my understanding of the Bird-Jones design is that it utilises a
spherical primary/modified secondary morror and a corrector lens in the
focuser tube to effectively squash a 1000mm focal lengthinto a short 500mm tube. So you get what looks like for example a 500/F4.4 scope
but is actually a 1000mm/F8.8. Some people seem to get on OK with them
-give or take some interesting colimation issues - but to my mind they
seem to add a layer of added complexity with no worthwhile pay-off. If
you want a slowish 114/130 reflector you can get decent one without the
bird-jones compromise. OK - not quite so compact - but that's the choice.
from astronomyforum
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.
from skyatnightmagazine
examples:
1)Celestron Powerseeker 127
2) 8 inch Bird-Jones Newtonian f=1400mm, from Aspheric Optics(now discontinued)
from astronomyforum
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.
from skyatnightmagazine
examples:
1)Celestron Powerseeker 127
2) 8 inch Bird-Jones Newtonian f=1400mm, from Aspheric Optics(now discontinued)
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