Astrophotography Chapter 7

You may notice that we have not recommended our standard short focus refractors to this point. These achromat designs can be used for PFA but will not provide the same high quality results as our premium ED refractors or our Newtonian reflectors. On the other hand, even our achromat refractor designs work well for daytime photography, making them a great low price solution for telephoto nature photography.
The difference is due to the design of a standard achromat refractor. Such designs, regardless of who makes them, are incapable of bringing all colours of the visible spectrum to a common point of focus. For visual use, this residual or secondary color that achromats produce can be quite effectively suppressed by modern filters, like the Baader Semi-Apo filter.
But for photography, the filters are less effective. The problem is that while they can sufficiently suppress secondary colour where our eye is concerned, they do not suppress it enough where a highly sensitive film emulsion or digital sensor is concerned. Consequently, images taken with achromat refractors will suffer from chromatic aberration, seen as large purplish halos around bright stars. There is simply no avoiding this with the achromat design.

Young Male Elephant by William Ronald, Canada. Taken with SK 804AZ3.

There is one exception to this rule, however. More advanced astrophotographers can get excellent performance from achromat refractors by using an advanced technique called tri-colour imaging. In tri-colour imaging, sensitive astronomical cameras are used with highly specialized filters that pass only a narrow region of the visible spectrum. Since only one colour is being passed by a given filter, it is possible to achieve a very sharp focus, even with an achromat refractor. So, three filters are used: One that passes red light, one that passes green light, and one that passes blue light. The telescope is refocused each time a filter is changed, and when finished, the three images are combined to produce the final colour image.

So the best choice for astrophotography is either an ED refractor, which is designed to bring the visible spectrum to a sharper focus, or a Newtonian reflector, which, due to its design, is immune to chromatic aberration.

Rosette Nebula by Claude Baril, Canada. Taken with SK 1025AZ3.