Spider vanes

[Kaptain Klevtsov]

I've been wondering about curved vanes as they give better images (allegedly) but it strikes me that they would have to be stiffer and therefore thicker otherwise the secondary mirror would "boing" quite a bit. I was going to make some curved vanes out of hacksaw blades (which look beefier than the standard vanes on my skywatcher) but a quick try with one blade showed the idea to be flawed. The choice is then to go for thicker stuff with more loss in image quality (not good I think) or maybe three straight vanes would be another option as there will be no opposite vanes to reinforce the diffraction pattern.

I await your opinions Gentlemen and Lady

Captain Chaos

[GordonCopestake]

Also curved spider vains are LONGER than straight ones (think of how the crow flies) and the difraction spikes caused by the obstruction is still there but smeared across the whole image. It's purely a esthetical personal preference. Some people like spikes, some people dont. Depends if you take images for esthetics or for scientific study.

[GazOC]

You'll lose more contrast over the entire view, whether thats better/worse than the spikes......??

I'd guess you wouldn't notice.

[Kaptain Klevtsov]

Its looking like 3 straight vanes then for the best imaging option, that way I get good contrast and three sexy spikes.

I'm sure that I've seen NASA or Hubble images with three spikes so there's the Kudos factor as well.

Captain Chaos

[GazOC]

Its looking like 3 straight vanes then for the best imaging option, that way I get good contrast and three sexy spikes.

3 vanes give 6 spikes thats why 4 vanes are so popular.

[Kaptain Klevtsov]

Hmmm, going to have to get used to six then with the new project.

As it progresses I'll post pictures in the workbench section as I go on and on about the technicalities that I can't make decisions on. Its nice to be able to blame somebody

Captain Chaos

[iancandler]

I'm not qualified to make a judgement on whats best for imaging but have to admit I have always liked the look of the curved spider..

have a look at this page it shows both side by side so you might be able to get an idea of the thickness needed.

http://www.dobsonians.co.uk/Options.html

[Kaptain Klevtsov]

The curved vane looks wobbly. It must be made from something exotic to have enough torsional stiffness otherwise the tilt of the Dob. would make the secondary move about. He's even got the secondary on an extension for extra leverage.

As I have a hexagonal cage tube, it going to be three straight vanes for now at least. Once I need to get rid of the spikes I might get into fancy composites and try a curved vane.

BTW Ian, qualified doesn't come into it IMHO. Your link was much appreciated and the guy makes a living out of making nice telescopes so I think that that would make him qualified to make a judgement. It is going to be my design, as in my responsibility WRT the construction techniques, materials etc. Your advice is welcomed as much as anybody else's.

The free advice available here is a wonderful resource because it is based on personal experience for the most part and links to technical reports by experts makes up the rest. Thanks for the link.

Captain Chaos

[Astroman]

Curved spiders introduce less diffraction spikes compared to straight vanes. The wavefront of the light entering the telescope is affected in a non-linear manner with the curved vane, therefore producing more contrast, not less. Strength is gained by adding material in along the optical axis. This allows the thickness to remain the same, thus the same optical footprint. Views through both types, side by side, the curved spider wins every time, imho. I have a curved vane in my design for the 16" F/5.

[The Warthog]

Here is a photo of one made by an acquaintance of mine. He used aluminum hex rod for the secondary holder, and 1/16" x 3/4" aluminum sheet for the vanes. He says it is very rigid. The photo is before he painted it flat black. He got the shape by bending the aluminum around a suitably sized cylindrical object.