Collimation woes - HELP!!

[stash_old]

To honest, you always need the centre spot to collimate a Newtonian. Collimation is less critical for large focal ratio reflectors. The focal ratio is the critical number, not the mirror diameter: there are plenty of f/5 130 mm EQ Newtonians out there and they will have the same collimation tolerances as a typical 10" or 12" Dobsonian. You have a bird-jones, though, and that makes things different. The alignment of the secondary is likely to matter much more than with a regular Newt, for the same reasons that it also matters more when using a coma-corrector on a regular Newt. Look at the first graph in this PDF: http://www.catseyecollimation.com/Newtonian%20Axial%20Tolerances.pdf The first graph is secondary tilt tolerance with and without a coma-corrector. If I'm right, your situation is equivalent to a coma corrector. So you have to nail the tilt alignment on the secondary. Perhaps the primary being spherical means that this alignment matters less. I did a Google search and came up with this link: http://www.cloudynights.com/ubbthreads/showflat.php/Number/4004900 It seems that might be the case, but I didn't read the stuff there in detail. 

Remember the two most important points :

1. Never ever use tools while the OTA is vertical  - slip and thats crying material.

2. Too much  collimation means less actually looking which means less fun - its impossible to do 100% accurate collimation or so the "old timers" tell me.

If you study Astro Baby's instructions as others have said  you will be fine  I was as nervous and  frustrated as hell the first time(s). (still am truthfully) 

[umadog]

Remember the two most important points :

2. Too much  collimation means less actually looking which means less fun - its impossible to do 100% accurate collimation or so the "old timers" tell me.

It doesn't have to be 100% accurate. Like everything else it just has be to be within tolerances for the magnification you're using. You can generally get it within tolerance in under 5 minutes if you're practised and you understand how your tools work.

[F1Bird]

Should I have the focusser racked in or out when I'm doing this? Different guides say different things and some don't make any reference to it at all. Does it matter?

[umadog]

When you're adjusting the primary and secondary tilt you should probably have the focuser at roughly the position it's normally in when you view. This because there is some motion of the focuser when racking in and out, so you'll minimise your errors by trying to keep the rig as close as possible to state it's in when in use.

[NGC 1502]

Should I have the focusser racked in or out when I'm doing this? Different guides say different things and some don't make any reference to it at all. Does it matter?

What I do is this - when centering the secondary under the focuser, I have it racked well out, so I can see a space around the secondary, together with the far end of the focuser draw tube, and judge how centered it is.   I have used a sight tube for this part, slid well in, to just 'contain' the secondary.

For aiming the secondary at the primary, I rack the focuser in so the reflection of the primary within the secondary, shows a bit more than the whole primary mirror.  That way I can more easily judge that the edge of the primary, or the retaining clips, are equally spaced.   You may have to read that last bit again, maybe there's a clearer way to say it 

For me, it's always worked best to first sort the secondary mirror, centered under the focuser, and pointing at the primary.  When the secondary is fully sorted, do the primary, centre the reflection of the spider using only the primary adjusters.   Then fine tune on a defocused star, just using the primary adjusters.

I know there are loads of methods out there, but it works for me, and I've had very positive comments from viewers through my 10" Dob, as to the level of fine detail within Jupiter's cloud bands, full of low contrast detail that's lost if collimation is off.  Of course, a dodgy view can also be poor seeing or not sufficient cooldown.

Regards, Ed.

[Jason D]

Should I have the focusser racked in or out when I'm doing this? Different guides say different things and some don't make any reference to it at all. Does it matter?

For a scope with perfect mechanics, it does not matter. For such a scope, you can place the focuser at any position -- though ideally you want a position that allows the primary reflection edge to coincide with the secondary mirror edge when collimation is achieved. See attachment.

For a scope with less-than-perfect mechanics, you can either position it where most your EPs come to focus or just go half way.

Jason