[Tannehill]

Hi, welcome to the group and to amateur astronomy.

Short Answer: Not in any practical or meaningful way, most would reckon.

Longest Answer: For an f6 8" dob you can get perfectly fine and usable collimation using the non-offset, partial offset, or full offset techniques. (An f6 is probably not in the “fast” category for Newts, by the way. In comparison to SCTs, yes, but not compared to the now common fast Newt Dobs with f ratios <4.5 out there in abundance)

All the offset is doing is trying to make sure the optical axis of the telescope coincides with the physical centerline of the tube itself (mechanical center). The ideal tradition is “fully offset”, but even "non-offset" won't cause any meaningful image impairment at the eyepiece if you have the primary and secondary collimated otherwise. You could suffer some vignetting of the image if the tube diameter is small and/or if the disparity between the optical and mechanical axes is grossly abnormal in the non-offset situation. But that wouldn't affect image quality at the eyepiece for all practical concerns.

For some computer controlled equatorial or fork-mounted Newts, having the optical and mechanical axes coincide is more important. Critical in some cases. But that just falls away when talking about a visual-use Dob, really.

Depending on the tools you have (sight-tube, Chesire or laser +/- Barlow attachment, autocollimator, etc) you may gravitate toward full offset, partial offset, or non-offset. Full offset requires you to move your secondary mirror by a very small amount away from your focuser and toward your primary mirror: "down and away". The distance down and away, actually, for your 8" f6 is going to be a small fraction of a centimeter, like 1.5-2mm or something. If you secondary is about 1.5” (?) or so, that’d be right…Kinda hard to get excited about a 1.5 mm shift, eh? In the case of a very large very fast dob, that offset might approach 1 cm.

see http://www.skyandtelescope.com/howto/diy/3306996.html

Even most fussy folks with fast dobs are happy with “partial offset” of the secondary. This is moving the secondary "down" but not "away". Off-set toward the primary ("down") is easily done with a sighttube...you move the secondary down until the sighttube shows it to be centered under the focuser tube in the peep hole of the sighttube. You need a sight-tube to do it well. Really, even if you don’t care whether you are off-set or not, a sight tube is invaluable because often the secondary isn’t placed correctly in the first place. It might be centimeters off depending on the QA policy of the manufacturer. I had a cheap newt delivereed with the secondary installed 3 cm too high and backwards many years ago.

A crude sight-tube can be made with a 35 mm film canister in a 1.25” focuser tube with a hole poked in the dead center of the film cannister. Or you can buy a nice expensive commercial one. Chesires and/or laser collimators you are probably going to need to purchase unless you're an engineering wiz with a workshop. Ditto with an autocollimator. You can buy all of them if you want. The collimat-aholics out there (I think I’m one, now, sadly) would say you should.

A purist will also tell you to square your focuser, such that the focuser tube central axis points straight and also straight “at” the secondary, and not angled off to a side. (and so, it follows that squaring the focuser should really be your very first step in this whole process, even before doing the secondary). The sighttube can help you do this, also, as can a laser collimator (without the Barlow attached). It’s worth checking in your LB…..focuser squaring problems are a common unrecognized source of image degradation…often noticed with changes in eyepieces as the focuser racks farther in and out of collimation.

For the best compromise in your case, a sight-tube and Chesire could get you close enough. In truth, there are users who just use a sight-tube and can eyeball the primary collimation (provided they have a mirror center spot) close enough with that, and forego the Chesire step. That’d be fine with an f8 or f10 newt…

One can argue the practical value of going the last step and using an autocollimator with an f6 scope. It will be better if you finish with the AC but it won’t be as obvious as it would be in an f4 or something, where the coma-free field (sweet spot) is much smaller.

If funds are tight (and aren’t they for all of us?) I’d say get a sight-tube and a Chesire, find a helpful veteran in Adelaide to guide you, center the secondary mirror under the focuser using the sight tube and thereby accomplish your partial offset, dial in your primary with the Chesire, get that routine down on your own (since you’ve a truss and will need to do this often) and start observing!

You're a physicist, so you'd pick up this concept far faster than most of us. Join the Yahoo group Collimate_your_telescope, and you can "hear" the gurus of collimation (Nils olof Carlin, Vic Menard, etc) discuss and spar about the general concepts and the bewilderingly complex nuances. You can search the old threads to answer questions. And ask questions for free, and get very erudite and practical answers in return.

Scott

Originally Posted by a1120028

Firstly though Hi to all, I'm totally new to telescopes and on a whim decided to go buy one after doing a filler astronomy course at uni. My main area of study has always been theoretical physics never "hands on".
I must say I'm hooked, have aperture fever already and am going mad trying to work out how to properly collimate my 8" light-bridge. I must of read 10 articles on collimating all say different things so I really need help. Especially on how the offset of fast focal length scopes effects collimation. HELP!!!!