Skyliner 200p Tube Not A Perfect Circle And Affecting Collimation

[lestergibson]

This one did my crust in and searching hasn't turned up an answer so here goes. Sorry for the detail!

I decided to check my collimation after transporting the scope over rough ground for the past few nights. I've been getting what I thought was a decent enough  image in the ep but I thought I check it anyway, just to be sure.

The thing is, my OTA doesn't seem to be perfect circle; it measures 234mm, north to south and 237mm, east to west.

I sussed this out when I made a stiff cardboard disc, as per AstroBabys' instructions to make sure that the spider vanes were centred.. The cardboard disc is bang on, according to my tape measure, but when I removed the rim cover from the end of the OTA and inserted the disc, I noticed a gap between the edge of the disc and the OTA at the 'west' position. I think this would explain why I was having a real nghtmare trying to get the secondary centred while getting the prmary clips into view. If I centred the secondary, there was nowhere enough range in the secondary allen screws to get the primary clips into view. The only way I could get the clips into view was to move the secondary closer to the primary but, of course, this threw the secondary off centre. 

I recentred centred the secondary between the ends of the OTA and took a photo through the colli cap to post here for advice  but when I looked at it, I could see that, although the secondary was pretty much centred, top to bottom, it wasn't centred between the left and rght sides of the OTA. I don't know how to do this but guessed that the vanes must've been off somehow.  I then started playing with the adjustment screws for the vanes and managed to get the holder centered left to right. After doing that, getting the clips into view in the secondary took minutes. This was after grinding away at it for four headbangingly frustrating nights  in a row. After that, getting the primary into line was simple.

The thing is, although the collimation looks good (I think), the secondary holder is now off-centre along with the OTA seeming not to be a perfect circle.

I've attached a new post-collimation photo, taken through the colli cap and another with the cardboard disc inserted into the OTA for you guys to check. I'll be getting the scope out later for a star test but can you tell me what you think of the collimation and whether or not my issues with the OTA and the now slightly off-centre secondary holder are going to cause problems?

Cheers!

 

 

 

 

 

 

 

[Peter Drew]

The collimation looks good to me. To be honest, 3mm out of round, 1.5mm a side is pretty good for a rolled tube on a mass produced item, the end trim could probably pull it nearer than this once fitted.

[acey]

OP: you say the collimation looks good, so that's fine. As to the tube, its shape is irrelevant: some scopes have no tube at all. The secondary doesn't need to be at the exact geometric centre of an exact circle described by a telescope tube. Instead it needs to be at the centre of the optical axis of the primary mirror - which you do by tilting the mirrors. That's what collimation is all about. Looks like you've achieved collimation with offset, as expected, so good luck with your observing.

[lestergibson]

The only reason I asked was that I've read on numerous threads and articles that the secondary holder had to be centred so this is good to know, guys; thanks.

[brantuk]

The secondary does need to be centered - but relative to the primary mirror (not the tube). Some tubes are square shaped or octagonal/hexaggonal, but the key thing is lining up the mirrors properly irrespective of tube shape.

[billyharris72]

I spent a lot of time on this as well, trying to get the secondary holder dead centre in the tube (took the lining off the front and did it with Vernier calipers in the end). The end result was absolutely fine - just like it was before I started! The spider adjustment is mostly about getting the secondary aligned with the focuser tube, and this is important, but - unless you're working with a really minimally sized mirror to increase contrast - you can afford a milimetre or so of deviation from dead centre. Visually centred should be fine - a Cheshire does help, but don't let it drive you insane chasing spurious precision when the gains will be minimal at best - and the mirror as circular as you can get it. The secondary is not an active part of the scope's optics, so as long as all the light from the primary is being aimed down the centre of the focuser tube you'll be fine.

Getting the primary centred in the secondary is definitely more important, and aligning the primary more important still. From the images you've posted, your scope looks spot on. The sort of deviation that youi can spot (e.g. the secondary is closer to the top of the tube than the botom) are well within the tolerances required for good performance.

What's the view like when you get it under the stars? In the end, that's all that matters.

Billy.