Curved secondary vanes

[-Dougal-]

Can anyone point me towards anywhere that supplies curved secondary mirror vanes or any advice making my own? Also does it matter which way they are orientated? For example if using 2 rings or curves are they best placed vertically top and bottom or horizontally side by side?

I am currently working on an old 12" Revelation Dobsonian that I picked up quite cheapily but requiring a lot of work on astro buy and sell. Whilst the primary mirror is away getting recoated I figured that I may as well do further upgrades as well.

I am interested in replacing the cross shaped spider vanes with curved versions because I feel it will make the scope slightly more versatile by reducing the defraction spikes and my existing vanes have somehow become extremely bent and twisted.

I realise that they can reduce the contrast a little but I hope to limit this by fully flocking the inside of the OTA

[cjdawson]

Surely all that curved vans will accomplish is more material blocking the light path.  You'll still have defraction spikes, only they'll be curved instead of spikes.   I could be wrong though.

[John]

My dob has curved vanes. I don't see diffraction spikes. There is diffraction but it's spread more evenly across the FoV. I don't think they are revolutionary but personally I like what they do and the views seem good 

Mine were fitted by Orion Optics when the scope was made back in 2006 but I don't think they are offered as an option now due (I believe) to lack of demand. There are a couple of firms in the USA that make them as upgrades but none in the UK as far as I know. David Lukehurst makes great dobs and uses a single curved vane on his up to 14 inches in aperture. See the 3rd heading on this web page:

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

Here are the secondary vanes on my 12" Orion Optics dob:

 

[Stu]

1 hour ago, cjdawson said:

Surely all that curved vans will accomplish is more material blocking the light path.  You'll still have defraction spikes, only they'll be curved instead of spikes.   I could be wrong though.

So long as the vanes turn through multiples of 180 degrees, the diffraction cancels out. That doesn't mean it disappears completely, rather it is spread out evenly around the object rather than being focused into spikes.

Protostar make curved vane spiders which offer the minimum vane length to keep the total diffraction down. Theoretically John's supports have a longer total length, so will scatter more light but whether it is noticeable or not is another matter.

http://www.fpi-protostar.com/crvmnts.htm

[Tonk]

"You'll still have defraction spikes, only they'll be curved instead of spikes.   I could be wrong though. "

 

Second sentence is the correct one

 

[cjdawson]

10 hours ago, Stu said:

So long as the vanes turn through multiples of 180 degrees, the diffraction cancels out. That doesn't mean it disappears completely, rather it is spread out evenly around the object rather than being focused into spikes.

Protostar make curved vane spiders which offer the minimum vane length to keep the total diffraction down. Theoretically John's supports have a longer total length, so will scatter more light but whether it is noticeable or not is another matter.

http://www.fpi-protostar.com/crvmnts.htm

Gotcha.  That makes sense.  See every day is a school day.

[Moonshane]

I tried curved vanes but felt the view had more diffraction in the general view. I therefore changed back and anything critical e.g. A tricky double I just place away from the spikes.

[-Dougal-]

I need to at the very least replace or remake my spider vanes anyway due to how bent and twisted they are so might experiment a bit. First time I've seen 2 full circles used instead of semi circler curved vanes so might try those first? If I can't get on with them I can always go back to a more conventional design. What should the ideal material be to make them with? Stainless steel ruler perhaps the painted matt black?

[Alien 13]

There is a thread somewhere that does use steel rulers for curved spider vanes and it did look good.

Alan

[-Dougal-]

Thanks I'll try to find it

 

[-Dougal-]

25 minutes ago, Alien 13 said:

There is a thread somewhere that does use steel rulers for curved spider vanes and it did look good.

Alan

Is it this one? 

 

[Alien 13]

5 minutes ago, -Dougal- said:

Is it this one? 

 

Thats the one, I think people have also used hacksaw/bandsaw blades with the teeth ground off although they are more brittle.

Alan

[-Dougal-]

I keep reading that when using curved vanes the angles need to add up to 180 degrees? Could someone explain more about this? How is this angle measurement made?

[Moonshane]

Broadly it needs to form more than a semi circle.

[Stu]

10 hours ago, -Dougal- said:

I keep reading that when using curved vanes the angles need to add up to 180 degrees? Could someone explain more about this? How is this angle measurement made?

I think it basically needs to make up multiples of 180 degrees in order to cancel out evenly. That could be for example a single semi circle (as per the protostar), three vanes with a sixty degree curve each, or something like John's two circles which are effectively four x 180 degrees.

The shorter the total length of the supports, the less diffraction there will be. If you don't have exact multiples of 180 then there will likely be some strange imbalances to the diffraction.

http://www.fpi-protostar.com/cm-faq.htm