Robin & Richard,
Thank you for your replies. It is good to hear it is not a crazy idea, though one that might only give negative results.
I had been thinking that one exposure might be sufficient with the slit appropriately aligned. Though it is a good point that 2 positions might be needed due to the contrast between day side and night side of Venus.
Robin, I have one of the newer LHIRES IIIs, do you think the flat field calibration lamp is sufficiently good?
If aurora were the cause, then I would hope there would be an “obvious” green line due to oxygen at 5577 Angstroms, or at least strong enough to be above the noise.
The other issue would be knowing when to look, as ideally this would be when CME has been ejected in the direction of Venus.
I’ll try it out with an early morning in September when Venus is better placed. See how feasible it is to get good data.