by brianb » Tue Jul 21, 2009 8:50 pm
[quote]On the basis that it's probably much easier (and more accurate) to create a spherical mirror in the first place, does the correcting lens in a B-J do a good enough job to provide a comparable image to a standard Newtonian? Is collimation made any more difficult with the lens arrangement? [/quote]
Who knows what the correcting lens is like optically, other than the designer & manufacturer, without testing or reverse engineering?
However the Bird-Jones design has some downsides:
1. The primary mirror is much shorter focus than it normally would be in a reflector of that focal length. This means the secondary had to be bigger, reducing light grasp marginally and contrast significantly. The shorter tube should however tend to make the mount more stable - unfortunately the manufacturers of the chep B-J scopes capitalise on this by substituting a lighter & less steady mount.
2. The corrector lens will inevitably absorb & scatter some light. It also introduces some chromatic aberration, complete freedom from which is the main point of having a Newtonian.
A scope with a spherical primary mirror is immune to misalignment of the optical axis, however the components still require to be centred and there are many more of them - those inside the corrector lens assembly can't be adjusted, either. The bit of collimation which seems to stump many users is getting the secondary centred & positioned correctly, the B-J design has no effect on this adjustment.
Personally I can't see the point, I'd rather have a Schmidt-Newtonian or Maksutov-Newtonian if I wanted a short focus spherical mirror reflector.