Re: [ATM] Secondary Offset

[Alexis Cousein <al@sgi.com>]

Mitchell R wrote:
>      Thank you for the website link/explanation/picture. It is much clearer
> now.
> 
> Another question,
> 
> 	If I needed a 2.5" secondary minimum, and my offset is .1", and I
> get a stock secondary size of 2.6", does that take care of the same problem
> offsetting does?[/quote]

Yes. On the other hand, unless you have digital setting circles, there's
no reason not to adjust the offset towards the primary to get a centred
fully illuminated field.

> 	Even if the above is incorrect I think for visual use, not
> offsetting the secondary wont matter much?
> 
As I've been trying to say, normally you *will* offset your secondary
during the collimation procedure. A secondary that *looks* centred
under the focuser in a sight tube is actually offset optimally towards
the primary for the given offset away from the focuser, for exactly the
same reason (perspective) you need the offset.

If for some reason you want *no* offset at all (e.g. because you've
centred the secondary and primary in the tube and want an optical
axis parallel to the tube axis), you usually have to centre dot
the secondary; not offsetting at all is actually *harder* that
offsetting.

-- 
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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