How to make an artificial star?

Hi gang,

I was tinkering with the idea of making an artificial star myself. I can get my hands on bright white LEDs, can most probably make a box for it too, but how big should the hole be?

Anyone here has experience doing this? If I use the smallest drill bit, say 1/32" to make that hole, is that too big or too small?

Thanks for any help here.
Vin

The best way I know to make a precision pinhole with basic materials is to "drill" them in brass shim stock with a needle. I've used this technique to make numerous pinhole apertures for pinhole cameras. If you do it correctly (use a pin vise to hold the needle and turn it by hand) you can get precision pinholes down to the .005" range that will still be much cleaner than one drilled with a bit. For reference, the pinhole aperture used in the Digitec Optical artficial star is about .004" in size. If you want the cleanest possible pinhole I would get a commercially made one. I'm not sure how much rough edges would effect the usefulness for an artificial star but I think it's safe to say the cleaner the pinhole, the better it will work.


Joplin

A reflective artifical star is much simpler to make than a tiny precision hole. Just shine a very bright (but very small) light at a chrome ball bearing or a silver christmas tree ornament. A dot-type laser pointer makes a good illuminator, but so does a bright halogen flashlight with a 1cm diameter off-axis aperture mask, and so does the sun.

1/32" is way too big. Suiter suggests that angular size of the pinhole is half that of the Airy disc. It also needs to be placed far enough for not to induce significant spherical aberration (since telescopes are optimized for objects at infinity). Parabolic mirror generates 1/4 wave of spherical aberration when object is placed at 28(D/F)^2 feet away, for D(aperture) in inches. If you don't want to have more than, say, 1/20 wave in your setup, you need to place the star at about five times greater distance.

Whatever distance (S) you choose, pinhole diameter should be about S/1500D (make sure that both distance and diameter are expressed in same units).

The above "distance" formula doesn't apply to refractors and Cassegrain-like systems. 1/4 wave distance varies with the system, but will be significantly greater. With these systems, distance to artificial star shouldn't be less than a couple hundred feet.

To produce a pinhole small enough, you can use a high-power microscope objective or eyepiece. For instance, if you place a 40x (4mm f.l.) microscope objective 50mm in front of 0.8mm (1/32") pinhole (so that objective's back faces the pinhole), it will form an image of the pinhole that is 50/4=12.5 times smaller, or 0.064mm in diameter.

Or, placing, say, 5mm eyepiece 50mm in front of the pinhole (facing it with its field lens) will produce a pinhole image in front of the eye lens that is about 50/5=10 times smaller, or 0.08mm in diameter.

Such artificial star should also be brighter than one produced by a real opening of that size.

If you use a reflective sphere, the "pinhole" distance/diameter requirements remain identical.
A circular light source of the opening diameter "L" directed at a reflective sphere of radius "r" placed at a distance "S"(in a setup in which the light source directed towards the sphere is by the observer's side) will produce artificial star of diameter ~r^2/2S.

I poked a bunch of holes in tin foil and held the sheet up to the light. I found the smallest good quality hole, cut out that section and put it over a Rayovac 3AAA LED flashlight. Tape all the way around and done. I get good diffraction patterns on both sides of focus at about 150 feet. The flashlight even comes with a little clip holder so I can clamp it on stuff!