The optical performance of a well-made achromatic refractor may offer the best performance of any telescope type on many grounds.
1. The lack of central obstruction yields the highest contrast possible.
2. The closed tube of a refractor minimises temperature changes and resulting tube currents.
3. The relatively thin optical elements cool down faster than thick correctors or apochromatic triplets.
4. Once the objective has been squared-on to the optical axis, either in a fixed cell or preferably a high quality push-pull cell, collimation is maintained and rarely needs attention, even if the telescope is roughly handled during transport, etc.
5. The traditional achromat uses well established, stable glasses that are less susceptible to degradation over time than may be some of the more exotic glasses used in certain types of apochromat. This is evidenced by the large numbers of vintage and even antique refractors still in use, where the objective lenses have rarely been dissembled.
6. Longer focus achromats - over F10 - also have greater depth of focus, and are therefore less affected by wavefront changes caused by atmospheric turbulence.
Of course, the main perceived drawback of the achromatic refractor is colour correction, but this is very much overstated. The rash of 6 inch short focus, bulk manufactured Chinese telescopes ( F8 or shorter) do exhibit an alarming amount of false colour, particularly on bright objects such as Venus or the Moon, and have given achromats bad press. However, when focal length is F10 or above and with the benefit of the latest optical design, materials and coatings, the effect is minimised.
In addition, field correctors offer certain advantages, particularly at shorter focal ratios - in essence, the corrector may be considered as part of a multi-element apochromatic design. An example of this is the TAL 125 Apo which has sub-diameter correctors - we have not tested one of these and so cannot comment on performance.
Many of the short focus apochromats or so-called semi-apos exhibit blue fringes on bright objects. I have to say when viewing Venus through a premium apochromatic triplet, colour separation was just evident. This may be minimised in very high quality instruments, but at a significant cost, especially in large apertures of 6 inches and above.
Peak2Valley lenses are hand corrected for the visual C-F range, and are minimum 1/6th wave pv with smooth surface. The smoothness of a lens is important as it minimises light scattering, which causes reduction in contrast and definition.
For photographic or imaging purposes, it is highly desirable to have as short a focal ratio as possible, and this is where the apochromat comes in to it's own.
We have standardised on Istar apochromats, designed by their own Master Optician, which offer outstanding performance at very competitive prices.
Depending on the optics selected, lenses are interferometer or autocollimation tested to ensure quality. All Istar and D&G lenses are finished to an exceptional standard of finish and represent the highest quality available in the industry.
For 6 inch and larger objectives we supply a collimating cell, unless otherwise specified by the client. However, for smaller diameters a fixed cell may be appropriate.
Provided the tube and cell flange are accurately machined and the cell is mounted square to the optical axis, an adjustable cell is not needed for lenses that exhibit uniform edge thickness, ie having no wedge effect. For example, axial runout of 0.003" on a 6 inch lens amounts to 0.03 degrees of tilt, which is insignificant for an aplanatic lens.
The size and weight of the telescope is dependent on focal length. Clearly an 6 inch F12 will be shorter than an F15, with 72 inch and 90 inch tubes respectively. Despite this, and the thick walled tubing used, it is perfectly possible for an adult to carry the F15 telescope and attach it to a mounting.