Help please : Collimating large scopes

[rfdesigner]

Simple question really

If your scope has substantially better diffraction limit than current seeing, how does one get the last step of collomation done? (the one where you're looking for the airy disk when looking at a star)

Derek

[andrew s]

I use the the Catseye collimation tools ( www.catseyecollimation.com ) the autocollimator doing the final step. Andrew

[sixela]

Simple question really

If your scope has substantially better diffraction limit than current seeing, how does one get the last step of collomation done? (the one where you're looking for the airy disk when looking at a star)

Derek

You don't. You make sure that you have tools that are more precise at showing miscollimation than a star test, so that even if the seeing improves for a fraction of a second, the scope is already collimated.

It's a fable that you should always end with a star test. You can validate collimation with a star test once, just to make sure that e.g. your centre spot is well placed. For that, you use an aperture mask over the primary (close to the primary) until you see the diffraction pattern clearly enough.

But that's the last time you'll be doing a real star test to collimate. If something appears wrong during a session (e.g. if I forget to tighten some of my truss poles), I always have my BlackCat Cheshire in my pocket, and that's what's used to fix things. I could use a Howie Glatter laser collimator plus TuBlug as well, but that's a lot harder to fit in a pocket .

[rfdesigner]

You don't. You make sure that you have tools that are more precise at showing miscollimation than a star test, so that even if the seeing improves for a fraction of a second, the scope is already collimated.

It's a fable that you should always end with a star test.

Thanks.. so my homebrew collimator has probably done the business, and any collimation issues I have are with me, not the scope:icon_eek:

Derek

[sixela]

Well, as I said, many things can go wrong. Your collimator could itself not be collimated well, could not register well in the focuser, you might not be using it at the correct location, and finally, the centre spot could well be somewhere else than where it should be (in the exact centre of the blank).

So it's certainly worthwhile to do a star test *once*. But as I said, once you're confident the tools agree, there usually is no more need to do it again.

[mdstuart]

Agreed. Also try a nice double star like caster in Gemini. If you get two very sharp stars with a nice dark space in between then you are well enough collimated.

Mark