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woodmeister

My OTA is not circular.

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My skywatcher telescopes tube is not circular and is out by 5mm shorter on the horizontal axis than it is on the vertical axis. I tried slackening the spider vanes off so they were very lose, but the tube remains 5mm off regardless if my spider vanes are all equal length of completely slack.

What would I be better of doing out of these 2 options.

1. setting the spider vanes at different lengths on one side to compensate for the tube not been completely round.

2. Trying to shape the tube back so it is fully round.

Any comments or other suggestions would be most appreciated.

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As long as is does not interfere with acquiring good collimation it will not have a significant impact on the view/performance of the scope. :smiley:   

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Ignore it!  The tube is made from rolled steel and is unlikely to be truly round.  It will have no effect at all on the performance of the scope.  Just tighten the spider enough for each arm to be "tight" but no so tight as to deform the tube.

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Your scope is all about lightpath. This can be perfect whatever path the tube chooses to take. People make scopes with square tubes, with truss poles, with one pole...

Olly

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Just make sure the secondary holder is central in each axis (i.e. Both opposite paires should be equal but you will have a long pair in a line across the centre and a short pair across the centre) and you will be fine. It is one of the least critical aspects of collimation.

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Mine is worse! I was struggling with collimation, until Shane had a look at it at the last SGL - and just centered the secondary under the focusser by eye. I'd been measuring the center of the tube, which wasn't working very well. As long at the secondary is in the right place, the rest doesn't matter.

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Cheers for the advice, I like to double check to make sure I get it right. Sound like an idiot asking such simple questions.

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Not at all. That's how you (and others not confident enough to ask the questions) learn !

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I don't think it's daft. It didn't take you 6 months to realised that the tube wasn't round...

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I don't think it's daft. It didn't take you 6 months to realised that the tube wasn't round...

Yeah when I got my scope, since both the 2 and 1.25 adaptors were in the focuser I put my eyepiece in and tried to focus. Which worked really well at making Jupiter all fuzzy. I am guessing you did not do that?

I have re centred by just looking though the Cheshire and turning the spiders in whichever direction moves the holder toward the centre. My crosshairs and peep hole all line up with the centre spot. However My secondary does not get all 3 of the mirror clips in view. I tried rotating and tilting my secondary but as 1 of the clips enters my view another falls out of view just before. I can see the whole of the secondary mirror so I think it would come down to one of these 2.

1. My secondary might be a bit to close to primary and need pulling back up the holder, or is not quite centred.

2. the secondary is to small maybe (very unlikely)

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Yeah, I can't see all 6 clips with my cheshire on my 250px. I have to use  a little collimation cap - it seems to give a wider view.

I'm going to go with option 3 - it's all fine, everything's probably central as the clips are all on the edge of what you can see, and the secondary is (I hope) central and round, and the Cheshire hopefully has everything aligned.

Mine are like that - just on the edge of the field of view, but I can't get all 6 at once.

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Yeah when I got my scope, since both the 2 and 1.25 adaptors were in the focuser I put my eyepiece in and tried to focus. Which worked really well at making Jupiter all fuzzy. I am guessing you did not do that?

I have re centred by just looking though the Cheshire and turning the spiders in whichever direction moves the holder toward the centre. My crosshairs and peep hole all line up with the centre spot. However My secondary does not get all 3 of the mirror clips in view. I tried rotating and tilting my secondary but as 1 of the clips enters my view another falls out of view just before. I can see the whole of the secondary mirror so I think it would come down to one of these 2.

1. My secondary might be a bit to close to primary and need pulling back up the holder, or is not quite centred.

2. the secondary is to small maybe (very unlikely)

Make sure the focuser is wound all the way in, this will maximise the amount of primary mirror seen in the secondary. :smiley:  

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Yeah I usually collimate with the focuser racked all the way in as the gap between the secondary.

I tried today while it was still light. I could see all the primary mirror clips, I guess that is because I was collimating in daylight rather than a torch. However I could not for the life in me centre the primary. I loosened two applied turns to another, even tried loosening them all then starting from there but there was a very slight gap between the primary and the peep hole and I had no play left.

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woodmeister......hi, My Skyliner had a gap on  its arrival?

I rotated the whole end cap/spider assembly around the OTA until the gap disappeared, but due to the alignment of the screw holes, this may not work for you. It did for me. The secondary needed to be collimated after rotation of the spider cell assembly.

I could have just wrapped some black insulation tape around the perimeter, and be done with it, and  it was pointless/costly sending it back, but was a quality control issue nevertheless.

Edited by Charic

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Cheers Charic, I will definitely check that out on the weekend. I have had some fairly good views through my scope but never had anything to compare them to. Seen syrtis major and utopia on a night of good seeing.

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Even my tal 100 refector is not round.

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Hi guys. Now I am going to sound dumb! ! What is spider vanes? And how and when will you know your scope needs to be collimated?

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Spider vane is the name given to the assembly that is attached to the end of the telescope which centralises and holds the secondary mirror in place, the (normally) four vanes or 'arms' are the 'Spider'

Check this out about collimation  http://popastro.com/documents/PA_jan-mar2009_p12-13_telescopetopics_startesting.pdf There is more on the subject on the web, but suffice to say, if your Star test shows an un-even pattern? Collimate your telescope!

And http://www.galaxypix.com/Stargazing/collimate.html from the same page.

Edited by Charic

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