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 - unfo...