Fast vs Slow scope collimation pattern

[kirkster501]

Hello,

On astro-babys collimation pages (near the bottom) and on some other collimation resources it shows scopes that are F5 and faster with a collimation shadow that is not concentric. Why is that? I'd always thought they'd be concentric?

Steve

[Steve Ward]

It' non-concentric if the secondary is offset and not central.

Good article here . . . http://www.catseyecollimation.com/mccluneytext.html

[Jason D]

Hello,

On astro-babys collimation pages (near the bottom) and on some other collimation resources it shows scopes that are F5 and faster with a collimation shadow that is not concentric. Why is that? I'd always thought they'd be concentric?

Steve

Because you are looking at the secondary mirror from two different perspectives. The first perspective is from the focuser end and the second from a virtual point behind the primary mirror (see attachment). The secondary mirror will appear dark from the perspective behind the primary mirror. Mathematically, you can't see the secondary mirror concentric from both perspectives. We typical center the secondary mirror from the first perspective point (focuser end) and live with whatever we get from the other perspective point. The faster the scope the greater the shift of the secondary mirror shadow.

Jason

[Jason D]

It' non-concentric if the secondary is offset and not central.

Good article here . . . http://www.catseyeco...cluneytext.html

Non-concentricity will be the same regardless whether the secondary mirror was mounted centrally or with an "away-from-focuser" offset for a properly collimated scope.

Jason