Collimation and the 2ndry Mirror

[Marko of Oz (Mark)]

Hi all,
I've read reams of articles and how to's on collimation for Newt's including the ones on IIS and they all seem pretty vauge on one point in particular and that is the part where you "rotate" the 2ndary on its center bolt till its shape looks round through the focuser, not oval.

Ok,that sounds easy enough, but there's a lot of margin for error there between what looks oval and what looks round. (to my eye at least) So my question is:

Is there an accurate/easy way to do this or is near enough good enough for this part of the collimation?

I have a laser collimator(collimated) and an Orion combo sight tube cheshire tool to work with.

cheers

Mark

[bird (Anthony Wesley)]

G'day Mark, actually it's not all that critical, if its close enough that you can't tell by eye then it's close enough.

The rest of the collimation steps will make sure that the secondary is optically in the correct plane, and all that will happen if you didnt have it rotated quite right is that the very edge of the field will lose light a bit unevenly, nothing to worry about.

cheers, Bird

[Starkler (Geoff)]

Originally Posted by Marko of Oz

Is there an accurate/easy way to do this or is near enough good enough for this part of the collimation?

Marko if the secondary is centred under the focuser as seen through the cheshire/sight tube, and the cross hairs are over the reflection of the primary centre spot, you're right to go.

[erick (Eric)]

Just thinking aloud here - if the Secondary is rotated (along the long axis of the OTA), not only will it look oval rather than round, but wouldn't the rays from the primary not be able to reflect off the secondary along the axis of the focusser - so either with cheshire or laser, you'd be having a major problem collimating?

[Glenhuon (Bill)]

The way I've been checking the secondary is to look through the cheshire and if I can see all 4 primary mirror clips at the edge of field its pretty close to spot on.

Bill