Curved spider vane for my 6" f11 dob

[Moonshane]

Been meaning to do this for a while. John's recent purchase with such curved vanes and his successes have made me want to try this method.

I basically took the existing spider completely apart and then reused all but the spider vanes for the new one. I bought a piece of 1.5mm x 300mm x 12mm brass off ebay (about £3 delivered). I drew a plan a la Seronik http://www.garyseronik.com/?q=node/62 and this allowed me to gauge the length and correct curvature of the single vane.

I wanted to permanently modify the scope as little as possible so created a curve which met two of the four current holes. I honestly don't know if the curvature is critical but we'll see when I get first light I suppose. I used a honey jar of all things to get it about right.

I had to drill a couple of holes in the bar in the middle to house the spider centre and also on the folded tabs on the end to allow attachment to the OTA. I left one end of vane with an open noth to allow adjustment if required but I think this set-up once fixed will not move at all.

Getting the secondary central to the focuser/ota was the most tricky bit and needed several 'in and out's of the spider before I got it right but then it was a straight forward collimation. I think if I were making one for a larger secondary / ota I might possibly try four bent vanes but the same process. Might try this with my 12" next if it works out well.

I am hopeful this will provide diffraction spike free views of doubles in particular to give me a chance of splitting some of the trickier ones a little easier.

[Moonshane]

I think I might actually take it out at some point and file down the corners of the secondary centre. the seem to intrude into the light path and it bugs me that OOUK don't see this sort of thing.

I may even make another one from scratch from round rod as I noticed today that one of the secondary adjuster holes is drilled and tapped so badly it is about 2mm out of square and only just hits the secondary.

[John]

Neat Shane. Thats the sort of shape that David Lukehurst uses in some of his dobs.

Hope you get to 1st light it soon so we can hear how it fares 

[emadmoussa]

Genius. Neat job!

[JamesF]

I wonder if, for the sake of rigidity, it wouldn't be possible to add a mirror image of your existing vane using the other two holes, Shane.  There'd be very little of it that might be in alignment with the first, so it may well be possible to do without creating diffraction spikes.

On the other hand, if it's rigid enough as it is, why bother?

James

[Stu]

Some useful info on curved vane spiders and different configurations in this link.

http://www.protostar.biz/cm-faq.htm

Stu

[Stu]

Can't seem to edit posts after the updates last night.....

Shane, the only possible issue I can see with your configuration is that I don't think it goes through 180 degrees and that is necessary to fully cancel out the diffraction. Might need a few tweaks, but a nice project to do.

Stu

[Moonshane]

cheers guys. I'll report back as soon as I have used it. ultimately, I could (using the same materials) created about 10 different versions for the cost of a bought destiny version so I have some capacity for fiddling.

unfortunately, the curse of the planned night out with my wife struck again last night and I missed one of the clearest nights for ages.

[JamesF]

Can't seem to edit posts after the updates last night.....

Shane, the only possible issue I can see with your configuration is that I don't think it goes through 180 degrees and that is necessary to fully cancel out the diffraction. Might need a few tweaks, but a nice project to do.

Stu

My understanding is not that the curved vane cancels the diffraction effects out.  My understanding is that any spider causes diffraction.  When you have a spider with straight arms the diffraction effects reinforce along the length of the spider and thus you end up with diffraction spikes.  By having a curved spider you avoid the diffraction effects reinforcing each other because each part of the spider is in a different orientation to the rest, so you don't get spikes.  You still get diffraction, but it's probably more perceptible as slight scattering of light over the entire field of view.

James

[Moonshane]

that's my logic James but as with any system I suppose there's an optimum and then compromise to meet the circumstances. this way I avoided drilling holes in the OTA and thus giving me the option of making a curved four vane spider and using the same holes in the future.

I am hoping this works out but if not, I'll keep trying - we all need something to do on these soggy days and nights.

making spiders seems easier than I thought. I'll probably never buy one again.

[Moonshane]

ps, the secondary on this scope weighs almost nothing so this is more than sturdy enough in this case.

[Stu]

My understanding is not that the curved vane cancels the diffraction effects out. My understanding is that any spider causes diffraction. When you have a spider with straight arms the diffraction effects reinforce along the length of the spider and thus you end up with diffraction spikes. By having a curved spider you avoid the diffraction effects reinforcing each other because each part of the spider is in a different orientation to the rest, so you don't get spikes. You still get diffraction, but it's probably more perceptible as slight scattering of light over the entire field of view.

James

Yes, that's my understanding too James and that's explained well in the link I posted.

My point to Shane was that you need to have the curve covering the full 180 degrees in order to cancel out any reinforcing effects. Looking at Shane's support it seems near 90 or 100 degrees.

You will still get the diffraction as a scattering effect but there's less chance of a small secondary star hiding behind a diffraction spike.

Stu

[Moonshane]

it probably is Stu about 100 degrees as I come out a bit and the connections to the tune wall are at 90 degrees give or take.

either way it will be interesting! another option is to connect on the 'far side' of the central hub. we'll see in due course. if it provides the sort of effect I'll leave it for now but will be good to see how it performs on Sirius in the winter.

[Moonshane]

one issue with this spider design that I have just discovered is that if you use off axis masks (yes, I have even made one for this scope!) and put them on the spider for use / storage then they (the masks) don't sit properly and when moved can fall down the OTA - although sometimes thankfully, they get stuck halfway down. just a theory, this did not just happen at all, honestly.

[John]

My understanding is not that the curved vane cancels the diffraction effects out.  My understanding is that any spider causes diffraction.  When you have a spider with straight arms the diffraction effects reinforce along the length of the spider and thus you end up with diffraction spikes.  By having a curved spider you avoid the diffraction effects reinforcing each other because each part of the spider is in a different orientation to the rest, so you don't get spikes.  You still get diffraction, but it's probably more perceptible as slight scattering of light over the entire field of view.

James

Thats my understanding too and I have tempered my expectations of my 12" OO dob with that in mind. In practice the scope has delivered the most "refractor like" images I've seen with a newtonian but without switching to a conventional spider pattern for a while it's very difficult to quantify the contribution the curved spider design is, or isn't making to this image quality. My suspicion is that the performance of the scope is being improved by a combination of the .987 strehl primary, good collimation (F/5.3 is easier), a modest 21% secondary obstruction and the curved vanes. I've yet to flock the scope.

I'll be interested to see what difference Shane notices with his new design of spider.

Here is the design OO have used on my scope:

[JamesF]

That design is the one I'm most familiar with.  I look forward to hearing about Shane's opinion on his mod too.  I think it's an interesting approach.

James

[Stu]

Yes, the double circle vane obviously has two complete loops so ensures there are no reinforcing effects.

The amount of diffraction scatter is then defined by the total area of vane in the light path, so it is still beneficial to keep the vanes as thin and short as possible whilst ensuring necessary rigidity.

Will be interesting to see the results Shane.

Stu

[Moonshane]

took the scope out - clear skies

got it set up and the finders aligned - clouds rolled in

came in to check weather

went back out as looked pants - rain started so I brought it back in

this must be a record and seems typical of my observing experience in the main for the last month or so.

[Astrobits]

I would just like to clarify a couple of points here.

1. Any object in the light path will cause diffraction from it's EDGE only. The solid component, between the edges, will simply reduce the amount of light in the image, it will not increase diffraction. Therefore, contrary to the statement in the link above, thicker vanes will NOT cause more diffraction provided the length of the edge in the light path is the same. Bigger secondaries cause more diffraction simply because the edge is longer than a smaller secondary.

2. The diffracted light from a curved spider vane will be a diffuse area around every bright object reducing the resolving ability of the 'scope. Just imagine the 'spiky' diffraction from straight vanes smeared out 360 degrees around the objects. The effect is to decrease contrast in the fine detail of the image and it will not significantly affect the overall image contrast.

These are highly simplified explanations of a very complex optical situation.

Nigel

[JamesF]

I would just like to clarify a couple of points here.

1. Any object in the light path will cause diffraction from it's EDGE only. The solid component, between the edges, will simply reduce the amount of light in the image, it will not increase diffraction. Therefore, contrary to the statement in the link above, thicker vanes will NOT cause more diffraction provided the length of the edge in the light path is the same. Bigger secondaries cause more diffraction simply because the edge is longer than a smaller secondary.

2. The diffracted light from a curved spider vane will be a diffuse area around every bright object reducing the resolving ability of the 'scope. Just imagine the 'spiky' diffraction from straight vanes smeared out 360 degrees around the objects. The effect is to decrease contrast in the fine detail of the image and it will not significantly affect the overall image contrast.

These are highly simplified explanations of a very complex optical situation.

Nigel

Thanks for the clarification, Nigel.  What a shame it is that Mak-Newts aren't really feasible for a home-build

James

[Laurie61]

It's a nice scope to carry out this spider conversion on, at f11 it already has zero coma and tiny secondary, having a long tube should reduce stray light effects. The only thing left to address is diffraction spikes, I wonder whether you could have been braver still and used 1mm material for the spider, this would reduce further the amount of scattered light being distributed around the image.     

[Moonshane]

cheers laurie - it wouldn't actually. see Nigel's point above. the issue with thinner vanes is stability in that if e.g. the spider bends, it reduces the collimation accuracy. so for me an imperceptiple loss of light is better than loss of collimation. as I said above compromise is what scope modding / building is all about.

[John]

....2. The diffracted light from a curved spider vane will be a diffuse area around every bright object reducing the resolving ability of the 'scope. Just imagine the 'spiky' diffraction from straight vanes smeared out 360 degrees around the objects. The effect is to decrease contrast in the fine detail of the image and it will not significantly affect the overall image contrast.

Nigel,

This reads to me like you feel that curved spider vanes could actually deliver worse contrast / resolution than conventional X pattern ones. Am I interpreting this correctly ?

I can't wait to see what Shanes findings are   

[Peter Drew]

In the 1970's at Astrosystems we experimented with all known designs of secondary supports and concluded that for up to 8.5" aperture a single semi diameter strut gave the best overall performance, particularly on planetary observation. 

[Laurie61]

In the 1970's at Astrosystems we experimented with all known designs of secondary supports and concluded that for up to 8.5" aperture a single semi diameter strut gave the best overall performance, particularly on planetary observation. 

Hi Peter, you wouldn't happen to have a pic of your design. I feel a bit of modifying coming on with my 8.75"