centre dotting secondary

[earth titan]

Any benefit or problem when collimating or using?

[kenmyers]

it will benifit you when collimating.

[kniclander]

I wouldn't centre spot the secondary as it would potentially affect the image (the cenrtre spot on the primary is in the shadow of the scondary so does not affect the image). Also, why do you think it would help collimation?

[earth titan]

Also, why do you think it would help collimation?

From the reading I have done around the subject, it suggests it helps with aligning the optical axis. It seems a small indelible marked spot in the exact middle makes it simpler and more repeatable.

Interested to know what others think.

[Jason D]

First, it would not be the geometric center. It would be few millimeters above the center.

But I would not recommend marking the secondary. Just use a sight-tube to center/round the secondary mirror under the focuser.

Jason

[cathalferris]

There's no real advantage to spotting the secondary, and the spot will degrade the image contrast a bit when slightly off-axis. Not worth the effort especially when our brains are pretty good at guaging when something is concentric.

It may be worth looking at the catseye collimators and how to use them correctly especially the autocollimator.

[earth titan]

Sounds like consensus then!

[kenmyers]

Arrgh I read your post wrong. Only need to spot the primary, didnt realise you meant the secondry.