Re: [ATM] Secondary Offset

[Alexis Cousein <al@sgi.com>]

Mitchell R wrote:
>      I know this question comes up often but I want to understand the
> workings of Secondary mirror offset better. 
> 
>  
> 
> Why exactly is it optically correct to offset it? 
> 
It's a perspective effect. If you draw two lines from the focal point
to the edges of the secondary, because the far edge is further away,
the bisecting line of the spanning angle doesn't go through
the middle of the secondary.

> How much does it affect performance?

On axis, usually not, unless you've sized the secondary to yield a
very small fully illuminated field. All it does is shift the outline
of the secondary, and also shift the fully illuminated field slightly
off-axis.

If you do offset the secondary in only one dimension (which is what you
automagically do if you centre it visually under the focuser using a
sight tube), lack of offset in the other dimension (away from the
focuser) makes the collimated scope have an optical axis slightly
tilted towards the focuser and a secondary intercepting it a a
slightly more grazing angle. Contrary to some tenacious rumours,
the reflected optical axis and focuser axis will still overlap perfectly.

The only optical drawback is that the outline of the secondary as
seen from the focuser is very, very, very slightly elliptical
(shorter along the tube axis). That makes the fully illuminated
field centred but slightly elliptical. But that's
usually completely negligible.

It may also affect DSC accuracy, as the optical axis will no longer
be exactly orthogonal with the mount's last rotational axis.

> 
> Is it much more crucial on a larger mirror where it is usually a greater
> offset?
> 
Depends on the sizing of the secondary. Usually, there is less incentive to
take the secondary aggressively small on a large scope because the "natural"
central obstruction ratio is smaller (because when you scale a scope up,
the 2" focuser doesn't scale). Which means the fully illuminated field might
be sufficiently large even when it's not centred. And if you offset
the secondary towards the primary, there's almost never an issue.


> Does a faster focal ratio need more or less offset usually? 

More.


> Is it more
> important in fast focal ratios?

Yes.
> 
> How do you calculate offset?
> 
Use Mel Bartel's diagonal calculator.

http://www.bbastrodesigns.com/diagonal.htm

> Is there a "rule of thumb" when it reaches a certain amount it is necessary
> to offset?

There's a rule of thumb, which is the dimension of the secondary's minor axis
divided by (4*f/ratio). It can be slightly off, but usually not by much.

> Mel Bartels says a 2.5".

Mel Bartels will also tell you the offset (which by the rule of thumb will be
close to 3.5mm or slightly more than 1/8").

If you build a scope, there's no reason not to offset the secondary that
amount away from the focuser. You can still use a centred spider, by the
way - just glue the secondary offset by the correct amount on its holder
(just remember that in the plane of the secondary's face, you need 1.4x
more offset).

-- 
Alexis Cousein                                  al@sgi.com
Senior Systems Engineer/Solutions Architect     SGI/Silicon Graphics
--
<If I have seen further, it is by standing on reference manuals>

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