Re: [ATM] Secondary Offset

[Alexis Cousein <al@sgi.com>]

waldo kitty wrote:
> 3. the angle at which the secondary is mounted.
> 
> AIUI, #3 should be 45 degrees...

No. First of all, that's not a degree of freedom in secondary placement.
Collimation should ensure that the tilt is set so that the reflected focuser
axis passes through the middle of the primary. That doesn't *always* yield
a 45° angle; it only does when all offsets are perfect and there is no other
positional or rotational error of the secondary.

There's nothing magical about that 45°, except that the diagonal is cut with
a major to minor axis ratio optimised for 45°. Some large Newts even use an
angle different from 45° on purpose to lower eyepiece height. Newts with
secondaries centred in the tube (i.e. not offset away from the focuser) will
also end up with an angle slightly different from 45°.
> 
> #'s 1 and 2 [the two positional coordinates on which offset is needed]
 > would depend on the focal length of the setup as well as the size of
> the secondary and the desired FOV...

The position doesn't depend on the desired FOV.

You optimise the secondary placement so that the intercept angle is 45°
(because that's what will yield a circular fully illuminated field)
and so that the fully illuminated field is centred, and then
*get* a certain fully illuminated FOV as a result. You can't increase
the fully illuminated FOV by placing it differently (at least not if you
want a circular and centred one).

If that FOV is too small, then you either increase the size of the secondary
or try to place the focal plane somewhere else (by increasing the
distance from primary to secondary, which places the focal plane closer to
the tube, which may require another lower profile focuser or a smaller tube or 
UTA diameter  if you still want to be able to focus).

  in all my calculations, i've always assumed
> that the secondary would be mounted dead center of the tube (#2 above) and that 
> it was the distance from the primary that was critical in determining the 70% 
> FOV and the FI FOV...

Yes, it's still critical. That's more or less what determines the size of the
fully illuminated field.

Lack of offset doesn't give you a *smaller* fully illuminated field, but it 
gives you one that is not centred around the on-axis object. In other words,
your widest field eyepiece will have more illumination at one side than
the other, which is a bit of a waste.

Lack of offset away from the focuser does give you a fully illuminated field
that is centred, but slightly elliptical (shorter along the tube axis) because
the secondary isn't angled exactly 45° with respect to the focuser axis.

> FWIW: i generally strive to work out a FI FOV with no loss of light pathway as 
> calculated in NEWT... now, i dunno if this is the proper viewpoint on this or 
> not but it has always seemed to be that one would rather have a FI FOV vise a 
> 70% FOV...

You want both. First of all, for planetary observation you want a fully
illuminated field of view that isn't point sized: you still want
the on-axis object to be fully illuminated if the actual geometry is slightly
different from the computed one (because you want some tolerances on
mechanical placement). But in this scenario, you don't care about the
illumination beyond that field.

For looking at DSOs, you usually both want to illuminate the field of a
2mm exit pupil Nagler completely (or a fixed FOV completely, like "the
diameter of the full moon"), *and* you want the edge of your often
used widest field eyepiece (say a 26T5) to still have 70% illumination
at the edge.

In slow scopes, you'll optimise for the second and the first will take
care of itself. In fat scopes, you'll optimise for the first and the
second will take care of itself.


-- 
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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