Spider Spikes: Diffraction or Refraction?

[herbhighstone]

Hello to all the folks --

We all know about the bright spikes caused by straight spider vanes in reflectors. And it's customary to state that these spikes are caused by diffraction. This cliche has become such an article of faith that nobody, including myself, ever thought of questioning it.

However, there's another point of view here. Recently I was looking through a delightful old book called "Telescopes" edited by Gerald Kuiper. By the way, if you'd like to get your own copy of this 1960 book from the University of Chicago press, just search abebooks.com for "Telescopes" by Kuiper. Used copies start at less than $7 plus shipping, and some of those grand old scopes might give you a few ideas. You will also be totally astonished by the highly detailed, full-page reproductions of Russell Porter's drawings of the Palomar 200-inch telescope. The artistic genius of that guy was supernatural.

Now back to our spider vanes: On page 127 of "Telescopes" you can read a contarian view of what really causes spider vane spikes. Andre Danjon of the Paris Observatory states that in his experience, spikes are caused by REFRACTION, not diffraction.

Monsieur Danjon, did I hear you correctly? How could this happen? Danjon says that the spider vanes are almost always at a different temperature than the surrounding air. Thus each metal vane is surrounded by a heated or cooled air layer that REFRACTS light out of its proper path, and in his opinion, this refraction is the major cause of those annoying so-called "diffraction" spikes, at least in larger telescopes.

To minimize spider vane spikes, Danjon reports that Andre Couder would cover the metal secondary mirror supports with polished aluminum. (Was this thin aluminum foil from his wife's kitchen? Maybe so, but Danjon doesn't say.)This treatment works because a polished metal surface has a very low thermal emission as compared to blackened metal. Thus the shiny metal heats (or cools) the surrounding air much less than the dark metal. And so the refractive air layer that's causing the spike problems is greatly reduced.

Nobody is denying that diffraction takes place at a spider vane. Of course the physics of the situation dictates that diffraction must occur. However, it would appear that in at least some cases, refraction due to thermal air layers causes much more mischief than diffraction. In other word, diffraction isn't the entire problem by any means.

Here we're running into another counter-intuitive idea. Because today, we have this notion that everything inside the telescope should be thoroughly blackened to reduce scattered light. This has become a total obsession as we continually exchange information about the best black paint, black velvet, black flocking material, and etc. So the idea of shiny spider vanes sounds so absurd that we would never think of trying it.

However, now that amateur Newtonians have broken the 30-inch aperture barrier, curved spider vanes are becoming very difficult to engineer. Straight vanes under tension are, from a mechanical point of view, much more logical. So I'm proposing several alternatives to the present ideas concerning the reduction of spider spikes with straight vanes. One, you could try shiny stainless steel vanes with no coating at all.

Or two, you could try using black flock paper to blacken the vanes, and at the same time insulate them. Yes, I know that the paper will add to the thickness of the vanes. But in your own personal observing conditions, the thickness of the flock paper may be much less than the thickness of the thermal air layer that the paper will prevent.

There is yet another possibility. Research on solar heat collectors has shown that even after a shiny metal surface has been covered with paint, the thermal emissivity can remain low. Apparently the "shininess" is still there, even though it may be covered by paint. But if you etch the metal before painting it, thus making it rough and non-shiny, the thermal emission will become (for our purposes) undesirably high. So if you want black spider vanes, you should paint directly over the shiny metal without etching it or impairing the shininess in any way whatsoever.

Why have extremely thin spider vanes proved helpful in the past? This may not be due to the reduction in their thickness and thus the reduced diffraction, but instead to the decreased heat capacity of the thinner metal.

Once you get into the correct frame of mind, these thermal design refinements can be remarkably simple. Just remember to minimize the heat transfer between spider vanes and the surrounding air, and you'll be on the right track.

FANS, FANS, FANS! It's interesting that people are surrounding their Newtonian mirrors with all kinds of fans to get rid of thermal layers. However, I don't know of one single Newtonian telescope that has fans aimed at the spider vanes! If you don't want to rebuild your spider just yet, why not try a fan or fans to break up the thermal air layers around the vanes? This could be a very simple A - B test to see whether thermal air layers are a problem at your own observing site. Just turn the spider vane fans on and off, and note the results.

Or here's an even simpler test. You could get some shiny aluminum foil from your kitchen and wrap it around your spider vanes. That would probably take less than 5 minutes.

Okay, I realize that a lot of this is completely contrary to the established wisdom of telescope making. But even so, some of these ideas might work well for you, once again depending on your local climate conditions.

Best wishes from Herbert Highstone

[Alan French]

I thought the thin layer of warm air along spider vanes and resulting refraction was fairly well known, and it was very old news to me. I know some of us in our local club have talked about it.

I've always felt the spider vanes easily lose heat and it wasn't worth worrying about. It would certainly be easy to experiment with, however.

Clear skies, Alan