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Lightbridge Spider Vanes Wrong - Help!


Jim-a
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While I was trying to sort out some double diffraction spikes I'm seeing, I took the spider vane assembly out of my lightbridge.

I'm pretty sure that there is a problem with the vanes - and I'm pretty sure that it was probably an assembly error in the factory. I'd be grateful if you guys could double check that I'm not barking up the wrong tree.

Here are a couple of pictures of the spider assembly that show the problem.

post-23126-0-57452500-1348916181_thumb.j

post-23126-0-77507100-1348916197_thumb.j

and heres a sketch I did that shows the problem exaggerated

post-23126-0-60754500-1348916251_thumb.j

and heres a sketch to show what I think it should look like

post-23126-0-90950800-1348916463_thumb.j

The end effect of how the vanes are means that the secondary is skewed with respect to the focuser. Which would explain why I've never quite managed to line up all the collimation-ducks in a row (e.g. when the centre-spot was centred in the secondary, the primary mirror clips didn't look equidistant from the edges of the secondary).

So question 1 is... am I talking rubbish... or is there something amiss?

Assuming I'm onto something here, I reckon this must have been an assembly error in the factory - all four of the vanes are identical, but because of the way the bolt holes are positioned on the secondary holder two of the vanes will always be upside down. Heres a sketch of how the vanes are attached looking down from the top.

post-23126-0-91091100-1348917401_thumb.j

My assumption is that either there should be two 'flavours' of vanes and that the factory fitted all the same ones, or the bolt holes to attach the vanes are in the wrong place.

So question 2 is... can any Lightbridge owners have a look at their vanes and see if there are in fact two different types of vane, or if the way its bolted on is different from my sketch.

Question 3 is... what can I do about it? Its not a new scope so whatever warranty was there would have run out long ago. I reckon the easiest option would be to find someone who can drill and tap new holes so that all the vanes can be fitted the same way up - anyone have any comments / alternative suggestions?

Sorry to be long-winded and thanks in advance to anyone who can shine some light on this.

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Looking at your last sketch, you can spin 2 vanes half a turn..... EITHER the north and east positioned ones, OR the south and west positioned This will make them all the same orientation and take the skew out of the secondary.

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Would

Looking at your last sketch, you can spin 2 vanes half a turn..... EITHER the north and east positioned ones, OR the south and west positioned This will make them all the same orientation and take the skew out of the secondary.

Would't that offset the vanes though?

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Looks like as you say a factory botch up! Possibly contact the dealer/ shop you bought it from for a replacement, or if out of warranty it shouldn't be too difficult to drill and tap a new hole in the right position if you are reasonably handy at DIY.

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Hi Jim. Actually, I don't think you have a problem here.

Your opening post mentions 'double diffraction spikes'.

I can see what you mean in the upper four pics, but any skewing here can be sorted with the adjustment screws on the secondary itself, to put the secondary mirror in the correct place, relative to the focuser and primary. The spider vanes can be all sorts of shape, a single stalk, a single full diameter vane, three vanes, four vanes, curved vane etc, and it's debatable as to the pros & cons of each, but what really matters is that the secondary mirror is in the right place, and if the vanes are like yours, I wouldn't worry too much.

If the skewing bugs you, (I suppose it would me a bit) you could do a DIY fix on the vanes, and refit & collimate. But the secondary can be adjusted to the correct place either before or after the fix.

In the lower pic, if the vanes are offset, it would give either a thicker diffraction spike, or a double one, so I'd make sure the vanes are inline with the opposite one, as viewed from the front of the tube.

But I think that you don't need to replace any parts, just a DIY fix.

Regards, Ed.

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Nevy - thankyou so much for taking the time to take those pics and post them - really useful.

Lorne - Eureka!! I think you may have solved it - just couldn't get my head round it (defeated by a combination of rotational and reflection symmetry) - I'll give it a go tomorrow and let you know how it pans out.

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Lorne was right !! - I swapped two of the vanes and now everything is as it should be, haven't had chance to collimate it yet, but hopefully it will be easier now the secondary holder doesn't have a skew to it.

Thanks for your help everyone

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