Optical design

Apochromatic refractors

Three or more elements bringing red, green and blue to a common focus. The sharpest small telescopes available, and the most expensive per millimetre of aperture on this site.

Best suited to:Visual observing Nebula imaging

How the design works

An apochromat uses three or more elements, at least one of extra-low-dispersion or fluorite glass, arranged so that three wavelengths come to a common focus rather than two. In practice this removes visible false colour entirely: a well-made triplet shows no violet fringe on Venus or the lunar limb even at high magnification. The design also allows faster focal ratios without the colour penalty an achromat would incur, which is why apochromats work both at the eyepiece and on a camera. Field curvature remains, so imaging still needs a flattener or reducer.

Strengths

No visible false colour at any magnification, which gives the cleanest high-power views of any small telescope design.
Unobstructed aperture preserves mid-frequency contrast, so planetary and lunar detail looks crisper than a larger obstructed telescope suggests.
Sealed, collimation-free and quick to cool, and usable equally well for visual observing and imaging.

Limitations

The most expensive design per millimetre of aperture. A four-inch apochromat costs more than an eight-inch Dobsonian by a wide margin.
Still needs a flattener for imaging, and the reducer options add spacing complexity.
Three heavy elements in a long tube means more weight and more leverage on the mount than the aperture suggests.

What you are actually buying

Correction, not light. A 100 mm apochromat gathers exactly as much light as a 100 mm achromat and about a quarter as much as a 200 mm reflector. What it delivers is a cleaner, higher-contrast version of the light it does gather, with no colour artefacts and no diffraction from a secondary mirror or its supports.

On the Moon, on the planets and on double stars that produces a visibly better image than the specification comparison suggests. On faint galaxies it does not, because there the limitation is aperture.

The contrast argument, fairly stated

An unobstructed aperture preserves mid-frequency contrast that a central obstruction removes. This is real and measurable, and it is why a 100 mm apochromat can look “better” on Jupiter than a 200 mm Schmidt-Cassegrain that is nonetheless resolving twice as much detail.

Both observations are true. Which one matters depends on the night: in poor seeing the smaller, cleaner image often wins; in good seeing the larger aperture pulls decisively ahead.

Imaging with an apochromat

Excellent star shapes, no colour halos, and a flattener that is usually available from the same manufacturer with a documented backfocus. The limitation is speed — most apochromats sit between f/6 and f/8, which is slow for extended nebulosity.

The common solution is a reducer bringing the system to around f/5, which also widens the field. That is the configuration most owners settle on, and it makes a 100 mm apochromat a genuinely capable medium-field astrograph as well as a fine visual telescope.

Maintenance and setup

Sealed optics with no collimation routine and no cleaning schedule beyond removing dust. Cooling is quicker than any reflector but slower than a small doublet, because three elements have more thermal mass. Dew on the objective is the practical recurring issue, and a dew shield with a heater handles it. A well-treated apochromat should need nothing at all across decades of use.

What you will need alongside it

A mount with real margin — these tubes are heavier and longer than their aperture implies
A quality star diagonal, since a poor one wastes the optics you paid for
A matched flattener or reducer for imaging