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Mick, If I may, 35 years ago when I started in deep space photography I was told, by the then amateur experts, that 2x the focal length of the instrument used for taken photos is a must.
My instrument in them days was a 12.5" F6 Reflector. Guiding was done offaxis with a 12mm plus 2.5x Barlow. Any movement was detected and corrected before it was picked up by the mainscope.
Someone from IIS saw some of my old black/white prints from that time, not so long ago, and their first comment was on the pin point images of the stars. Guiding and good focus.
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