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Hi Joshua & Alistair,
I can't imagine refraction causing any elongation over such a short period. My understanding is that refraction is absent at the zenith but results in about half a degree of position change at the horizon. And I guess that the amount of refraction might follow the sine rule for anywhere in between (based on degrees above the horizon).
Thinking about this for a minute or two more, the formula for the relative diffraction might be of the form:
Refraction is proportional to 1-Sin(theta) where theta is the angle between the target and the horizon (either Western or Eastern).
For an elevation of 30 degrees (sin(30) = 0.5), the amount of diffraction would be about (1-0.5), or half, that of the refraction observed on the horizon, that is, about 0.25 degrees.
Let's assume a 1 minute exposure for our target star at 30 degrees elevation. Due to the Earth's rotation, the star will move about 0.25 deg across the sky. In that time the amount of refraction will have changed slightly due to the 1-Sin(theta) factor. Using a calculator, I estimated that the angular change due to refraction will be 0.001885 deg, or 6.8 arc-seconds. For a shorter exposure, the change will be approximately proportionally less. So for a 300 ms exposure, the change would be about 0.03 arc-seconds; not something that would cause elongated stars.
The above working is very much a quick estimation, so happy to be corrected!
Chris
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