Amateur Astronomers Association of Princeton

by Ted Frimet

Almost every amateur observer wants to get the biggest bang for their buck. This is especially true when they sit down for a spell, to look at the wonders of the universe. Quite a few of us want to get a better view. Telescopes, with all their power, have their own inherent limitations; however, some limits can be overcome. Other destructive imperfections, can only be met passively, and accepted with muted understanding.

Computer-generated image of an Airy disk. Credit: https://en.wikipedia.org/wiki/Airy_disk

This, though, is not that story. If you are seeking the with and without of collimation, there are, and have been, very well written, and dare I say, “concise” instructions on how to go about your business. This tome, is in response to a thought experiment, timeless reading, and a parents question, during public night. The thought will be exposed below, the reading will have citation, and the parent – who asked simply, “How can I keep my child’s interest in astronomy?” – was met with, “use the telescope as a tool to complement your child’s learning, and their education”.

Neal deGrasse Tyson notes in a lecture licensed by The Great Courses, something that we are all too familiar with. If you could travel faster and faster, approaching the speed of light, your time slows down. He also speaks to the point that the light that we perceive, from distant stars, take a very long time to reach our eyes. Star light travels at light speed. For photons, time has no meaning. That is, if you ‘could’ travel at the speed of light, time would stop for you. But of course, this will probably never happen, as it would violate Special Relativity. However, it is still interesting to note that time has no meaning for a massless photon. The moment it is created, it appears at its destination. Yet, you, the observer, might be observing that very same photon millions, or maybe even billions of years after its instantiation.

I had a dream, a few nights after my first draft of this essay. A slice of a squashed faced astronomer was stuck in an oval cutout. It was if he were pushing his entire face and a partial hand against a window. And I was on “our side” looking at him. Akin to “Alice Thru The Looking Glass”, I was seeing the observer at their eye-piece. Their light was pushing out of their eyepiece, and thru their telescope tube, and into the void. It was the only explanation my less than conscious brain could offer while waking from a dream. You think you see old light. I see you as you were, or are, or will be. Keep in mind, that the light you emanate travels instantaneously to another observer…that is if the observer were massless, or perhaps made of neutrinos. A massless being is not time constrained. Their time simply pays no part in the image you emanate. That is, the massless observer sees your light, instantaneously.

Point your telescope at a star, and rack your focusser, until you get the airy disk. Note the diffraction circles. Each circle is light from your distant star. Your actions focus each solitary circle into a starry point of light. And if done well, we are very happy observers.

The airy disk light rings are the cumulation of the lights three dimensional wave package. You see, almost every illustration I’ve seen shows light as a 2D drawing appearing on a cartesian plane. And it’s an incomplete view of reality. Even in Harold Richard Suiter’s manual, “Star Testing Astronomical Telescopes”, c1994, p58 fig 4.1 shows part of a wave. And then Suiter asks the reader to “try to point out the location of the light wave”. It isn’t that the question is fundamentally flawed, it is that the visual aide is incomplete. One might argue the imposed limitations of graphic art in book production. If the light wave were graphically presented as a three dimensional wave, you might be casting a different light on the subject matter and answering Suiter’s question, forthwith.

Below is my attempt, thru GNU Image Manipulation Program (GIMP ) to show a three dimensional wave. It ain’t pretty- looks more like pasta. What is missing in my model, is that along the length of my pasta, there should be undulations of small and large diameters.

Forget the pasta, and bring in the play dough. Work the play-dough out of an circular extrusion. Along the dough’s length, squish it with your hands, allowing your fingers to make indentations. If you were to cut thru the dough, at say, even intervals, you would have many circles in varying diameters.

Slice the light cone of special relativity and create many circles. They vary in size from small to large – and large to small, too! Except my cone is a wave, it is three dimensional, and has undulations in diameter, throughout.

More than a few years ago, I had the benefit of speaking with, briefly, a man named Charles. Charles was the son of my soon to be mother-in-law’s neighbor, Betty. Nice people. Charles, as a Bell Labs Engineer told me about how I needed to change my perspective on radio frequencies appearing to me as a 2D wave plotted on a x-y chart. And to start thinking of these waves as being in three dimensional space. Many years passed, and now I am reading Harold Suiter. Within the very first couple of chapters, Suiter makes the comparison of light to audio waves. And rightly so – as it no doubt it assists the reader in making the jump from a technological understanding of one established science to the next. So, you might infer from this, that Suiter has encouraged me to make the jump from radio waves to light waves. But after all, we really have not, since radio frequencies are just another component of the electro magnetic spectrum, shared of course, by its cousin, visible light. Star light is a three dimensional wave front.
So, to answer Mr. Suiters question, to point out the where the light is on the light wave, we answer, gingerly – it is on the incoming light wave’s outer circumference of the conic section.

Since photons instantly reach their destination, at no time at all, the airy disk is the result of the wavefront crashing, and smooshing down onto your observers lens. It is as if light’s arrow was thrust in your direction, down your tube, and splattered onto your lens, leaving behind in every instant of your observation, ring, after ring, after ring.

The observed radii of the airy disk increases with the telescope aperture. Not only do you have access to more photons, with a larger aperture, you have access to older light. But you already knew that. You see farther into the time based dimension of the universe and always look at old photons.

But here is the conundrum. You are now in my deep end of the pool. That out of focus light that you see isn’t old light. It is the result of the instantaneous impact of quanta produced, far, far away- for time has no meaning to the traveling array of quanta that makes up the airy disk.

Rack your focus in, and see light as it was. Rack your focus out, and see light as it will be. Did I cross the line? Maybe you could too, if you were slightly out of focus, like me.