Collimating a Bird-Jones (Jones-Bird) reflector telescope
I got a Bird-Jones telescope in for some maintenance the other day. It needed its mirror realigned. It's a bit of a pain to work on because it's got an additional lens in the focuser that disrupts the laser we use to guide the adjustment.
What's a Bird-Jones?
The Bird-Jones reflector telescope (also known as a Jones-Bird) is a variant of the Newtonian design.
These guys look pretty much like normal Newtonian telescopes, with an open aperture at the front, a mirror down the bottom, a flat secondary at the top and the focuser at the side front.
So what's the difference? The mirror is a subtly different (and cheaper) shape. A true Newtonian design has a parabolic primary mirror at the bottom of the tube. A Bird-Jones uses a spherical primary mirror.
Have a look at my two scribbled diagrams. The first one shows a parabolic mirror. Parallel light rays from a star are coming in from the right, and bounce off the mirror. No matter whether they hit the mirror at its edge or near the middle, the reflections all pass through the same point - the focus of the lens. Parabolic mirrors are cool.
The second diagram shows a spherical mirror. Light rays from the star come in as before, but the ray that hits the edge of the mirror crosses the centreline close to the mirror. The ray that hits the middle of the mirror crosses the centreline further away from the mirror. The mirror doesn't bring the light to a nice focus. This is called spherical aberration.
To correct the problem that the spherical primary causes, the Bird-Jones uses a lens installed at the focuser, a bit like a pair of glasses. As well as fixing the spherical aberration, this lens also doubles the focal length of the telescope. Because of this, the lens is often (incorrectly) called a Barlow.
Good for manufacturers
Bird-Jones telescopes have a number of benefits - most of them are for the makers of the telescope.
A spherical mirror is cheap. If you rub two flat bits of glass together with abrasive in between for long enough, they'll turn into a spherical pair without your having to do anything complicated. Put a layer of aluminium on the concave one and you're done!
And you find them everywhere
Bird-Jones telescopes are sold as a saxon 1141, a Sky-Watcher 114/1000 or 130/1000, Celestron AstroMaster 114 or PowerSeeker 127, as well as a number of other brands. I've even seen a 10" version.
However, across the world, these models vary, and I've heard that some may or may not be of the Bird-Jones design. It's all very confusing, but a reliable rule is that if the focal length of the telescope is more than the physical length of the tube, it's probably a Bird-Jones.
Good for users, too
Typically, Bird-Jonses are small and inexpensive, and they come on lightweight mounts. They make great beginner scopes.
They give you a long focal length in a short, light-weight, and portable tube that's easy to put in the car and take camping. Because of the long focal length, you get a good cheap view of a planet or some other small object (as long as it's bright) - great!
So what's not to like?
But this brings me to the disadvantage of the Bird-Jones design - or at least, the Bird-Jones as they're currently produced. When their mirrors go out of alignment, they a total pain to readjust.
The lens in the focuser means that collimating with a laser is nearly impossible, as the lens disperses the laser light. Rather than a point, the laser ends up as a blob.
Typically, these telescopes do not last long, because when faced with the task of adjusting the mirror, a lot of people simply put their "toy scope" out with the rubbish.
Collimating a Bird-Jones
A while back, a client brought in a Bird-Jones telescope and asked us to collimate it. Up for a challenge, or possibly foolishly, I agreed.
I took a photo down the focuser to see how bad it was.
While it's obvious that the scope is out of collimation, it wasn't dramatically bad. If a kid has been playing with the adjustment screws at the back, sometimes you can't see your own eye in the mirrors.
Finding the centre of the mirror
I looked at the primary mirror and realised I had to mark the centre spot - so I had to remove the mirror cell. Honestly, this task was fighting me every inch of the way. Spherical mirrors like this normally don't have a centre spot marked.
I prepared a paper mask with the right diameter to find and mark the centre of the mirror. I used a compass to draw the mask, but if you put the mirror onto the paper and trace or cut around it, you can find the centre by carefully folding it twice.
I carefully put the paper onto the mirror, and, with a marker pen, put a dot on the mirror through a little hole in the centre of the paper.
The focuser and correcting lens
Next, I had to disassemble the focuser.
I loosened (but didn't remove) the two screws at the bottom of the focuser. This allowed the pinion to disengage from the rack on the focuser draw-tube and so I was able to withdraw the inner part of the focuser.
There's a little triangular tool you get with the scope in one of the photos above. This allows you to release the lens by hooking it into the notches in the retaining ring. I removed the two-element corrector lens, which is at the eyepiece end of the draw-tube.
I carefully marked the corrector lens so I could reassemble it in the correct orientation.
Then I had to put the focuser draw-tube back into the focuser without the correcting lens so I could install the laser collimator.
And now, a normal collimation
Without the irritating corrector lens in the focuser, I was able to collimate the mirrors in the normal way. After that, I had to pull the focuser apart, reinstall the corrector lens and then reassemble the focuser ... again.
An improvement, at least
The final photo shows the collimation is better, although I'm still not sure it's perfect. Let's just say the collimation has been improved. It was quite a struggle.
So what have we learned?
We've learned that a Bird-Jones telescope is a small, portable and cheap scope for a beginner astronomer. It's cheap because of its spherical mirror.
Because of this, maintenance on the scope is difficult and pretty time-consuming. Shifting the cost of the telescope from the manufacturer to the user often results in the telescope being turfed instead of being fixed.
As economists tend to say, there's no such thing as a free lunch!