Many of you don't know me, but I bought a scope because I had 70x15 bino's that I loved using; I spent 15+ nights learning the scope and eyepieces and practicing focusing with barlows etc (this was all before christmas as I've now had a kid who needs alot of attention - I'm now planning to get a night a week or so set aside just to use the scope).
I have a 8" GSO Dob from andrews and a couple eyepieces etc.
But I read quite alot about the mirror in the GSO dobs being held too tight - so I'm thinking of losening the screws that hold the mirror and making sure they not too tight - but will I wreck the Collimation on the scope? I've read a couple things about Collimating without a tool - just looking into the eye piece and seeing your reflection to test - But is this accurate? should I buy a Collimating tool before I loosen screws?
Sorry for the mass block of text post I hope someone replys
Heres an exerpt from my upcomming how-to you might find useful.
You may not have to do anything. Check first.
Star testing for collimation and astigmatism
We can check for correct collimation and detect astigmatism in our scopes optics by performing a “star test”.
Make sure you put your scope outdoors with covers off for at least 60 minutes before starting because to successfully star test requires a properly cooled scope and also steady atmospheric conditions.
Take your highest power eyepiece (shortest focal length) eyepiece, find a nice medium bright star and focus in the center of the field of view.
Now the adjust the focuser outwards slowly until the star defocuses and you see a series of rings like in fig 1 below.
(insert startest.gif)
If you see nice concentric round rings like in A. your scope is well collimated (assuming your secondary mirror is correctly placed).
Most likely it will look more like B, indicating some tweaking is required. If it looks like C, your scope is way out of alignment!
If you are getting nice round circles all is well. But what if the rings are not perfect round circles?
Often the outermost ring will appear a bit “hairy” and waver about a bit. This is due to either atmospheric conditions, or a scope that hasn’t cooled fully down to ambient temperature.
If the image is reasonably steady, take note of the general shape and rack the focuser in and out on both sides of focus.
Does the image look a bit like a three/four leaf clover or triangular/squarish?
This usually indicates astigmatism in the primary mirror caused by the three/four rubber holding clips being clamped down too hard.
Does the image appear oval or some other shape and shift plane by 90 degrees either side of focus?
This could indicate a pinched secondary mirror caused by its plastic holder being too tight. Try moving your head to rotate your eye relative to the eyepiece. Does the plane of the image follow the movement of your eye or stay fixed in relation to the scope?
If it moves with your eye I’m sorry to inform you that’s where the astigmatism lies. If its in the scope there’s still hope ! J
MintSauce, You can safely loosen the lock screws without effecting the collimation. These should NOT BE OVERLY TIGHT anyway, just nipped. Check article in S&S.
*http://skyandspace.com/printable/how...ticle_790.asp*
HTH.
Just the facts m'am.
ps. similar/same way as Geoff's.
pps. A Cheshire is an easy, cheap aid.
I've not been away, only posting less - I've just not been using the scope much at all (still don't know many targets, I look at the same things each time with amazement).
Thanks for the tips, so if I centre spot the mirror then I can use a film can? (I've read about a pin hole in film can collaminating)
if I get a chesire do I still need to centre spot the mirror?
Yes, you will need to mark the centre of your mirror to collimate it with a collimation cap or cheshire...most folks do a "spot" (some already come that way).
I read somewhere on the net about someone who used a triangle (instead of a circle) aligned to the collimation screws...they claim it helped them speed up the process (interesting idea).
There are several opinions about the best way to mark the centre of your mirror, mine came marked so I don't have an opinion on this.
I did get a chesire collimating eyepiece (Orion). I highly recommend it as it works a treat for me...
Some people will recommend a laser collimation tool but, for me a simple solution is better. It does not get much more simple than no moving parts and no batteries.
I'm the sort of person who wants to take it apart and figure how it works tho - even tho it's not broken. Same goes for the focuser - I want to dismantle and clean out that grease that I hear about, it is a little sticky.
So I may centre spot the mirror and try a film can, I really might just order a cheshire actually - anyone got and links to good sellers?
My scope already came centre spotted as well but I did read somewhere about how to mark the centre of your mirror by folding a circular piece of paper the size of you mirror into quarters. You cut out a hole in its centre and then overlay it on the mirror. Place some sort of marker there (like a cirular sticker or something) and there you have it, centre spotted. But how to get the paper onto the mirror?
I hope someone replys 
