How to Cheshire and center spot primary mirror

[jackson.arw]

Hello again all,

I have a Sherwood DS 144 and have got a laser colimator and have just recieved a Cheshire from FLO as well. How do I use it though ?? Look through it and twiddle some stuff seems the obvious starting point but what should I see?

Also

My primary mirror dosent have a center spot and I can see no way of taking it out of the bottom of the tube ... I can loosen the 3 fixing screws and it comes loose inside the tube but I guess that isnt ideal .. any ideas other than a hammer ?

Thanks

[barkis]

Placing a centre spot on the main mirror is not really necessary in my opinion.

You more than likely would place the spot on the physical centre, which is not guaranteed to be the exact centre of the optical figure.

Having a spot is a guide to collimation, but final adjustment is best done on a star. If you insist on have a spot placed on the mirror, be careful when removing it from the scope, and when sticking the spot on. the aluminium coating is easily marked. A sleek, or scratch can not be removed. If you are worried about doing it, then don't.

Ron.

[ZOG]

Mel (Astrobabe) has written an excellent tutorial on collimation. Can't find the link ATM but I'm sure someone will be along who can.

Edit, found it

http://www.adur-astronomical.com/collimation/astro%20babys%20collimation%20guide.htm

[Astro_Baby]

The perfect centre spot is a punch hole reinforcing ring. Finding the exact centre of a mirror can be done by making a template of the mirror then cutting it out and folding it into quarters. Cut a small notch at the end and voila - when unfolded that should give you the mirrors physical centre. You need to be pretty accurate on that.

Sont know the scope you have so no idea how the mirror may be taken out.

[jackson.arw]

Thanks for the advice everyone - read Astro Baby's guide some time ago, superb Ill just make the best I can out of the mirror as I dont fancy breaking it and there is no screws/bolts to remove the cap at the bottom of the tube ... not like I can see much other than slowflakes atm anyway ...

I think the scope is a precursor to Skywatchers but I may be wrong there.

[acey]

If the 144 refers to aperture then it's not going to need a great deal of re-collimating. Don't try removing the mirror to centre-spot it until you've had the scope long enough to not feel nervous about it (though it sounds like you're there already). Best do it when you think the mirror needs cleaning (ie it's really dirty). You won't really be able to use either the Cheshire or the laser without a centre spot (or ring in the case of the laser), but you can use a punctured film cannister to get things as good as you can manage, and a star test to check, as suggested. Maybe you've been doing that anyway - so if you're really brave enough to remove the primary then you'll probably just need brute force once the holding screws are removed, and do exactly as AstroBaby suggests. You may find it hard to get the primary back in the tube - whenever you take it out, note how the screw holes are aligned so you can replace it in exactly the same position if your tube happens to be not perfectly circular or the holes not exactly spaced.