Hi Andy
The airmass for Regulus was around 1.35, that for Castor 1.15…
They were taken on the same night.
I am using a C11, and bright stars inevitably straddle the slit (already the slightly wider one at 35mu).
I might try an experiment with moving the collimator axis a little.. you know, the screw on the bottom of the Lhires.
Keep safe
Kevin