Basically a prism, a refracting lens bends (refracts) color wavelengths in different angles thus missing a common focal plane.
The Result is Chromatic Aberration...
...appearing as red-blue-violet color fringes around the brightest stars.
1. Simple Achromat
...has two air or oil spaced coated lenses, an objective convex lens (crown glass) and a correcting concave lens (flint glass).
This arrangement reduces errors inherent to a single lens to some extend.
2. ED Doublet Apochromat
...has a two-element fully-coated objective lens made of extra-low dispersion (ED) glass. It clearly improves over an Achromat.
3. Triplet Apochromat
...has a three-element fully-coated objective lens often made of precious FPL-53 ('Super ED' or 'FD') glass which significantly improves chromatic aberration.
Abbe Number
Ernst Abbe's constant (Abbe number) is applied to classify the ability of glass to cancel chromatic aberration.
The higher the number (V) the better the optical performance.
Abbe's number = (nD-1) / (nF-nC)
whence,
nC refractive index for C-line (656nm)
nD refractive index for D-line (589nm)
nF refractive index for F-line (486nm)
consequently depending on the wavelength of light.
Refractive index is the ratio between the angle of light incidence and the angle of light refraction, a constant = sin(i) / sin(r).
The index of refraction in a vacuum is 1.000 (by definition), in air 1.000293, and the index for water is 1.333.
For pure Crown glass it is 1.50-1.54, for pure flint glass 1.60-1.62. The index is also temperature dependent.
In apochromatic refractors, FPL-51 glass offers V=81.54 while FPL-53 glass achieves V=94.93, which is almost identical to Fluorite (V=94.99).
In spite of the remarkable color correction improvements by modern Doublet and Triplet APOs, coma aberration in the edges is still present and require an external optical
element known as 'flattener', some of which reduce the native system focal length by 0.8 or so, and cost a little extra fortune.
4. Quadruplet Apochromat
...is basically a triplet APO with an integrated flattener lens, or an arrangement of two doublet lens groups.
This structure virtually eliminates coma and chromatic aberrations at less of the cost of a same-aperture triplet APO + optional flattener.
Starting from Ø50mm, quadruplets with up to 80mm aperture are available for under 1000 dollars, but rise in price exponentially with wider apertures.
An average objective lens diameter for a quadruplet is 70mm. This seems small for good resolution and contrast, yet, unlike visual observation through an eyepiece,
imaging does not require as much aperture as it collects light during total integration of several hours, even days.
Pros and Cons of Apochromats
Advantages
- Light-weight, portable, retractable hood
- Wide field of view
- Corrected colors
- High contrast
- Stray light resistant (with baffles)
- Almost pin-point stars
- Dual-speed, less focusing stress
- Fast temperature adaptation
- Less dew-prone
- No regular collimation required
Disadvantages
- Small aperture, less resolution for, say, double stars
- Flattener desirable for DSLR sensors (doublets and triplets)
Field of View (Image)
The field of view in astrophotography is a function of camera sensor size and system focal length:
[FOV = (2 * atan(sensor_width / (2 * focal length)) * 57.296)], variables in mm, FOV in degrees.
In combination with a digital camera featuring a given sensor size, the field of view in 250mm to 500mm focal length refractors is perfectly suited for wide spreading deepsky objects, such as the Orion, North America and Rosette nebulae, the Andromeda galaxy, as well as the Pleiades open star cluster.
The shorter the focal length the more forgiving to a mount's tracking errors and rough polar alignment. An anyway recommended optional 0.8x reducer/flattener for doublets and triplets can further widen the field of view.
There are also flatteners which do not change the focal length. Quadruplet, quintuplet and Petzval apochromats typically sport an integrated flattener.
Regarding the disadvantages, a lower resolution is often compensated for by using excellent low dispersion lens glass such as FPL-53 nearly
eliminating color fringes. For instance, a 70mm aperture triplet can outperform a Ø150mm plain achromat which washes out resolution and contrast with its excessive color fringes.
Please also take into consideration a Newtonian starting from 130mm aperture. Newtonian mirrors are free of color fringes. The four vanes holding the secondary mirror produce four diffraction spikes over bright stars but else are excellent
astrographs with high contrast and clarity. Since the mirror is curved, edge coma needs to be reduced using an optional coma corrector lens. Also, Newtonians are heavier than comparable refractor, therefore
demanding a sturdy mount with high payload capacity.
Conclusion
A simple low-cost achromat produces excessive color errors disqualifying this design for serious imaging. A fine optically coated ED doublet APO can cost as much as a triplet APO
especially when it sports quality lens glass, such as the praised FPL-53. However, for best possible imaging results, both, doublets and triplets still require an external flattener
ending up in costing more than a, say, 70mm 'self-contained' quadruplet or quintuplet which eliminate need for an optional, external flattener. Besides, most flatteners
are designed for specific telescope models or F-numbers only and won't be optimal for other scopes you might own, or for your next one.
A Good Choice
As of September 2019, the author's pick is the latest Ø71mm/450mm/F6.3 Quadruplet APO TS-71SDQ from TS-Optics, Germany, which is made in China by Sharpstar sporting a fully multi-coated FPL-53 air-spaced triplet objective
with an integrated single element (minimizes likelihood of optical misalignent) correcting lens. The focuser is 2.5" (63.5mm) in diameter with an Ø44mm image circle which entirely
illuminates a full frame (36 x 24mm) DSLR sensor. All it requires to attach a DSLR is a bundled extension tube which threads to the 2.5" focuser's M63. Less parts count entails less weight and the least wacky the optical train.
According to the manufacturer, the focuser accepts imaging gear of up to 4kg while holding focus well. Most modern dedicated CMOS cameras for deepsky or planetary work weigh less than a kilogram.
Update December 2023: The TS-71SDQ is discontinued and replaced by other 70mm refractors, such as from Sharpstar/Askar and other manufacturers.
