Collimation of Newtonians - a further query

[Halle]

Hi Everyone,

I have recently bought a SW 10" Newtonian OTA from Steve at FLO and have consequently been refreshing my memory on collimation. I found the video suggested by Rob extremely useful and informative. On refering to a very old copy of Norton's Star Atlas it stated that the circular outline of the flat itself should not be exactly concentric with the eyepiece tube and mirror circles, but slightly displaced towards the upper end of the tube for perfect collimation. Does anyone know the reason for this and how much difference it makes to the quality of the star image?

[George]

Its called offset and its designed to be that way

http://www.oarval.org/collimatE.htm

[Doc]

It's more noticible in faster scopes as well.

[Halle]

Thanks George. The link is very useful and I now understand why offset is needed.

Geoff

[dpastern]

I'm not sure if this is the video you were referring to or not, but I found it good:

http://www.andysshotglass.com/Collimating.html

Dave

[Astro_Baby]

Andy shot glass is a good first pass but he doesnt cover offset in his tutorial - over at Cloudy Nights there was a huge thread on this ( aren't all cloudy nights threads huge ) There was a view that in some ways Andys Shot Glass had confused as many Newt owners as it had helped because fast Newts always have offset. I have to confess this was one of the bugbears I had. Fmailiar with Newtonians but not fast ones when I restarted the hobby and blissffully unaware of the issues regarding offset.

[dpastern]

Interesting. I had a look at the link in the 2nd post in this thread and can see why they've suggested it, although to me at least, it's not terribly well explained on why, but more centres (pun intended) on how.

Dave