optical and tube alignments - how do I do this?

[bkm2304 (Richard Brown)]

The other day I was collimating the Lightbridge and I started turning the knobs on the main to line things up. As I did so I realised that I had no idea whatsoever of whether the main was aligned with the tube assembly. I also realised that I had no idea how to make sure his was the case - and I'm sure it wasn't.

Does anyone know how to check this on a truss dob?

Does it matter all that much if it is out by a wee bit?

Thanks

Richard

[Blue Skies (Jacquie)]

Isn't that why you do the collimation thing? To line the mirrors up?

Ideally your primary mirror cell should be square within the mirror 'box' (round as it is on the Lightbridge) and the focuser should be 'square to the secondary cage and the truss tubes fitted so that the mirror box and secondary cage go together square, or near enough.

This is one of the things I love about telescopes - while things need to be ultra precise to work at their best, there is an amazing amount of error margins built into them!

This is why we allow all that movement for the adjustment of the two mirrors and the focuser goes in and out - it doesn't matter if things are not super precise, we've allowed all this adjustment room in there.

Of course, if you do make it precise as possible you'll find that set up time is less, because you've got less adjusting to do each time.

So in my opinion, no, it doesn't matter if things are out a wee bit at all.

[Tannehill]

Is it critical that your optical axis coincide exactly with the long axis of the tube? No. To do that "perfectly" in a Newt means you have to offset your secondary a smidgeon (like a few mm only) away from the focuser. The other direction of offset (moving the focuser "down" toward the mirror) is actually automatically done when you use a sighttube to center the image of the secondary mirror under the focuser drawtube. This latter shift you should do as part of collimation. Not doing it will result in fair amount of vignetting of your light cone and detract from image quality.

There are many references to this issue, but a brief discussion by Bryan Greer (the owner of ProtoStar, a sponsor of IIS) on the skyquest-telescopes Yahoo group (message number #36822) explained it well. I won't paste it verbatim here, but you can PM me and I'll send it to you. The long and short of it is, you can just ignore that "full offset" issue and go simply with your "partial offset" set up (partial meaning you've only moved the secondary so it's centered under the focuser drawtube) and lose nothing.

There are some GOTO and tracking setups wherein alignment of these two axes is important, but it'll be impossible to notice it in a visual use newt.

Scott

Originally Posted by bkm2304

The other day I was collimating the Lightbridge and I started turning the knobs on the main to line things up. As I did so I realised that I had no idea whatsoever of whether the main was aligned with the tube assembly. I also realised that I had no idea how to make sure his was the case - and I'm sure it wasn't.

Does anyone know how to check this on a truss dob?

Does it matter all that much if it is out by a wee bit?

Thanks

Richard