Collimation help needed

[vinny]

I hope that i have posted this in the correct thread but i am baffled with how to collimate my scope,

I think i have got my head round on the principles but the one thing thats standing in my way is neither of my scopes (114 newt and 200 dob) have a centre mark on the primary mirror.

I seem to have watched a video at sometime on how to find the centre of the primary and mark it so a laser collimator can be used ,but i have searched and searched and cannot find any info on the subject

[Mark at Beaufort]

Vinny

I think this is the link that you require.

Collimating a Newtonian

Mark

[vinny]

iv seen that one but it assumes that you already have a mark on the centre of your primary

[acey]

If you want to collimate accurately then you need a centre mark, so you need to take the mirror out. Various ways to find the centre - e.g. a couple of strips at right-angles over the mirror, then you push down gently at the place where they cross to fix the mark. Or cut a piece of paper to the size of your mirror, fold it twice to find the centre and make a little hole, then place this over your mirror so you can see where the centre is.

For the centre mark use a ring-shaped hole-reinforcer. Don't be too nervous about sticking paper on your mirror - the centre is obstructed by the secondary and nothing reflects from there into the eyepiece. You could scratch the centre of the primary and it'd have no effect on the image.

[crovax]

That's my guess guess only but I think you'll have to do that by eye then.

[Astro_Baby]

To find the centre measure the diameter or calculate it from the circumference ( bear in mind mirrors are not always exactly what they say on the scope - ie a 200mm mirror may actually be 203mm).

Draw that onto a sheet of paper and cut it out very carefully. Now fold the paper into four. Where the fold lines meet is the centre.

An alternative ( and very cunning way ) of doing it is get some good quality tracing paper. Draw the circle onto it and calculate its centre (do a google search as there are various methods of doing this). Cut the tracing paper a good inch and maybe two LARGER than the circle you have traced. Once done offer it up to the mirror and compare the mirror to the circle you have drawn on the tracing paper. It must be an exact match.

Now apply a small piece of low tack tape thats folded over so its sticky on both sides across the centre mark on your tracing paper. You can do this with a small piece of low tack scotch tape thats folded into a loop with its sticky side facing out. Press this to the centre of the tracing papers circle and make sure its nice and flat.

Apply a reinforcing ring sticky side up to the low tack tape. So you now have the reinforcing ring on the center of your tracing paper held by the low tack tape and with the rings sticky side up.

Carefully offer up the tracing paper to the mirror with the reinforcing rings sticky side facing the mirror and get the edges aligned so the mirror fits exactly the circle you drew. Lightly press the tracing paper behind the reinorcing ring to transfer it to the mirror.

Voila - job done. the Catseye collimation system uses a similar way to get its red triangle placed onto the mirror.

[vinny]

thanks for all you help people i think i have sorted it now and it gave me a chance to dust my mirror off with a soft paint brush as well