Re: [ATM] Secondary Cage Calculations

[Alexis Cousein <al@sgi.com>]

Robert Houdart wrote:
> The largest Field of View is limited by the focuser size. For example,
> the largest field stops in commercial 2" eyepieces are about 45 mm
> (1.8") wide. The upper cage rings should not block any ray for this
> largest field of view.
> 
That's if you have the luxury of also being able to size the secondary accordingly.

If the secondary is in danger of vignetting the edge of field of the
wide eyepieces, it may be interesting to actually reduce the cage size just
to get the secondary to be closer to the focuser - which lets
a given secondary size illuminate a larger field.

It's a choice of where you want the cage to start vignetting.

You have a couple of choices, one being never for any field stop that
lies within the field stop of the largest eyepiece, one being at the
same off-axis angle as your secondary, and one even earlier than
the secondary (remember the drop-off in illumination is much more
gradual for vignetting by the front, so at the edge of the field
this actually tends to pay off for a given secondary size, even
though the 100% illuminated field is smaller).

The sbolute minimum size is the aperture size, in which case
only the on-axis object is fully illuminated (and only if
secondary offset is perfect) but you can get away with a
smaller secondary to get decent edge of field illumination.

But nobody seems to be that radical (another reason not to use
that is that you don't want a focuser draw tube to start intruding
in the light cones for objects close to the axis).

It's interesting to note that the vignetting from the front and
secondary are more or less additive; unfortunately, the light cone
is clipped by the front at the other side from the clipping
by the secondary.

And then you get to decide how much extra you're going to factor in
for lack of perfect offset of the secondary, i.e. collimation tolerances -
usually only if you use the more aggressive criteria.
> 
> Simple geometry then shows that the ring ID should be equal to aperture
> + the size of the field stop for the largest Field of View.

Criterion #1: "never". But as I wrote, unless your secondary is so large
that it doesn't introduce vignetting, that may not be the compromise
you necessarily want to pick (even though it's a sane one; the only
drawback will be a slightly larger central obstruction for a given
fully illuminated field size).

We don't all have the luxury of having (or designing) scopes as large as
the Cruxis, i.e. large enough to combine a secondary that's large enough
and a small central obstruction ;).


-- 
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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