Yet another collimation question

[IckleImp]

Hi all

I realise this will have been posted before. Just been out observing tonight, only the moon visible so far but great views anyway. Turned my scope round to point at a star to check the collimation with the out of focus test. If I move either side of focus I can see the spider vanes and a black dot where the secondary mirror is. Should the mirror be central?. At the moment I have it to the left one side of focus and the right the other.

The star itself is blurry when I try to focus on it.

Many thanks in advance

[Peter Drew]

If the shadow of the secondary changes position inside and outside of focus it suggests that your focuser might not be squared on. 

[IckleImp]

Can this be rectified?

[NGC 1502]

Can this be rectified?

You mention spider vanes, so is your scope a Newtonian reflector ?   If so, yes it can be rectified with basic hand tools.

Here's the best collimation guide I know to sort it  -    http://garyseronik.com/?q=node/169

Ragards, Ed.

[Jason D]

Hi all

I realise this will have been posted before. Just been out observing tonight, only the moon visible so far but great views anyway. Turned my scope round to point at a star to check the collimation with the out of focus test. If I move either side of focus I can see the spider vanes and a black dot where the secondary mirror is. Should the mirror be central?. At the moment I have it to the left one side of focus and the right the other.

The star itself is blurry when I try to focus on it.

Many thanks in advance

You are going about it the wrong way. A fast scope that is well-collimated will show the same thing.

Basically, you are defocusing too much. When you see the shadow of the spider vanes it means you have defocused too much for star collimation. You are seeing the offset of the secondary mirror. When you defocus on the opposite side of focus, you are inverting the image, hence, the secondary shadow shift will be on the opposite side.

You need to defocus by a small amount, enough to see only few rings, at high magnification. If you do that and you observe the relative size of the secondary shadow changes at opposite ends of focus then that is more a reflection of the optics quality rather than collimation.

Jason

[IckleImp]

I think maybe I am trying to run before I can walk. I will use the guide and start from scratch.