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The Small Obstruction and the Curved Vane Spider

When it comes to image quality the ideal that's bandied about is almost always refractor-like. Why refractors yield these ideal, standard-setting images is the key to at least partially understanding why reflectors mostly do not. When you look into the front end of a refractor you see a clean aperture definition, a smooth edge to the cell that cleanly defines the pupillary edge, and nothing in the middle of the tube. This is very important. The very nature of the edge of an optical system and any obstructions are critical to its performance. Often, reflectors seem to be made with an almost reckless disregard for what happens when the incoming light path is interrupted and disrupted by obstructions (both central and edge) ranging from thick, straight vanes, mirror clips and miscellaneous protrusions to nuts and bolts, ragged edges and even electrical wires; not to mention the size of the central mirror obstruction itself, which is often huge. And the impact of these obstructions are  not just subtle but frequently vividly annoying. If you really want refractor-like performance you can't go cluttering up the light path with large obstructions and their diffraction-producing attachments, but if you must place obstructions in the light path (and we must) it should be done carefully and with full consideration of their impact on high-resolution image formation. I love refractors, and they do produce pristine - refractor-like - images. But try and get one that's 10 or 12 inches in diameter and you start thinking a lot about how to improve reflectors. The interesting part of this problem is that it can be significantly mitigated relatively easily and at minimum cost. Mostly it's about telescope design and just applying a few common-sense improvements.

The picture above was taken through one of my 12.5" f/20 Dall-Kirkhams. A camera was placed at the focal plane and a plastic bag draped over the end to diffuse the light. This is actually what the telescope sees when it looks at the sky. The size of the obstruction as well as the spider vanes can be easily seen. The vanes are curved and thin, the secondary is small and cleanly defined, there is no monkey business at the edge - no protruding objects, no straight lines, no  mirror clips. I have tried to make this reflector as refractor-like as possible.

As to the effects of a clean system with a curved vane spider take a look at the image below. This is Vega! No spikes, and nearby faint stars are clearly visible. The image was taken by Mark Jordan with an 8" Newtonian telescope he made using a curved vane spider design very similar the ones I use. It also contains one of my 8" f/5 mirrors.

Below is seen one of my 12.5" spiders. Vanes are .064" thick and 1/2" wide. Keeping the vanes narrow as well as thin in cross section is important because if the vanes are wide any off-axis light will effectively see a thicker vane. With wide vanes, placement of the vanes also becomes critical because if the vanes are tilted in any way they will become effectively thicker.

If you wish to read more about the impact of obstructions and diffraction, there is an excellent publication on the web by Vladimir Sacek at http://www.telescope-optics.net/. Specific discussion relative spiders and obstructions is at http://www.telescope-optics.net/spider.htm