Thank you Davo, I dont know but I love wide field

.. got a fair few the other night. They seemed to work without guiding so I just kept shooting leaving the camera unattended while doing housework.
I note your projects include a hartman mask.. I used a cornflake box for the cardboard and placed four holes not two.. seemed to work well and I would even photograph with it on (on the assumption I raised the F ratio) when doing the Moon.
Thanks for your encouragement
Alex