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Unfortunately not all reducers are born equal.
Some are just two element achromats - binocular objectives!
Others correct for flatness of field, spherical aberrations, chromatic coma and distortion etc...
Peter,
The positioning of the reducer can change the reduction ratio BUT can also introduce some severe aberrations!!
(One of the secondary benefits of spectroscopy is that almost any reducer can be used - we're only interested in the target on-axis star image)
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