Collimation cap and primary mirror

[Colossal Plossl]

I bought a rigel collimation cap and have collimated my 130p but only by adjusting the knobs on the primary mirror.  At what point do I use or need to adjust the secondary mirror knobs?

Thanks.

[Pixies]

Hi. You need to do the secondary first. Initially you align it under the focuser, then you align the secondary to the centre of the primary. You may find thatthese are already OK. Once that's done you can align the primary mirror.

See the following:

 

http://www.astro-baby.com/astrobaby/help/collimation-guide-newtonian-reflector/

https://garyseronik.com/a-beginners-guide-to-collimation/

[NGC 1502]

+1 for the Gary Seronik guide.
 

Gary cuts through the unnecessarily complex and confusing info out there.

 

Ed.

[Colossal Plossl]

Thanks guys

[Colossal Plossl]

not a great photo.  Right side is cut off.

 

Edited August 29, 2020 by Colossal Plossl

[Pixies]

That's fine. You are looking at the silhouette of the secondary mirror (the dark circle). The smaller grey circle is the reflection of the underside of the collimation cap.

The secondary mirror has what's called an 'offset'. It is supposed to be extended in one direction away from the focuser. It's hard to describe what the offset is for without diagrams. I'll try and find a good article.

[Pixies]

Hi. have a look at this:

 

Edited August 29, 2020 by Pixies

[Pixies]

OK. here you go.

The issue is that when you intersect a cone, the ellipse is not centred over the axis of the cone. If you imagine a cone of light from the primary up towards the secondary, you will either need an oversized secondary mirror to capture the full area of the intersection, or an offset elliptical secondary. 

Follow this link and find the interactive diagram at the bottom of the page:

https://www.geogebra.org/m/NnxHw6gY

If you rotate the image so that you are looking down the axis of the cone, the elliptical intersection is offset.

 

Edited August 29, 2020 by Pixies

[Pixies]

Sorry - one final comment for the OP.

You don't have to worry about the offset. If you follow Astro Baby or the Seronik guides or any instruction using a collimation cap (or Cheshire or sight-tube) when aligning the secondary, it just works. The only time you would need to worry about it would be if you were building your own secondary mirror holder.

[johninderby]

Modern scopes have the secondary mirror mounted offset on the holder by the right amount anyway so no need to bother about it. If the secondary holder is centered in the tube then the offset is right.