Artificial Star from a led light torch

[kbrown]

Bought this weather proof led light torch from B&Q for £12 and thought I'd try to turn it into an artificial star. I just replaced the front "window" with a 3D printed ring and the reflector with a black cone with a small hole in the front (also printed). 

The cone had another hollow cone inside which converged right at the front. I then sanded it just enough to get the hole visible. Don't know if it's small enough though. Remains to be tested. 

[kbrown]

Had a go at it indoors with my diy 70mm refractor. The light was only about 4-5 meters away so I don't know if this tells me anything useful other than I've got dust in my optics ?. I had to add an extension to be able to go both sides of focus. Took these images through a 10mm EP with my phone. Guess the chromatic aberration seen here is typical for an achromatic objective...

1) Inside focus

 

2) At focus

 

3) Outside focus

Edited September 30, 2018 by kbrown

[Chriske]

kbrown,

Pinholes for testing optics should be as small as possible, but most of all it needs to be absolutely round. Another important point is using a thin object to allow light to pass through.
I always use a (very)thin sheet of brass and make a hole in it with a needle. Then I carefully sand it to remove all burrs that are made by the needle. Next I gently push the burrs that are forced into the hole(during sanding) outside and sand again. I repeat all this until the hole is completely clean and absolutely round.
During this operation I take care not to push the needle deeper every time the hole is 'cleaned up' by the needle.
Do not make that little hole to small of course.

Checking is done with a magnifying glass.
Lots of info is found on the net. This is one of the many. It does not tell how to make that hole, but gives lots of info about that hole and what to expect from it.
Info 'how to make' I got from Ingalls's books a very long time ago...

A printed pinhole is not a good idea. Because that hole is not 100% round and it is made out relative thick material it will result in extra and unnecessary diffraction.
 

[kbrown]

Thanks Chris. I did start wondering about this after my last post. I then had a look at the hole I made under a USB microscope and it actually wasn't even all the way through. The light was just shining through a very thin layer of plastic so this is another downside of just using plastic -> it might not be opaque...

Anyway carrying on with my experiments I pushed a needle through the plastic just enough to feel the tip on the other side. I then took a piece of 1mm fiber optic and pulled it slowly under heat to thin it out as much as possible. Took a few attempts but I managed to get it hair thin in one end. I then pushed the fiber optic through the hole and cut and sanded it flush to the front of the cone. This resulted in a very round light dot. It's just a bit too big now after all my attempts.

I'll probably try the same again but with a thin sheet of aluminium or brass like you suggested in front of my 3D printed cone.

Edited October 1, 2018 by kbrown

[John78]

So the technique we got taught when we were boys at school involved a coke can, biro, needle and wet and dry paper.

Cut a section of coke can out, push it flat

Push the biro into the can to form a small depression

Went and dry the hill down until its thin

Push through the needle as shallow as you can, if you cant get the needle through - wet and dry some more.

[kbrown]

Old topic I know but I had another go at and another approach to this after reading some posts using a weak laser diode without a lens under its lasing threshold instead of a regular light source and a pinhole. I got a bunch of cheap laser pen lasers with lenses from fleabay. Removing the lens was super easy. Just pull it out carefully. Turned out the laser modules had a 330 ohm series resistor on them. Bit of calculating and experimentation I decided to add a 2.2k resistor in series with the module. The torch has 3xAAA batteries so this results in just over 1mA current flowing through the diode. Well under the lasing threshold. I could go higher if needed but at the moment I don't know how bright this needs to be. Seems bright enough already but I haven't done any tests with it yet.

There is a bit of a glow around the diode itself due to reflected and scattered light in and on the PCB where the diode is mounted. Might try to do something about it but it will be difficult without breaking the hair thin wires going into the laser diode.

 

 

 

Edited July 10, 2020 by kbrown

[Kevin Kretsch]

Can I suggest slightly different approaches?

1) Tin foil very gently pierced with a small sewing needle. You can literally make them in seconds, with practice you can get very good at making tiny holes. Fold the tin foil over the front of the LED torch and hold in place with tape or rubber bands. Yes, it's a bit DIY and fragile not a lovely robust thing you can pull out of a case and go (although with a little ingenuity you could make it so).

2) A ball bearing. The reflection of a light source in the surface of a convex mirror always appears reduced in size.  A 1cm diameter ball bearing produces a tiny reflection image. Makes a great artificial star when illuminated by a small flash light a short distance away.  Again, with a little creativity you could make a fixed setup that could easily be mounted to a photo tripod.

[kbrown]

I decided to continue my experiments. The laser led without lens is a very small light source but it isn't round. I decided to go with a fibre optic cable route. Now I have a very round 50 micron daylight white (about 6000K) light source. Remains to be tested but I can already tell it's the best version so far. It looks bigger in the photo than it actually is as I wasn't able to focus  on the light properly.