Curved v. straight secondary support

[BinocularSky]

Redirected from http://stargazerslounge.com/topic/212861-explore-scientific-16-dob/page-4#entry2285111

BinocularSky, on 08 Apr 2014 - 09:11 AM, said:

All else being equal, curved vanes will be longer than straight ones, so will have more diffraction.

That is interesting thanks,

I had followed, over the years, the debate about straight vanes concentrating the diffraction effects into 'thin' well defined areas removed out of some other parts of the image - cf. curved spreading these effects diffusely over the whole image.

I had not thought, till now, about your length point ! Interesting !

So, now I am thinking [all else maybe not being equal ! ]  there is the possibility that just two or three curved vanes will be mechanically adequate to support the secondary instead of four in the classical form ?

then which is the shorter ?

I made one many years ago in the shape of a circle; in this case the total length was shorter 4 straight vanes, but slightly longer than 3 straight vanes. (Circ = 2*pi*r)  Not as rigid as a 4-vane.

[emadmoussa]

I'm happy to ignore the spikes on planets or suppress them with filters (I don't observe planets that often anyway). But somehow like the spiky stars  

Whether or not spiked vines vs curved vines make a difference in terms of light grasp, I pretty much doubt it. 

[Ptarmigan]

Good move !

and thanks for the info. (good for if I get round to building another 'scope, I used 4-vane previously)

Might be worth mentioning, if spikes are liked then 3 vanes in a 'Y' shape will give 6 spikes so I am told and good rigidity.

Agreed, any effect on light grasp/obstruction is going to be small to marginal or even negligible cf. the secondary mirror ! I think it is all about where the diffraction effects end up.

Either in/on the planet  image and in the Airy rings of the star, Or well removed from the image into spikes which is good for the double star hunters.

[acey]

Even thick vanes have no significant impact on light grasp (and nor do mirror clips) - it's all about the diffraction they introduce, not their shadow. Any number of straight vanes will produce an even number of spikes (a single strut produces 2 spikes, 3 vanes give 6). Curved vanes don't make spikes but still produce diffraction - you're just spreading the light in a different way. It's case of balancing that against stability of the secondary, ease of construction, and personal preference.

[8kids]

Does the spider arrangement on the ES dob still produce 4 diffraction spikes?

[BinocularSky]

Does the spider arrangement on the ES dob still produce 4 diffraction spikes?

Yes. Opposite vanes are parallel; diffraction spikes are perpendicular to the vanes, so 4spikes.

On the batphone, so expect weird autocorrect