collimation HELP!

[colesmat]

Last week I "started" collimating my 200p dob. After a few hours of gave up. Anyway, I got back to it tonite and had better luck (or more patience). But..... looking through the cheshire, every thing is well out! I have the primary circle centred to the cheshire but the reflection of the secondary is way off. It is so far off of can see quite a bit of the tube (like a crescent). Does my focuser need adjusting? The spider vains are all even and the secondary central but I'm looking down the tube at a really weird angle. I've followed astro babys guide to see tee but there's something not right here. Any advice is appreciated. Thanks.

Matt

[NGC 1502]

Hi Matt, I'll try to help, but there are lots of differing opinions out there..........

Do you have a simple collimating cap ? You can get good collimation with one, then fine tune on a defocused star.

With the cap in the focuser, get the secondary central under the focuser, equal space all around. You may or may not need to adjust the secondary closer to, or further away from the primary, or adjust the spider vanes. You may also need to rotate the secondary. Having got the secondary under the focuser, use the 3 secondary adjusters to get the reflection of the primary centered. It's easiest to do that if you can see the primary mirror clips all equidistant at the edges of the reflection.

It's crucial at this stage to IGNORE the reflection of the spider vanes.

Having got the secondary accurately under the focuser, and the reflection of the primary central (using the mirror clips) then without touching the secondary again, centre the reflection of the spider vanes by adjusting the primary, and using the centre 'doughnut' on the primary mirror.

You are now close to being collimated. You can fine tune it with a defocused star at high power (150-200x), by tweaking the primary mirror to centre the secondary shadow within the image of the defocused star. Check that with the defocused star in the centre of the field of view.

Now, this can be contentious, but it works for me and Gary Seronik of Sky & Telescope uses this method (especially if the scope is not too fast, like the 200p Dob).

Please guys, if you have a better method, let us know, but like I said, it works for me...........

Hope you sort it, Ed.

[astronymonkey]

Hi Matt,

It sounds like the primary is out a bit, try tightening the collimation bolt which is on the opposite side of the tube to where you can see the "crescent" . When you look through the focusser move your hand around the top end of the tube until you can see it opposite the crescent, now slide your hand straight down the tube to the bottom and see which bolt you need to adjust .... if its between two then adjust them both equally.

Its easier if there is someone to help, they can do the adjusting while you peer down the focusser to see if the adjustments make it better or worse.

good luck, you'll get the hang of it !

[colesmat]

Cheers guys. You helped alot. Managed to collimate in time to get out and see saturn.

Matt