A different technique for star testing collimation?

[pat]

I read about this test some time ago in a book. So the details could be in question.

First you center polaris. Then focus outward till the dark disk fills the inside circumference of the eyepiece. If there is any variation in the roundness of the disk, then the collimation is off.

The part I forgot is how to correct this. Pat

[Merlin66]

I think if you threw the star that far out of focus it would be of limited value.

Easier to see mis-collimation a couple of mm either side of focus.

[John]

You only need to de-focus a little - so you get a disk with the secondary shadow (assuming a newtonian, SCT or mak). You don't need that disk to fill the whole field of the eyepiece by any means - I believe the larger it gets the less accurate it is.

Edit: Merlin66 got there 1st !

[Mr Q]

I read about this test some time ago in a book. So the details could be in question.

First you center polaris. Then focus outward till the dark disk fills the inside circumference of the eyepiece. If there is any variation in the roundness of the disk, then the collimation is off.

The part I forgot is how to correct this. Pat

Any star test should be done with a bright star near the zenith to reduce scintilation to a minimum. Use a medium power EP and rack the focus out or in so that you can start to see the difraction rings. Go a bit further till you see the secondary shadow in the center of the airy disc. Then examine the rings (after the scope has cooled down to the ambient air temp.)

Slight off collimation will show up as the rings not being concentric while severe off collimation will show part of the airy disc missing.

The following link shows what to expect and the effects of bad collimation on planets and the Moon.Thierry Legault - The collimation

This next link shows how to collimate a newt or SCTCollimating a Newtonian

This is the correct and standard test results of a star test and the proper way to correct a bad collimation.

[pat]

I do use the basic star testing methods. Which is good enough. What I tried to remember is why the author also included this in his book. It sounded like a sound principle the way he explained it. So I must have missed something in my explanation. Pat