Refractors (revised Dec 2006)

scope

Achromat vs. Apochromat

After a lot of studying the writings of some excellent opticians and telescope makers and some comparative observing and imaging, I have come to some conclusions about the strengths and weaknesses of the two available refractive systems for modern telescopes.

Achromats:

Even with selective filtering, the two designs are not equal when it comes to Lunar and planetary observing. The achromat loses all light that is unfocused when using a filter such as the Baader Fringe Killer which has relatively high transmission compared to other brands that go as low as 78% transmission. You are in effect losing the advantage of a larger aperture to illuminate the image, visually the image appears a yellow color due to the focus being closer to red light than blue. Some manufacturers of short focal ratio achromats have moved the focus closer to that of the red end of the spectrum in order to reduce blue and violet fringing effects, this contributes to a softer focus. Filtering blocks this light giving a sharper image with the appearance of better fine detail and contrast, but there is less light in the image. The longer focal ratio achromats, ideally 5 times the aperture in inches reduces the size of the image in unfocused light to smaller than the image in focused light, yielding a sharper more white colored image. Some people seem more sensitive than others to unfocused color. As can be seen, a 3 inch refractor can be f/15 without becoming a major annoyance but a 6 inch refractor at f/30 would require a large installation and mount would not normally be a portable instrument.

Today’s affordable achromats are all built with focal ratios much below this minimum standard and thus all suffer to some degree from chromatic aberration, it cannot be helped, it is the nature of the beast. These short focal ratio (f/10 to f/5) telescopes are great for deep sky imaging as long as the blue light is filtered out so as to prevent bloated star images. They are also great for the thousands of DSO’s that are within reach especially in apertures above 4 inches. They are, with appropriate filtering, great solar scopes as you are usually working with heavily filtered light and often using additional narrowband filters.

The fit and finish of these telescopes may be described as adequate but most owners will upgrade focusers, finders, paint finish, and even the objective lens. Many are inadequately mounted for anything but low power visual observing so they should be obtained as OTA’s only or with a separate but more adequate mounting. Anti-reflection coatings are usually single layered even if all 4 surfaces are coated, in some cases only the front element is coated, resulting in lower overall light transmission. If you are fortunate enough to own an older long focus achromatic refractor from the 60’s 70’s or early 80’s then you have an excellent planetary telescope especially in apertures of 3 and 4 inches, many of which are better in quality than the majority of imported achromats available now and will give much better image contrast and resolution.

Apochromats:

The instruments available today are truly marvels of optical engineering, although in many cases a lot more money than the equivalent aperture achromat. However the advantages gained are well worth the price for several reasons. The 3 major designs available all are designed to do one thing and that is to bring the red, blue and yellow (+ green) light together at the same focus, resulting in very white appearing images of both stars and naturally colored planetary images. Whether the design is a doublet of fluorite or composite low dispersion glass, a three lens objective or the 2 doublet petzval design they all function very well for most applications. As focal ratio is independent of color correction, the ratio can be designed according to the application….short fast scopes for imaging and ultra portability or longer ratio instruments that yield a larger image scale for lunar and planetary observing.

There is a great deal of variation in the pricing of these telescopes based on materials and finishing and special attention given to close machining tolerances, accurate and smoothly operating focusers, and accurate optical alignment. Most of these instrument’s optics are hand polished in the final stages of manufacture and the anti-reflective coatings are many layered over all optical surfaces, optical light transmission is in the 96% to 98% region as compared to reflectors that are often less than 90%. Many of the manufacturers have reputations of excellence in their products that come from years of use by many very experienced amateurs and astronomy authors, thus the reason for the higher dollar value, many of these instruments retain their value if well cared for over the years and will definitely give a lifetime of observing pleasure. Fit and finish is also very good with attention to precision fitted parts that are machined to very close tolerances and operate very smoothly, you rarely see the owners of these instruments retrofitting focusers, etc. There are also many fewer cast parts.

Conclusions:

Applications for long focus achromats: (f/12 - f/15 or more), lunar and planetary imaging and observing, solar with proper safe filtration, double and variable star observing.

Example: 3 or 4 inch f/15, older Meade’s, Tasco's, Mayflower, Sears, and Unitrons. Older Jaegers and D and G long fr. refractors.

Applications for short focus achromats (f/10 or less) deep sky object observing, comets, imaging the deep sky in ratios below f/7, low power lunar observing, solar with proper safe filtration.

Example: 120mm and 150mm Skywatcher or Celestron C6, AR5, AR6, Various Antares, Short tube 80, WA80, etc.

Applications for apochromats…everything, especially great for imaging in shorter fast focal ratios, and in longer planetary designs for high resolution planetary views that with good seeing, can be pushed to very high magnifications due to their very good contrast often exceeding Dawes limit by a considerable margin.

Example: TeleVue NP101, TV102, NP127, Takahashi F102, F106, 'Q' series and larger, AstroPhysics Starfires, 130, 155, 160, 180, TMB, various William Optics and Vixen Telescopes.