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Moonshane

Curved spider vane for my 6" f11 dob

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I spoke to Orion Optics about fitting curved vane spiders a while ago and they were very negative about them. I didn't understand why at the time as they seemed to be a good idea. Obviously things are more complex than first appears.

The proof will be in any improvement or otherwise that Shane sees.

Perhaps the old, two thick vane support on my old SX250 isn't so bad after all :-)

Stu

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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.  :smiley:

Assuming I'm understanding this correctly as being a support similar to that used in the Heritage 130, I guess perhaps shouldn't be entirely surprising.  If the diffraction is a function of the "edge length" of the spider rather than the cross-sectional area perpendicular to the optical axis (if you see what I mean) then a single arm supporting the secondary could be two or three millimetres thick to give high rigidity whilst at the same time minimising the "edge length" to reduce diffraction.  The light loss due to the extra thickness of the spider arm would presumably be minimal compared with that lost because of the size of the secondary or the end of the focuser draw tube poking out into the OTA.

Why is there a limit at 8.5"?  Was that an optical issue, or just the engineering problem of supporting the secondary sufficiently well?  Or did you not test larger apertures?

James

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John,

If the same length of spider vane was used but bent then the amount of diffraction, that you would have seen if they were straight, will now be spread evenly around the object. Therefore at any given point around the object there will be a similar amount of light "scatter". With straight vanes the "scattered" light is concentrated in the spikes leaving clearer gaps where a companion object can be detected and brighter areas where a companion could hide. The result is that close details can be less obvious with bent vanes. So Yes, High frequency contrast and resolution are reduced.

However, if a single curved vane is shorter than the straight vanes it is replacing, then there will be less total "scatter" and that will give better image contrast and resolution than the double ring often used.

The curved vanes should go through 180 degrees as near a semi-circle as possible to give equal weight to all orientations. If a single vane only goes through, say, 90 degrees then the two diffraction spikes will spread through 90 degrees leaving 90 degree gaps and there will be many "mini-dumbells" to look at. I wonder if Shane will see that effect.

Interestingly, there is an article in the October Sky & Telescope, p30,  where there are a couple of photos of Sirius A and B by Dennis di Cicco from 1975. They were taken using a 14" SC telescope with and without a hexagonal mask. Without the mask it is difficult to positively see the Pup but with the mask scattering the light into six spikes the Pup is obvious. It is also clear that if the mask was rotated then the pup would have been invisible behind a spike.

Nigel

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John,

If the same length of spider vane was used but bent then the amount of diffraction, that you would have seen if they were straight, will now be spread evenly around the object. Therefore at any given point around the object there will be a similar amount of light "scatter". With straight vanes the "scattered" light is concentrated in the spikes leaving clearer gaps where a companion object can be detected and brighter areas where a companion could hide. The result is that close details can be less obvious with bent vanes. So Yes, High frequency contrast and resolution are reduced.

However, if a single curved vane is shorter than the straight vanes it is replacing, then there will be less total "scatter" and that will give better image contrast and resolution than the double ring often used.

The curved vanes should go through 180 degrees as near a semi-circle as possible to give equal weight to all orientations. If a single vane only goes through, say, 90 degrees then the two diffraction spikes will spread through 90 degrees leaving 90 degree gaps and there will be many "mini-dumbells" to look at. I wonder if Shane will see that effect.

Interestingly, there is an article in the October Sky & Telescope, p30,  where there are a couple of photos of Sirius A and B by Dennis di Cicco from 1975. They were taken using a 14" SC telescope with and without a hexagonal mask. Without the mask it is difficult to positively see the Pup but with the mask scattering the light into six spikes the Pup is obvious. It is also clear that if the mask was rotated then the pup would have been invisible behind a spike.

Nigel

Thanks Nigel.

My 12" scope uses the double ring approach as shown in the photo I posted earlier in the thread. As the scope has resolved Sirius for me and shown the central rille in the lunar Alpine Valley, amongst other things it must be doing something right !

Of course it's now tempting to wonder if it would do better still with a conventional spider design instead but, as a refractor fan, I don't miss the diffraction spikes to be honest  :smiley:

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Our 8.5" limitation was imposed due to potential stability issues, I have used a single straight semi diameter secondary strut on a 17" with good results. The struts had a central boss slightly smaller than the secondary, the strut tapered outwards from this to a radial foot, a bit like a traditional anchor which had a radius to suit the inner wall of the tube. These items were initially bandsawed from 12mm aluminium plate, turned in a lathe for the radius and the other surfaces were cleaned up with a file. Later, an extrusion was obtained that just needed cutting for depth.  :smiley: 

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maybe one day I'll actually try it! I've been waiting a whole two days now and it's very frustrating :grin:

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well, my first light with this was tonight and I am somewhat disappointed really. looking at Vega the vane did remove the main four diffraction spikes and created a bloom either side of the star which was the diffraction being smeared across the field. however, it also created a number of other spikes all around the star making sharp focus difficult. splits of Epsilon Lyrae, Delta cygni and Pi Aquillae were routine and easy but not as sharp and clean as with my old four vane spider.

I think I'll be reverting back to that at the weekend. well, for the cost of £5 total, it's not the end of the world. I may just appreciate the fine scope that this is and use it in four vane mode or possibly try another format in the future.

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That's a shame :(

Perhaps what's required is a "rotatable spider", so you can move the spikes to a different orientation in case something is hiding behind one :)

James

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ha ha true James. I have just switched it back to the old spider and after an approximate collimation (in the dark of course!), it is back to normal and feels and looks a lot better I think. I'll be leaving it for now but may try again in the future with a different design.

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Disappointing Shane. Perhaps there is more to getting the correct shape ?.

Mine seem to work well and I'd soon be complaining if I was seeing odd effects and I guess the David Lukehurst ones must work well for him to use them.

Thanks for trying a design out a reporting on it though  :smiley:

Maybe try another design at some point when you feel inclined ?.

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no doubt I will John. in a way I did this on a whim and in truth did not appreciate the importance of the 180 degree factor. I suspect this will make a big difference. also the fact that my secondary is so small affects things I think as I suspect the bolts that hold the secondary vanes to the central hub intrude into the light path slightly with both designs. this sort of thing is what gives OOUK a bit of a bad name sometimes as it's a bit slack. I may still reduce the diamater of some parts of the central hub where I can. dob modding is such great fun even when it goes wrong!

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Though it's frustrating, I think we often learn more when things don't work than we do when they succeed   :smiley:

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Shame it didn't work out Shane. I'm sure it is to go with the need for a smooth, consistent radius 180 degree curve to ensure the canceling effect works best. Maybe worth another go in future with a different design?

Stu

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I am looking forward to your report.

By the way make the most of planned nights out with your wife. I got home the other night after going to cotswold astro society and my wife had set up my 14 inch scope to cool down ready for me when I got home!

Mark

Sent from my BlackBerry 9320 using Tapatalk

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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

Hi Nigel,

Thanks for the information and clarification.

I know that diffraction only occurs on the edges of objects and that the area of material does not affect the total amount of diffraction. I am, however, curious about the effects of thickness of vanes on what you see.

For instance, what would be the difference between two 2 vane spiders, one that has 0.5mm vanes, and one that has lets say 10mm vanes?

My expectation is that they would both exhibit the same level of total diffraction but the thin vane spider would concentrate the light into tight spikes, but the thicker one would show broader, less intense spikes. Is that approximately correct?

Stu

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Hi Nigel,

Thanks for the information and clarification.

I know that diffraction only occurs on the edges of objects and that the area of material does not affect the total amount of diffraction. I am, however, curious about the effects of thickness of vanes on what you see.

For instance, what would be the difference between two 2 vane spiders, one that has 0.5mm vanes, and one that has lets say 10mm vanes?

My expectation is that they would both exhibit the same level of total diffraction but the thin vane spider would concentrate the light into tight spikes, but the thicker one would show broader, less intense spikes. Is that approximately correct?

Stu

Interesting question and, having a brief think about this, I have decided that much deeper thought is needed before I can come to any conclusion.

My initial thought was that there would be no difference, but it might just be that different thicknesses of vanes would produce subtle  differences in effect.

Nigel

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Thank Nigel. I look forward to hearing your thoughts after further consideration.

It certainly seems to be a complex subject.

Cheers,

Stu

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Very interesting read Cath, thank you

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Yes, very interesting - thanks :)

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Thanks for the link - that's a helpful summary of the influence of the spider on diffraction. Ed

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