Sky @ Night Mag Collimation

[TopHouse]

Just reading the collimation guide in the latest sky at night mag and it's made my collimation OCD kick in again. In the step by step, step 3 says "adjust the secondary mirrors collimation screws to give you a good view of the whole primary mirror"

I never see the whole of my primary mirror!

[Merlin66]

Hmmm

Could be your secondary mirror is undersized for the size of the main mirror, the focal ratio and the distance between the main mirror and the focuser.

How much of the main mirror DO you see?

Ken

[Doc]

Do you use a sight tube if so I bet it's too narrow to see your whole mirror. Use a collicap and you should see your whole mirror and your mirror clips.

[TopHouse]

Thats with a collicap in, spun aluminium one, equivalent to a film cannister job. I see very nearly all of it, if i sort of tip my view a bit i can just catch the very edge of one of the mirror clips but i can never see them all at once. It is collimated though, both with laser and a star and i get lovely concentric circles. Just wondering if i loosen the primary off and adjust so it's pulled as far down the tube as possible (giving very slightly more distance between the two mirrors) and then collimate again if that will do it.

It's a 200P by the way.

[Merlin66]

Only problem with that strategy is that you loose the equivalent distance for focusing. Check the position of your eyepiece in the focuser to make sure you'll have enough inward movement to compensate.

The other (!) issue is: if you intend using a camera you may need that distance back again to get the camera body into focus... like many things in life.... a bit of a compromise.

Ken

[TopHouse]

It's never easy is it? lol

[beamish]

leave well alone and put the mag down

[TopHouse]

leave well alone and put the mag down

That's just what the police said!

[LukeSkywatcher]

What page of S@N is the collimation article on.....so i know to rip it out and burn it when my mag arrives.

[Astro_Baby]

Hmmm - you may not be able to see all of the primary depending on where the focus drawtube is.

On my scope I have to rack the focuser in to be able to see the whole primary mirror vie the collicap.

AN ideal view of the secondary would look like this.

[Astro_Baby]

YOu can see from that what the overall view should be like. (This was taken down a Chesire collimator witha cheapo digicam)....so you can see the edge of the focuser drawtube (the blackest bit at the edge), secondary mirror is the ghostly white shape, primary (the dark black ring with mirror clip notches) is superimposed on the secondary and is equally spaced around the secondary mirror.

The secondary is itself concentric to the focus tube.

A collicap usually limits you to just seeing the secondary mirror but you may need to pull the focus tube in or out to get the best view.