All About Curved Spiders

All spiders will cause diffraction. In a normal, straight, four-vaned spider, the diffraction will be in the form of four distinct spikes emminating from any bright object. We've all seen this in photos:

What is generally not well understood is that there are really eight diffraction spikes from a normal spider, not four. Each vane causes two spikes, perpendicular to the vane:

Add another vane opposite the first, and we get two more spikes, though it will still look like two since they fall on top on the first ones (if the vanes are parallel, that is). But even though it still looks like two, each spike will be brighter because of the added diffraction.

So, what does this have to do with the price of coffee in Miami? Nothing. What does this have to do with curved spiders? Everything!

Straight-vaned spiders reinforce the diffraction spikes making them brighter. A properly designed curved-vane spider will spread the diffracted light throughout the field, eliminating the spikes.

First off, a curved vane will not make a pair of spikes, it will make a pair of fans:

Like the pattern produced by a searchlight.

Because the diffracted light is more spread out, it's much less visible (usually invisible). The key is to design the spider so that the fans form a complete circle around the star with no overlap.

The arc of each diffraction fan is equal to the arc of the vane in the light path. For example, if the vane below has a 90 arc in the light path, it will form two 90 fans:

So you want to select the number of vanes and the arc of each vane such that the fans form a complete 360 circle.

Some combinations that will work, from a purely optical point of view, are:

1 vane with a 180 arc.
2 vanes with 90 arcs, set 90 apart. Note that if the vanes are set opposite one another the fans will reinforce and form a brighter searchlight pattern.
3 vanes with 60 arcs, set 120 apart.
3 vanes with 120 arcs, set 120 apart.
4 vanes with 45 arcs, set at 45, 90, 180, and 315.
5 vanes with 36 arcs, set 72 apart.

Other combinations are possible, but all designs follow the rule that no vane has a vane opposite it.

Mechanically, just about all curved spiders are inferior to straight ones. From a combined mechanical and optical point of view, the 3 vanes with 60 arcs is a reasonable choice.

Note that it seems to make little difference which way the vanes are curved. One design that takes advantage of this is two 90 vanes set 90 apart, but curved in opposite directions. This allows you to use a single 180 piece with ample mechanical strength.

Fellow ATM Gary Seronik, who uses this design, reports: I've found through expermentation that there is considerable lee-way in the designs. It seems so long as some kind of curve is present the diffraction effects are erased. At first I went to great pains to assure that the bends were precisely 90 degrees and so on, but later found out that it made little difference.

With most designs, the total amount of diffraction will actually be slightly more than with an equivalent straight spider for two reasons:

  1. There is more total vane in the light path.
  2. For equal mechanical strength a curved vane will probably have to be thicker.

The advantage is that the diffracted light is evenly spread throughout the field, and not concentrated into noticable spikes.

This brings up one additional item: fellow ATM Richard Schwartz points out that in certain circumstances the noticable spikes may be prefereable. For instance, if you are looking for the faint companion of a bright double star, the circular diffraction pattern from a curved spider could bury it, but with bright spikes you can place the star where the spikes aren't.

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