Width, not thickness, of Spider Vanes question

[Tommohawk]

Hi all - quick question.

Ive read a bit about spider vane thickness, but what about the width, or depth if you prefer. They are typically about 10mm ish, but does this make a difference?

What about if they were say 30 or 40mm?

Any thoughts? 

Thanks!

[John]

I think it's the profile that they present to the light path of the scope which matters so the depth of the vanes is less critical ?

[Stu]

Agreed. Provided the vane is exactly edge on to the light path then it is only the thickness that can have an impact. Perhaps it's the risk of them being slightly out of line that encourages keeping them to a limited width.

That said, I think some of the curved vane supports tend to be wider to give extra rigidity. 

[johninderby]

Remember seeing the results of tests made by OO uk a few years ago and they found the thickness of the vanes wasn’t important. In tests no one could see rhe differnce between thick vanes and thin ones. However if a vane was twisted it could cause problems if say one vane was twisted making it appear thicker so perhaps deeper vanes could be more prone to twisting.

Incidentaly they said the reason they stopped using curved vanes was cost as unless the curve was perfect it caused problems and it was expensive to make perfect curved vanes.

[fwm891]

The good thing about curved vanes is the reduction/elimination of diffraction spikes. As for depth the depth is needed at the central boss where two fixing points can be used to stop twisting and the third (forming a triangle) at the tube wall.

[Chriske]

We always use 3 curved vanes, 3mm thick and 40mm wide for a 10" scope. A smaller scope will have enough with 30 or even 20mm wide vanes. These curved spiders are very strong.
It is in fact way stiffer compared to the 3mm straight spider we did use years ago. If you do use curved vanes see to it that the 'blades' are mounted parallel with the optical axis. And of coarse, as always blackening is a absolute necessity.
The curved vanes should 'cover' 180° of curve above the primary mirror. So doing you redistribute the diffraction of the spider itself evenly all over the field of view.
 

[Tommohawk]

Hi all and thanks for the helpful replies. All agree that width/depth makes no difference. 

The reason I ask is I was considering a James Webb style support for the secondary of a travel scope for planetary use only. Unfortunately I can't quite afford launching it into even a low earth orbit, so am stuck with the inconvenience of gravitational forces. 

This design has a very small secondary obstruction, but I suspect that 1m (approx) vanes of sufficient stiffness will increase the obstruction by too much.

Any thoughts? Are there any ground based scope using this design?

 

[JamesF]

Curved vanes do seem a nice idea from the point of view of getting rid of diffraction spikes (assuming you're not a fan of them).  Making sure they're sufficiently rigid is important though, I guess.  Making them deeper might help, but I'd agree with Stu that it may well be more tricky to minimise their intrusion into the light path.  And of course diffraction will still occur, and more of it, given that the length of the vanes must be greater than for a similar telescope with straight vanes.  I believe this means that there will actually be a reduction in contrast by comparison with straight vanes, but I imagine it's far too small to be noticeable to the naked eye.

James

[Chriske]

6 minutes ago, JamesF said:

......but I imagine it's far too small to be noticeable to the naked eye.

James

Absolutely.

[johninderby]

The curved vs straight spider vane argument is a bit of a Marmite thing. Some like them and others find them a waste of time. Both designs are a comprimise anyway.

An optically flat glass plate would be better but just too expensive.

 

[John]

I like curved vanes. The ones in my 12 inch Orion Optics dob seem to work well and the scope has done everything that I have any right to expect of it

 

For a small aperture scope I reckon a single curved vane would be worth considering such as the 180 degree arch design shown here:

 

[Tommohawk]

Thanks for all the input. Anyone have any thoughts on the "James Webb" style truss mount for the "secondary"? A small lightweight colour camera would be placed where the secondary would normally go.   It would need quite long vanes of course, even for a fast-ish scope. Are there any examples of land-based scopes that use this kind of design? 

The advantage is that the whole assembly would be much lighter and more portable than a comparable truss style design.  Absolute rigidity isn't so much an issue as this would be used for high speed planetary work.

Any ideas?

[Peter Drew]

I don't think rigidity can be dismissed quite so lightly. Lack of rigidity leads to lack of collimation, the achievement and maintenance of which is essential for best planetary imaging results.   🙂

[Tommohawk]

11 hours ago, Peter Drew said:

I don't think rigidity can be dismissed quite so lightly. Lack of rigidity leads to lack of collimation, the achievement and maintenance of which is essential for best planetary imaging results.   🙂

Hi Peter. Fair point - rigidity needs to be maintained. Whats your view re this design idea? Do you think sufficient rigidity could be achieved with struts/vanes of this sort without causing too much central obstruction?

My example is a 350mm mirror with a 62mm camera - giving a linear CO of ~18%, which is very nice. By area the obstruction is just 3.1% 

But struts of eg 8mm thickness would almost triple the obstruction (by area), and even 8mm might be too flimsy.

Anyone got any thoughts on that??

[JamesF]

15 hours ago, Tommohawk said:

Thanks for all the input. Anyone have any thoughts on the "James Webb" style truss mount for the "secondary"? A small lightweight colour camera would be placed where the secondary would normally go.   It would need quite long vanes of course, even for a fast-ish scope. Are there any examples of land-based scopes that use this kind of design?

Don't you need further optical components in this sort of design to correct aberrations?

James

[Tommohawk]

3 minutes ago, JamesF said:

Don't you need further optical components in this sort of design to correct aberrations?

James

The JWST is a 3 mirror anastigmat, so yes, different optics. Its only the mechanical structure I'm borrowing.

My proposal is a straightforward newt, but with the camera placed where the secondary would normally go. Parabolic primary, no coma corrector because only interested in the centre of the field. It should be possible to include a CC but I think this would just add further weight/stress to the camera assembly esp because of the spacing between the CC and the camera.

[billdan]

The major problem you will have to resolve with the camera replacing the secondary is a method of accurately achieving focus. The Critical Focus Zone of a F5 newt is 70 microns and about 90 microns at F6.  (The width of a human hair is about 100 microns). Focus is not a set once and forget about it, you have change focus as the temperature drops during the night.

Also the Coma free zone of an F5 scope is only 3mm in diameter at the focal plane and about 5mm for F6. So you may be disappointed without a coma corrector attached.

If you use a cooled camera, warm air will be pushed out by the TEC cooler fans making the seeing worse than it actually is. 

My advice, don't bother, stick to conventional.

 

Cheers

Bill