More on Color Correction
by Roland Christen
"Is it true that there is range of achromat glass (crown/flint) and
there is a range of ED glass and there is a range of flourite lenses
as well? By "range", I mean the degree that the glass can correct chromatic
aberration (leaving everything else such as spherical aberrations as
The answer is NO and YES.
NO, there is no range of achromat color correction. Achromats are made from
ordinary glasses that have no special dispersion characteristics. An achromat
is made with a crown positive element and a flint negative element. These
glasses are made with various oxides, which give them their strength and
durability. Normal dispersion glasses will correct the colors from C to F
to 1 part in 2000, or the focal length will vary by .05% from red to green to
blue, and approximately .25% for violet. The amount of color halo you see around a
star with any aperture will be inversely proportional to the focal ratio.
Thus an f15 scope will have half the color halo of an F7.5. A 20"F15 achromat
scope will have the same size color halo as a 4"f15 achromat, but because the 20"
has 1/5 the size Airy disc, the color relative to the Airy disc max resolution
will be 5 times worse for the 20" vs. the 4" scope. What happens if you add a
third element? Nothing, the color correction is still 1 part in 2000. You can
add 50 elements, and you won't gain anyhting.
YES, there is a difference between various ED and Fluorite scopes, but it is
not really the ED or Fluorite that governs the amount of correction, but the
mating element. Normally, even the worst ED design will have 4 times better
color correction than a normal achromat, but it could easily be 20 times
better simply by choosing a different mating element. The more expensive the ED,
the easier it is to mate it to a corresponding negative element to achieve
perfect color correction. You can make an ED lens with an ED positive element,
and a flint negative element just like an achromat, but the color correction will
not be very good, perhaps a semi-apo. By choosing a Short-Flint or Crown-Flint
you can get better correction, but not perfect cancellation of secondary color -
perhaps 4 to 6 times reduction depending on the combination. To get perfect
cancellation over a wide wavelength range you would mate the ED with a negative
element made with Crown glass. These are the most difficult to make, so commercial
Apo makers avoid these.
What gives the ED material the ability to correct color? It is the chemical
composition of the material which is based on fluorides instead of oxides.
Fluoride based glasses have a different dispersion characteristic caused by
absorption bands that are further away in the ultraviolet (this also allows
them to pass UV light more readily).
The drawback of fluoride based glass is that the bonds that fluoride forms
with other materials are weak (oxide compounds are very strong and super
hard). The fluoride material is hard to polish and stains more readily in some
One more comment, it is possible to make a so-called "ED" lens and have normal
achromat correction of 1 part in 2000. There is one glass that some call ED -
FK5, which will produce the same correction as any normal achromat combo. It's
also exceedingly cheap. Schott has given FK5 the designation FK (fluoro-crown),
but it has no particular color correcting properties in the normal visual
Very confusing, yes?
Roland Christen, ASTRO-PHYSICS
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