oh god i swore I'd never do it

[kniclander]

I thought I'd try and check the collimation by a star test during an observing session. using polaris at x150 I found that when I racked the focusser in the doughnut looked perfect and round with the shadow of the secondary right in the middle; but when I racked it out, the shadow was off a little to one side. Why would that difference be?

Then I adjusted the primary so the doughnut was symmetrical with the focusser racked out only to find that when I went back to look at jupiter, it wasnt as clear as it had been before I started mucking about (er, I mean collimating). so I re-did it with the cheshire and it's still a little out on the star test (with focusser racked out) but jupiter was lovely and sharp again.

also, to my surprise, fairly large adjustments of the primary made very little difference to the star test - I had thought it would be extremely sensitive?

thanks for any help:icon_salut:

[barkis]

There can be an offest of the secondary mirror in faster scopes, so rely on your collimation result being correct. Don't be influenced by the shadow of the out of focus secondary on the primary, it will mislead you.

If Jupiter is displaying good detail, and a star test is showing a good Airy disc, leave well alone.

Ron.

[kniclander]

ah, thanks for that - it's an f4.7 so that makes sense.