Why achromats ? Or long focus refractors ?
Simple question, not so easy to answer .....
As already mentioned in the "About me" section, for moon and planetary observing I love and prefer the views through a long focus achromatic refractor telescope to the views through a shorter APO triplet refractor telescope. This is mainly because from personal experience in general the images seem to be more stable in the achromat i.e. the frequency of observing sessions with reasonable to good seeing conditions tends to be significantly higher.
What I also observed is that when images were distorted by turbulences in the air the sharpness and details remained better visible than in the APO where the image became completely soft.
Some of the possible reasons that can make the views in a long focus achromatic refractor so good, in so far as I understand, are :
- in a refractor the light has to pass only once through the telescope tube so that the view is less affected by air turbulences inside the tube. This is offcourse the case for ANY refractor.
- the lens consists of 2 lenses made of glasses with about the same expansion coefficients, much lower than those of the special higher dispersion glasses used in ED-doublets or triplets. Those higher dispersion glasses also have a higher density resulting in much heavier lenses. This means that the cooldown time of an achromat is much faster, certainly compared to triplets, and that the deformations during that cooldown are less important thus resulting in less deformed lenses and ultimately better images. In climates with rapidly changing temperatures at night this is in my opinion a very important advantage.
- due to the long length the lens sits higher in the air and farther away from the observer so that it is less affected by local, nearby air turbulences
- the long focal ratio makes the images less affected by certain types of air turbulences. Some say this is because of the higher focusing depth, so that the image remains sharper during moments of air turbulences moving along the telescope axis (as opposed to lateral movements) others propose incredibly complicated optical reasonings. I don't know what the reason is but to me it seems a real statement.
What is not so good about achromats is their chromatic aberration or their inability to bring all of the colors of the spectrum to focus at the same distance from the lens. So you tend to see a blue-violet halo around bright objects and supposedly some smearing and loss of contrast on small details.
I have to admit I don't really like that CA but I accept it because it is compensated by the aforementioned advantages. And besides: the CA in long focal ratio telescopes of F/12 - F/15 is sufficiently small to learn to ignore it. Also, the use of binoviewers (which is anyway a must to improve the view on small, bright objects) significantly reduces that CA, to a point where it becomes no longer annoying (this is, off course, a personal thing).
Now I know that not everyone agrees with this and that some are not willing to tolerate any CA, only a perfect APO will do.
They are right, if that APO is in complete temperature equilibrium and seeing conditions are excellent, the views are the best you can get for a given aperture. I have personally experienced some of those observing nights ... in fact 3 nights in many many years.
I feel that in a climate such as in North-Western Europe those nights are very rare and anything that can give me more instances of stable images gets my preference, even if that means less than perfect views with some CA.
It is for everyone to decide what he or she prefers, but at least decide on a good basis and try out a good long achromat once in a while,... you may be surprised.
Oh, and don't forget : an achromat is less expensive than an ED doublet or triplet, certainly in the bigger apertures. A fact that is not to be ignored....
Now, a long focal length ED doublet would make a nice compromise between an APO-triplet and an achromat : better color correction than the achromat combined with a faster cooldown than the triplet and the long focal ratio for the image stability. This is the reason that I will produce such a long focus ED doublet refractor : the FrT120dED !