Artificial stars - attempts and results.

I have recently been playing with artificial stars as collimation sources for my telescopes. The rationale for creating and using artificial stars for telescope collimation looks like this

• You always have good seeing over the short distances involved

• You can control the target brightness

• You're no longer fiddling with small set screws in the dark

• There is a greatly reduced or non-existent time for telescope cooling.

With all these benefits what possible drawbacks could there be ? Well - you need to guarantee that the target is bright enough, sufficiently far enough away to be not resolved by your telescope and illuminated by a source of suitable size again not to be resolved ( when using a reflective artificial star)

The most common source used for illuminating the target is the sun, where the target is a spherical surface, say a ball bearing or even christmas tree decoration. As long as these targets are smaller than a given size at a given distance ( mathematically described by Suiter in 'Star-testing Astronomical Telescopes' then they can be used as irresolveable sources just like stars.

A rule of thumb often quoted for minimum target distance is 20x focal length of the telescope to be tested. I tended to find that using the sun as a source in Good Old Blighty is not reliable enough and my garden orients in the wrong direction for good illumination of the ball - I get glancing illunination only so I wanted something a bitmore controllable.

So the next step was to take a diode laser as found in a pen-pointer or similar and try that. I rigged up a mounting in wood, where the target was again a ball-bearing and the laser was 12" away in the direction of the observer so get a good, almost direct-incidence reflection. I also added a transistor heatsink to the laser to ease the heatload on what is really a cheap diode not meant for always-on use. This solution was tested in the dark of the evening across 20m of garden and found to be OK for coarse alignment but no use for fine collimation. This was because the artificial stars linear size on the ball bearing was too big and oriented as a horizontal or vertical bar. Indeed, close inspection of the lasers spot output showed a 2mm or so bar with 3 bright spots along its length. The resulted diffraction pattern was too confusing to be of use and the out of focus pattern only of little use.

With these results I moved designs and went to a fibre-optic source.

The first stage was to use a piece of polyethylene strand from my childrens' toy wand, such as also found on 'fibre-optic christmas trees'. These are quite coarse, large diameter (0.5 to 1.0mm ) strands but all I had. I attempted to feed the laser into this and didn't get a good result. So I changed the design to use a mini-maglight torch as the light source and made an adapter with a fine drill and a few shiny washers and some hot-glue to fix it all together.

This was much better - the light output was better, the output size was smaller and I could position the fibre anywhere I liked to hide the glow from the torch from being visile in the telescope. The result in the eyepiece was much better too. Enough to expose the limitations of my testing environment. for the sake of comfort and environmental control I was using the telescope indoors through double glazing. Now in itself this is not too big a deal, plate glass is quite flat enough for gross testing but clearly not good enough for detailed testing . however , opening the doors means there is a large degree of cold air causing thermal optical effects so I'm back to testing at thermal equilibrium outdoors.

In the meantime, our good friends at ukastroimaging provided me with a real optical fibre pigtail for testing so more in that shortly.