Collimation Question

[Starwiz]

Does anyone know the scientific reason, why fast scopes require a slight offset (as per the attached image below from the 'Astro Baby's Guide to Collimation')

I'm an engineer by trade, so taking it at 'face value' naturally leaves me wondering 

Thanks

John

 

Collimation Offset.bmp

[andrew s]

It is a simple consequence of geometrical optics, if you want the center of the field to be on the optic axis and the optic axis to be parallel to the telescope tube. It is due to the fact the flat is at 45 deg to the optic axis the ray furthest from the focuser travels less distance than the one nearest to reach it.  The faster the scope the more obvious the offset as the extreme rays are steeper.

Regards Andrew

[John]

Fortunately the offset is usually built into the design of the secondary mount on many scopes so we don't need to worry about setting it. It's worth being aware of it though because it does affect the view down the drawtube when collimating a faster scope, ie: not all the circular elements are concentric as Jason Khadder's illustrations show.

 

 

 

[andrew s]

2 hours ago, John said:

Fortunately the offset is usually built into the design of the secondary mount on many scopes so we don't need to worry about setting it. It's worth being aware of it though because it does affect the view down the drawtube when collimating a faster scope, ie: not all the circular elements are concentric as Jason Khadder's illustrations show.

 

 

 

You also don't need to "offset" to have properly collimated Newtonian. To achieve this you just follow the normal procedure for collimating but the secondary will no longer be at exactly 45deg and the optic axis is displaced and no longer parallel to the mechanical axis. The secondary will then not be perfectly circular when viewed down the focuser tube.

In fact given the mechanical accuracy with which the mirror cell and secondary are positioned and squaring on of the focuser nearly all scope have a mixture of offset and displacement of the optic axis from the mechanical axis etc.

Regards Andrew s

[John]

1 hour ago, andrew s said:

You also don't need to "offset" to have properly collimated Newtonian. To achieve this you just follow the normal procedure for collimating but the secondary will no longer be at exactly 45deg and the optic axis is displaced and no longer parallel to the mechanical axis. The secondary will then not be perfectly circular when viewed down the focuser tube.

In fact given the mechanical accuracy with which the mirror cell and secondary are positioned and squaring on of the focuser nearly all scope have a mixture of offset and displacement of the optic axis from the mechanical axis etc.

Regards Andrew s

Does the light cone still come down the centre axis of the focuser with this approach ?

 

[Ruud]

6 hours ago, Starwiz said:

Does anyone know the scientific reason, why fast scopes require a slight offset (as per the attached image below from the 'Astro Baby's Guide to Collimation')

This is the reason. The grey zone is the shadow of the secondary.

The centre of the primary (1) is on the optical axis.  To black as little light as possible and catch the full light cone of the primary, the centre of the secondary (2) has to be displaced with respect to the optical axis.

The faster the primary, the farther the secondary needs to be displaced. This way the secondary can be kept at a minimal size and no light is unnecessarily blocked.

[andrew s]

7 hours ago, John said:

Does the light cone still come down the centre axis of the focuser with this approach ?

 

Yes it does.

Regards Andrew

[Starwiz]

Thanks all, for the excellent detailed responses and thanks Ruud, for the diagram.  I was about to draw my own, but you've saved me the trouble.  

I think I understand it now.

John

[Jason D]

Few more diagrams that explains the offset:

[Jason D]

On 3/28/2016 at 11:26, Starwiz said:

Does anyone know the scientific reason, why fast scopes require a slight offset (as per the attached image below from the 'Astro Baby's Guide to Collimation')

I'm an engineer by trade, so taking it at 'face value' naturally leaves me wondering 

Thanks

John

 

Collimation Offset.bmp

First, it is a common misconception that only fast scopes require an offset. All Newtonians can benefit from a secondary offset regardless of F-ratio (fast vs slow). The offset magnitude for slow scopes is smaller and not as noticeable compared to fast scopes. The left diagram (slow scope) is somewhat misleading since it does not show any offset. It should have shown an offset with a smaller amount compared to the right diagram.

Jason

 

[Jason D]

One more common confusion about the secondary offset. There are two of them. One involves offsetting (or shifting) the secondary mirror closer to the primary mirror. This offset can easily be achieved by adjusting the central bolt of the secondary mirror stalk. The secondary offset involves moving the secondary mirror away from the focuser. This type of offset is not needed for most scopes -- with few exceptions.

Jason

[jetstream]

I sure love my Catseye sight tube

[Starwiz]

On ‎30‎/‎03‎/‎2016 at 08:30, Jason D said:

One more common confusion about the secondary offset. There are two of them. One involves offsetting (or shifting) the secondary mirror closer to the primary mirror. This offset can easily be achieved by adjusting the central bolt of the secondary mirror stalk. The secondary offset involves moving the secondary mirror away from the focuser. This type of offset is not needed for most scopes -- with few exceptions.

Jason

Thanks for the explanations and great diagrams Jason.

John