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How to remove secondary mirror support from Dob


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Before you remove the secondary and the vanes. Measure the centre of the vane from each side. Then measure again or make a template out of card so you can replace the vanes in the correct position as near as centred as poss. Remove secondary mirror then remove vanes using the small locating screws you have identified. You could consider fitting Bobs knobs when you put everything back which will make collimating much easier. Best of luck. 

 

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Excellent, thanks. Do you mean measure the centre of the secondary holder from each side? I'm new to Newt maintenance!

Yes I've already fitted Mark's nobs (like Bobs but cheaper) and I intend to flock the secondary edges whilst it's out. Do people flock the vanes too?

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Not sure I've understood actually - do I remove the secondary holder by unscrewing the central screw before removing the vanes from the scope? Not sure how to remove the vanes from the scope.

It looks like if I twist the big knobs on the outside of the tune (pictured above) it will twist the spider vanes. Could be wrong, but they are very stiff.

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9 hours ago, Commanderfish said:

Excellent, thanks. Do you mean measure the centre of the secondary holder from each side? I'm new to Newt maintenance!

Yes I've already fitted Mark's nobs (like Bobs but cheaper) and I intend to flock the secondary edges whilst it's out. Do people flock the vanes too?

Yes I made a small template from thick card that spans the diameter of the tube opening and marked the centre of the card. Loosen the centre screw  holding the mirror first. Not heard of the vanes being flocked. You also need to hold the vanes tight while you unscrew the holding screws or they will twist.

 

 

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I don't see any reason to loosen the centre screw, it just makes it more likely that you will have more to do collimating it when you put it back in.

As Steve says, either remove the primary first or put the tube horizontal to avoid accidents. Hold each vane straight/upright as you undo the thumb wheel on the outside of the OTA so they don't twist. It can be a little fiddly getting the threaded vane holders backed through the holes but you just have to shuffle the whole thing one way to make space if that makes sense.

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4 hours ago, Sirius Starwatcher said:

Yes I made a small template from thick card that spans the diameter of the tube opening and marked the centre of the card. 

 

Thanks!  Is the purpose of this to make sure the secondary is exactly centered in the middle of the tube, or is it to ensure that we replicate the actual position of the secondary, i.e. repeating any existing offset from the exact centre of the tube?

Yes, I always operate with the tube locked in the horizontal position.  A safety feature of the dimensions of the 16" F4 tubes is that it's not physically possible to begin any kind of maintenance with the tube in the vertical position! Well not standing on the ground anyway...

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OK so if I've understood correctly, I'm trying to ensure I put the spider and secondary holder back exactly in the center of the tube. The secondary is already offset on its holder. Is that right?

 

 

 

I've made a cardboard template but I don't really know what I'm doing with it as I can't see through it to mark anything!

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18 hours ago, Commanderfish said:

OK so if I've understood correctly, I'm trying to ensure I put the spider and secondary holder back exactly in the center of the tube. The secondary is already offset on its holder. Is that right?

 

 

 

I've made a cardboard template but I don't really know what I'm doing with it as I can't see through it to mark anything!

Hi Commander I'm not in the uk at the moment so can't forward photos of my template would be happy to do so this weekend if their any use to you.

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If the vanes get twisted or damaged when being removed or refitted is there a right or wrong way of straighting them? Are replacement available? I understand curved vanes can reduce star spikes? (sorry don't know the correct terminology) 

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On 9/30/2016 at 16:53, -Dougal- said:

If the vanes get twisted or damaged when being removed or refitted is there a right or wrong way of straighting them? Are replacement available? I understand curved vanes can reduce star spikes? (sorry don't know the correct terminology) 

Don't know about untwisting the spider vanes, can probably do it with a pair of pliers.  Can buy new ones too.  Curved vanes can be purchased to combat diffraction spikes, but my understanding is that they do not remove the diffraction spike, instead they smear it across the image so the spikes aren't as obvious, but you may end up feeling that this degrades the view.

Personally I'm not planning on changing to curved vanes, I've got a lovely Apo frac for bright stars :). I also suspect that I'd prefer the  straight diffraction spikes to the curved vane alternative.  The straight diffraction spikes remind me of stars drawn in the illustrations of childrens books :)

 

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Whatever you do don't undo the centre bolt screw or play with the smaller collimation screws just yet.  The three tilt screws may need a slight 'tweak' afterwards, but they may not...and we wish for you to avoid a headache!

For the secondary mirror to 'look up' into the focuser, then vanes can only be in one plane.    Undo the four vane locking screws which are on the outside of the tube.  Be careful and do not let the assembly slip so that the mirror hits or touches the side wall. The whole vane and mirror assembly will then slide out in one piece.  When you want to put it back, just put it back so that the secondary mirror again 'looks up' into the focuser and make sure that the vanes are equal in length on all sides.

The central bolt of the secondary moves the mirror up and down the tube.  Do not touch this if you can help it.  It is the cause of much pain in collimation.

The three smaller collimation screws tilt the secondary mirror so that it points at the centre of the larger primary.  You may need to tweak these after you have replaced the spider vane / mirror assembly.  You will need a laser collimator, or a Cheshire eyepiece.  The information given by Cheshire is harder to grasp, but once understood it is in my opinion much better.  However, a laser will do you ok if you're just starting out in this.  It's ok for you to adjust these three screws if you're a bit unsure, there are some good vids showing laser collimation on Youtube and it's a simple process with a laser - provided that you haven't messed with the central bolt.

Finally - the vanes - you won't damage them doing this.  When you re-install them, they do twist slightly due to them being torqued by the vane screws on the outside of the tube..  All you do is use your fingers and give the vanes a little twist so that they are looking nice and straight again.

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Excellent, thanks Steve - that's exactly the info I need to prcoeed with some confidence before attacking the stiff screws holding the spider vanes.  

My tube is likely to be not quite circular too as it is rolled aluminium with a seam on one side (i.e. the tube may be slightly oval). I'll note the measurements of each vane as I suspect the two pairs may be different.  

A Baader Laser collimator came with the scope and I am happy collimating both primary and secondary with it.  The laser collimator itself is nice and central.  I've replaced the secondary collimation screws with knobs thus avoiding use of tools.

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OK so I drew up my template and took the secondary out.  Flocking is in progress  - it's a big ass tube so not finished yet!

I measured the distance along the edge of each spider vane from the tube edge to the edge of the secondary holder.  For reference, the focuser was pointing to the right as you look down the tube.  There is a seam in the tube on the left where it looks like the tube bows outward a bit.  AS you would expect, the vbane going up and the vane going down where almost exactly the same length from OTA wall to secondary holder.  There was a difference of 2mm which I take to have just crept in from transport, heating and cooling.  I will set them back to exactly even when I reinsert.  Now here's where it gets slightly odd.

The right hand vane, i/e/ the one going to the focuser, was the same distance from OTA wall to secondary holder as the upward and downward vanes.  However the vane opposite it, i.e. the left hand vane, was longer by about 1 cm (I've got the exact amount written on my template at home).  I note that this vane mounts close to where the seam in the tube is, where I think maybe the tube bows out a little, but this couldbe optical illusion due to there only being a seam in one place.

So the question is, when I reassemble, do I set this pair of vanes how they were before, or do I set them to be exactly equal lengths to each other?  Or does it not really matter?  Obviously whatever I do, I will adjust the secondary tilt during collimation when everything is reassembled.  The scope was colimating well and giving sharp views before.  Though I have read that the secondary being slightly off centre is not that big a deal if the secondary tilt and primary collimation are correct - is that right?  It's a big scope (400mm) so if the secondary is 5mm to one side maybe it doesn't matter?

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