly out of focus
waves. If, on the other hand, the blue end goes out of focus very
rapidly, the decreasing wavelengths are so far out of focus, that the
energy is spread out very far away from the disc, and contributes
very little to the fringing effect. Thus, an inexperienced tester
could easily conclude that a lens with very poor color correction in
the blue does not seem to have very much color error. The color error
is indeed there, but being so far spread out, it seems to disappear
into "thin air".

In lenses, the key is the color of the in-focus disc. The whiter it
appears at focus, the better is the correction. In a lens with lesser
color correction, the in-focus disc will appear more and more
yellowish (at reasonably high powers of course). The resolution is
not necessarily any different, but the main effect is that with
better color correction, there is more light concentrated in the
image, and fainter stars can be seen. Globulars, for instance, should
look more crisp and the individual stars in the globular should have
more of that sugar sparkle look.

One can indeed make a fast achromat that would appear to have "very
little color", simply by bringing the red end of the spectrum closer
to the yellow and green waves. This sacrifices the blue end. In other
words, if you cut the red error in half in a normal achromat, the
blue error will double. This is called Cde correction (red yel green)
as opposed to the normal CeF (red green blue) correction.  The old
time achromat designers tried to include as much of the visible waves
as possible with the CeF correction, and relying on long focal
lengths to increase the depth of focus and reduce the perceived color
error. If a 6" F8 were made that way, it would look colorful, indeed.
By skewing the color curve toward the red, and sacrificing the blue
end, the perceived fringe does not look any bigger (far out of focus
blue and violet waves have very little energy per unit area). Since
the red end is close to focus, you will not see the typical "purple
fringe", which is not really violet light, but the combination of red
and blue.

At low power, these lenses do have an almost Apo look to them,
especially on the Moon. With an object that bright, two things
happen. Your eye's color perception shifts over to the red end of the
spectrum, and because of the brightness entering your eye, your pupil
contracts. In effect, your eye stops down the aperture. So, instead
of seeing the Moon with a 6"f8, you may be stopping it down to 3 or 4
inches due to pupil contraction. If you move your eye around the
eyepiece, you may catch glimpses of the light originating from the
outer part of the lens' aperture, and will see various amounts of
violet and blue flaring in and out. On deep sky, the eye has no color
response, and everything looks white, no matter how well or poor the
color correction is. I would expect a 6" F8 achromat with Cde
correction to show bright globular star clusters buried in a thin
nebulous haze (the out of focus blue and violet light). The
brightness of the haze would be less, the worse the color is, and
would depend, of course, on the magnitude of the cluster. If this
background haze is below the detection limit of the eye, you might
not see any. It would be interesting to compare this nebular
background in CCD images taken with this kind of lens vs a mirror
scope, or high end Apo.

==============================

To clarify the situation on color correction a bit more for those
who'se eyelids have not already closed and are nodding away from all
this technical mumbo-jumbo, I will attempt to explain where achros,
EDs, fluorites etc fit into the scheme of things by comparing the
ability of each design to produce a reasonably focused image spot
diameter over its wavelength range. Reasonably focused being
somewhere around 25 microns.

Fast 6"F8 Cde achromat:  550 - 650 nm 
Long 6"F15 CeF achomat: 480 - 650 nm 
Fast 6"F9 ED doublet: 450 - 650 nm 
Fast 6" fluorite doublet: 420 - 1000 nm 
Fast 6" FPL52/53 triplet: 380 - 1000 nm 
Fast 6" fluorite triplet: 360 - 1000nm

It would be interesting then to divide the cost of each lens by its
useful wavelength range. For instance, a 6"F8 Cde achromat selling
for around $800 today would come in at $10 per nanometer. (our 6"
EDFS at $4900 comes in at $7.90 per nanometer). Interestingly, an 8"
SCT selling for around $900 comes in at $3.81 per nanometer. No fair
asking how a Newtonian would fare!

Seriously, why would you need correction well into the blue-violet
past 480nm? With black and white emulsions, this was necessary
because they have considerable sensitivity down to 380nm. Today's new
blue sensitive CCD cameras also need good correction in the violet.
Also, CCD cameras pick up lots of IR light below 650nm, so correction
to 1000nm is a distinct advantage. For pure visual use, it would be
quite sufficient if the useable range extended only from 440 to 650
nm. So, check the above table for your particular needs and make your
choice.


Roland Christen, ASTRO-PHYSICS

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