Collimating LightBride Netwonians

[Foxinsox]

I need to re-collimate my 12" Meade Lightbridge Truss Newtonian.
I was about to re-collimate it using available information to make the primary and secondary mirrors, spider and focusing tube all square and true but then read the following text in the LightBridge instruction manual which came with the telescope :
" the collimation procedure for the Meade Lightbridge Dobsonian is slightly different from that of other Newtonian reflecting telescopes, because of the "fast" f/5 to f/6 focal ratios's ......... correct collimation requires that the diagonal mirror be offset in 2 directions : (1) away from the focuser and (2) towards the primary mirror , in equal amounts . this offset is approximately 1/8"in each direction. Note that these offsets have been performed at teh factory prior to shipment .."
If this is the case then I have no idea how to re-collimate my 12" LightBridge.
Most newtonians these day have focal ratios of about 5 - 6 . Do others collimate theirs with this offset ?

[Baza]

Have a look at this https://www.youtube.com/watch?v=8G98RTP6jbY
If you haven't moved the spider it should be correct. These guys don't even mention the offset.
Good luck.

[Foxinsox]

Thanks Baza.
I saw this You Tube clip and it looks great. and I was going to proceed with my re-collimation using this method
But the wording in LightBridge manual stumped me. It says to apply significant offsets to the diagonal because of the "short focal ratio"
Í am inclined to just ignore it and just use the conventional settings .

I just wanted to know of others experience if the own LightBridge truss newtonians

cheers

[Jason D]

Lightbridge instructions are half-correct. Yes, you need to offset the secondary mirror towards the primary. To do so, you need to use a sight-tube to ensure the secondary mirror is rounded/centered under the focuser. The instructions are kind of incorrect about offsetting the secondary mirror away from the focuser. This step should have been completed at the factory when the secondary mirror was glued to its stalk. In other words, you do not need to worry about this offset. If the factory did it then you already have it. If the factory did not do it, then forget about it since fixing it would require you to detach the secondary mirror then re-attach it with the proper offset.
To summarize, worry only about the offset towards the primary mirror which is automatically taken care of when you center/round the secondary mirror under the focuser.
As far as the video in the link provided, there was no mention of the secondary mirror offsets because that video assumed the secondary mirror is already positioned correctly in terms of both offsets. The video only covered with is known as axial alignment which is ensuring the laser beam hits the primary mirror center and the returning laser beam hits the emitter.
Hope I did not add to your confusion.
Jason

[Foxinsox]

Hi, Jason

Thankyou for this excellent repsonse.
Last night I simply laser-collimated my primary & seconday, as i had done many times before, and just ignored the confusing instructions in the Meade Lightbridge document . It worked perfectly.
Your response help explain the rationale of the factory instructions .
Thankyou