Newtonian collimation - Astrobaby question

[dodi]

Hi,

I am a bit puzzled by the collimation paper by Astro Baby. In the drawing for fast Newtonians, with offset secondary, I see the (imho) correct schematic that shows the spider reflection (the arms) is offset and away from the center dot on the primary, just as the reflection from the secondary on the primary is. However, on the picture directly underneat, she shows and mentions the spider arms to fall exactly over the center dot. With the best of my imagination, I cannot get my fast Newtonian aligned this way. So... what is correct: the schematic (which I think) or the picture?

tx

Dodi

[dweller25]

For a fast newtonian it should look like the schematic.

[patomlin76]

I too was completely baffled by this, which meant that I was trying to collimate where it wasn't possible to...

Basically as far as I'm aware, the centre dot of your primary mirror should fall in the centre of your collimation eyepiece or cheshire. This however will be offset from the centre of the secondary mirror.

When I realised that all fast scopes are offset like this, I realised I'd achieved decent collimation much earlier!

[Merlin66]

This is a photo of a "fast" f3.6 scope - you can see the amount of off-set needed!

[dodi]

Yes, I know the diagonal needs offset, but I am puzzled by the two conflicting illustrations on the astrobaby collimation page. Obviously only one can be correct In my opinion, one sees the reflection of the secondary offset, but also the reflection of the spider itself should not be in the center of the main mirror, like in the schematic of the fast newton, not like the picture through the focuser of a fast newtonian.

[Jason D]

The simple rule is to ignore the spider vanes.

These could look centered or shifted. It has to do more with whether the secondary mirror was mounted with an offset or not. If it was, then the spider vanes will look centered. If not, they will look shifted.

Back to the simple rule: Ignore the spider vanes when it gets to collimation.

[Jason D]

See attachment. Figure "1" represents what you would see on scopes with the secondary mirror mounted with an offset. Figure "2" represents what you would see on scopes with the secondary mirror mounted centrally without an offset.

Jason

[patomlin76]

If like me you have a Skywatcher, the offset is built into the secondary mirror holder, so the vanes will appear central. However an aligned and circular secondary mirror will not be centered in your eyepiece.

[dodi]

Thanks for the clarification, makes sense to me now Cheers.