>> Hello Tom
>> At last Tuesdays workshop you asked why Ronchi lines are straight for a perfect parabolic mirror when viewing a star but also straight for a perfect sphere on the test bench.
>> The explanation is this. The primary mirror in our telescopes is a paraboloid because that shape has the property of bringing incident light from a point source at infinity to a single point in the image plane of the telescope. Recall the image plane for an object at infinity is located at a distance in front of the mirror equal to the focal length or one half the radius of curvature. A sphere, on the other hand, has the property of bringing incident light from a point source at the radius of curvature to a single point in the image plane of the optical setup we use in Broder. In this case the image plane is located at the radius of curvature.
>> This ability to produce a point image from a point object in these two different setups is due to the geometry of the two different surfaces. You may have studied this in geometry or algebra, wherever conic sections were covered.
>> In terms of wave optics the two different setups involve different geometry wave fronts. The light waves from a star at infinity has the geometry of a plane wave while that from the point source in our Ronchi setup constitutes a diverging spherical wave front. A paraboloid brings a plane wave to a point while a sphere does it for a spherical wave. After the light bounces off the mirror, if it's heading toward a single point in the image plane, it will constitute a converging spherical wave. This is the case for a plane wave bouncing off a parabola or a spherical wave bouncing off a sphere.
>> Now when you put a Ronchi reticle in a converging spherical wave the reticle will be the same distance inside or outside of the image plane and what you see will there for be straight lines.
>> Hope this helps. Jerry
> On Fri, Mar 20, 2015 at 4:28 PM, President SBAU
> Thanks for the explanation. Add this to your book of "101 optical explanations for telescope mirror makers"?
> However, I am not totally clear on why a star is a planar wavefront, that is, why is it different from the test point source which is a diverging spherical wavefront? Probably because the part of the sphere we see from the star is hardly different from a flat surface by the time we see it in our little telescope? A star test (coma?) must be the ultimate check on a telescope mirror compared to Ronchi or Foucault tests and guesses.
On 3/20/2015 5:23 PM, Jerry W wrote:
> Hi Tom
> Your supposition of why a star presents a planar wave is exactly correct. Close to the star it is also a sphere, but by the time it gets to us it is a plane for all practical and intensive purposes. The presence of spherical aberration, coma, astigmatism etc are all aberrations on the simple wave front explanation I gave. They are of a secondary nature compared to the primary one of producing a true point. We can get away with ignoring them because when we are squinting through the Ronchi grating with our naked eye we can't see them. If we were to greatly magnify the image then they would show up.