Re: ATM Artificial Star Idea?

[Aplanatic@aol.com]

Hi James:

As long as the laser diode is far enough away then it's only a matter of how small the spot should be for your testing needs.

When I use a distant point source for testing, I like the "geometric" image to be about half the size of the theoretical diffraction image size.  This allows me to see the first and second diffraction rings at focus, evaluate the focused and de-focused image characteristics for good symmetry and correction, perform very high contrast Foucault and Ronchi testing, etc.

As an example, let's take my 10" f/6 Newtonian.  The light source must be placed about 100 meters away to make the refocused system have less than an 1/8 wave of residual spherical aberration due to the finite distance to the source.  Recall that the Airy disk has a diameter of 2.44*lambda*f where f is the f/ratio of the scope.  Thus, for a point source at infinity and perfect optics, the Airy disk diameter for this example will be 8 microns for 550 nm light.

Since the focal length is about 1.5 meters, there is a geometric reduction is spot size of 100 meters/1.5 meters, or about a factor of 65.  If I want the "geometric" imaged spot size to be about 4 microns, then I need the light source diameter to be less than about 250 microns or about 10 mils.

I don't think that your laser will have a spot size that is less than about a millimeter, so you will have to mask it if you want to see the diffraction effects in their full glory.  In addition, you will need a way to diverge the beam from the laser in order to cover the full aperture of the scope even at this large distance, I think.

So, here's what I would do.  Use a positive lens to focus the beam onto a pinhole that's about 200 microns in diameter and let it diverge from the pinhole.  The lens should produce a beam with at least a degree or two of divergence so that it's easy to point it at the target from a long distance away at night.

I've made a light source for artificial star testing from a superbright white LED and a pinhole in aluminum foil.  I first ground and polished the plastic lens of the LED to get the pinhole as close to the diode as possible.  This light source works very well with a coated set of mirrors, but is too dim, in my opinion, to be very useful for testing/figuring an uncoated mirror.  I think your idea is a good one (with the above modifications) and I might give it a try myself.  You might also consider a superbright LED followed by a converging lens and a pinhole.

As a general note, I find all of the "full-aperture" tests to be very useful, especially for systems with multiple elements or full-aperture correctors.  By full-aperture tests I mean those tests that form a focused spot by using the full aperture of the mirror.  Examples are:

1) A distant point source
2) Star test
3) Autocollimation testing with a flat
4) Testing with an auxilliary collimation telescope

Because of atmospheric seeing limitations and the clear-sky requirement, star testing has its draw backs.  Autocollimation testing is out for most of us because of the need for a very precise flat.  So, I use a test telescope focused at infinity for most of my system testing because it can be easily set up in my garage and it requires only a well made parabolic mirror of any convenient f/ratio.  I also use the distant point source test for final inspection and collimation of the assembled scope.  If I had an easy and conveient place to set up a distant point source then I might use that method a lot more.

Dave Rowe