Collimation Problem....

Hello all!

I recently 'upgraded' to an 8" Newtonian.

It's a (second hand) Celestron G8N, so not strictly speaking a Newtonian as it has a lens in between the Primary and Secondary.

I am having a problem with Collimation.

I can get near perfect collimation on a star test either side of focus BUT NOT both sides of focus with the same adjustment (on the primary).

With good collimation on one side of focus the other side of focus show a Coma misallingment.

Sort of like this:



Any thoughts/tips?

Thanks in advance,
Peter

I'm clutching at straws here (ie I'm not confident that this is going to help) - but do you ensure that your test star is centred during your collimation tests?

I'm thinking that maybe its possible that your optics are exhibiting coma abberation anyway and your test star is off-centre so when you collimate on one side of focus you make worse the coma on the other side of focus?

You could move the test star further away fron the centre of the field of view and see if what you are witnessing is worse and if so might support this suspicion.

However I would be interested in seeing comments from others on this.

Mark

Thanks for this suggestion Mark.

My real star tests have been on Arcturus and Polaris.

I only have hand tracking on my mount so with Arcturus I have to regularly move to centre.

So, you could be on to something with this as I doubt I'm ever really centred.

Having said that, I also have an artificial star.

Thus far I have not really put it far enough away from the 'scope to allow me to check both sides of the focus (I only have it about 30 metres away).

It's so bright at this time of the year I may just leave it and enjoy what views I am getting with the current collimation till it gets darker again

But....your suggestion makes a lot of sense!

Thanks again,
Peter

Hi Mark,

How are you?

I invested in an RA drive for my CG5 mount a little while ago.

I've been spending a little time trying to get my Collimation right the last few nights, taking care to keep the star centered (Polaris and Vega)...AND....

It now looks like the pattern is pretty much the same (good pattern) on both sides of focus.
Not an identical image yet (one side is 'blurrier' than the other) but things ar eimproving

Many thanks again,
Peter

Glad you're making progress.

Peter - I had to re-read my original post - because I had to think about why I suggested either side of focus.
That was suggested as a diagnostic technique (and it was an educated guess) to see if we could tie down your problems to be due to collimating on a star that wasn't centred and coma affecting your collimation.

You don't actually have to do the either side of focus to obatin collimation (maybe it can help - I don't know - but I suggested as a diagnostic technique).

However, that said, if you are collimated you can look at the star image on either side of focus and this can tell about other forms of optical distortion that you may have.

See
starizona.com/acb/basics/using_startesting.aspx

Hope that helps and very best of luck.

Mark