Re: ATM 3-Vane or 4-Vane Spiders

["Nils Olof Carlin" <nilsolof.carlin@telia.com>]

Frank Myosin wrote:

>.....they found that TOTAL 
> diffraction is the problem, and nince a 3-vane has 1/4 fewer vanes, it has 
> correspondingly less diffraction.

The problem is to a large extent in the eye of the beholder - there are circumstances when the diffraction spikes are a technical problem, but for "normal" observing it is more a matter of aesthetics - it is up to you to think the spikes look cute or just irritating.

Let us take a practical example: my 333 mm mirror, and a 63 mm secondary supported by 4 (actually 3, but I calculate with 4) vanes, each 2 mm thick (I guess that is thicker than most would use). The secondary takes 3.6% of the area - the vanes take 1.3% of what is left. Suppose you have vanes that take up 4% of the total aperture: of the 96% passing by, 4% are diffracted into spikes (or other shapes with curved vanes). Thus, the diffracted light led astray by the vanes is appr. 1.3% of what passes normally. Most of this light will fall within +- 1 arcmin (the diffraction minima), or an area that is a few times the apparent size of Jupiter. It will of course lower the contrast some - and lower the effective Strehl ratio by about one unit or two in the second decimal. With spider vanes half as thick, half as much light would be spread out over four times the area. 

> The thinner the vanes the better, but best 
> are no vanes at all. 

Well what are the alternatives? a Schiefspiegler, an optical window, a refractor, no telescope at all? An optical window would need to be better than Strehl 0.98 or so not to degrade the image more (the reason to use it would be to combat tube currents - but if this is what you are after, you may as well make a Schmidt or Maksutov corrector).

> The mass of the vanes radiates to the sky and is a bit 
> cooler than ambient air and consequently the vanes are optically much 
> "thicker" than their physical thickness. 

Anyone seen this in reality? I could heat a vane with my fingers, and see how its diffraction image in a defocused star image swells, and shrinks again as the vane cools, but I haven't seen a minimum. With a reasonably generous dew shield, the vanes will see very little of the cold sky, and won't cool much. With minimum baffling, though, the effect may be significant - anybody with experience?


> a thin wire is best of all - very 
> small mass. 

The mass is hardly an issue, but people with wire spiders seem to be happy with them. I'm sure they can be great fun to make, but let us not overestimate the problem. For literature, Suiter's Star Testing... and his writings in the ATMJ are recommended.

Nils Olof