The best eyepiece types often included as standard equipment are the Kellner, MA (modified achromat
), and Plossl designs.
For decades observers working with achromats
have used color filters (usually yellows like Wratten 4, 8, 12, and 15 or greens like Wratten 11, 56, and 58) to block the secondary spectrum's defocused red, blue, and violet light.
(By the way, achromat
means "without color," while apochromat means "without color--but, we're serious this time." If someone invents a refractor with color correction superior to that of an apochromat, we're going to have to come up with a new name.)
It is a short-focus achromat
, which at first glance appears similar to other low-cost, short-tube refractors.
Far from overwhelming, this is generally quite unobtrusive and significantly better than in a number of other budget achromats
Fortunately, we are using two identical achromats
with a focal length of 100 mm each, which when combined provide an effective focal length of 50 mm.
This home-built telescope squeezes all the performance advantages of the classic achromat
into a wooden box half the length of a typical refractor.
The only lenses required are a pair of matching achromats
(lenses made of two components cemented together to form a single element) arranged so that their biconvex elements are facing each other and nearly in contact.
On the downside, refractors, especially conventional achromats
, create problems for CCD cameras that "see" wavelengths (colors) well outside the range brought to a common focus by the objective.
For the first four years optics from old photocopying machines served as objective lenses, but in 1990 Tyler found a source of inexpensive, high-quality air-spaced achromats
. Continuously refining their plans, Tyler and his team now build upward of 100 telescopes a year.
Since we wanted our spectrograph to be capable of operating at this extreme without wasting light or resolution, we selected high-quality f/2 achromats
of 80-millimeter focal length for the collimating and focusing lenses.
This was solved in the late 18th century by the invention of the achromat
, which uses a pair of lenses made of two different kinds of glass.