Sky at Night Magazine

[greywolf1958]

I have recently become aware of these reflecting telescopes - for those unaware, they are similar to Newtonian reflectors but instead of the parabolic main mirror, have a spherical one instead. To overcome aberrations (off-centre images will not focus to a common point so will appear blurred), there is a special lens fitted in front of the secondary mirror.

My question is one regarding comparative quality really.... 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?
Any experiences gratefully appreciated.

Thanks.

[brianb]

[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.

[chrisc]

I have a Meade DS-2114S-LNT which has this optical arrangement.

It is rubbish.

Collimating is almost impossible, the image is poor.

From a non-optical point of view, the LNT is useless, the tracking is poor (mostly non-existant).

It was a gift, I would not recommend the optics to anyone.

I use binoculars to observe, the scope sits gathering dust.

One day I will replace it, probably with a Skywatcher 150PL.

[greywolf1958]

Thanks brianb and meademan - your comments make sense and I guess any savings made by not having to figure the primary, would be lost (or indeed exceeded) in the extra work/cost required on the figuring corrector lens plus the coatings to overcome the chroma issues. More work than necessary, possibly.
That said, I'd be interested to hear anyone else's experiences - it seems odd that a manufacturer would throw money at a design variant these days if it was really so bad...?

Cheers.

[philipc]

I'd have thought that the amount of correction involved would be [i]far [/i]too small to introduce any chromatic problems. Also, like most designs, it's probably possible for there to be both good and bad implementations of such a design.

[brianb]

[quote]it seems odd that a manufacturer would throw money at a design variant these days if it was really so bad...? [/quote]
It's not [i]necessarily[/i] so bad. The incentive is to have a scope which is very compact by the standard of Newtonians, which can be a big selling point.