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
Quote:
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
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