Bart wrote: With elevation is meant the altitude of the object (as if using an alt-az mount), this -does- change during a session.

Also, if you take refraction into account, will you then -not- take it into account when using narrow band filters? There the effect isn't measurable.

Now a question, would you consider to focus during imaging or quickly adjust the focus in between lights?
Personally, I don't use very long exposure lights, especially with Starlink up and screwing up our images.

Elevation usually refers to the geographic distance from the earths center, most often expressed as altitude above sea level. What you're referring to is most often called the zenith angle, or the angular displacement from zenith.

The Refractive Index applies to all electromagnetic radiation. It is usually calculated for yellow light for use in astronomy. However, the actual energy (frequeny and amplitude) of a received photon is an independent variable in the calculation for angular deflection produced by the refraction index.

I refocus on every 2 degree change in temperature, or guider crapout, or at the start of any sequence. Focus while taking lights isn't critical to the process, but shouldn't be changed between exposures.