Canon R5 R5 and RF100-500 Astro Focus

[Deepgreen]

I have done quite a bit of astrophotography with the R5 and the RF100-500mm lens (in manual focus mode). I am finding that, in a large sequence of shots (say 100-150) using a star tracker the first several frames are in focus and then the rest seem as if the focus has slightly shifted to produce blobby stars rather than the sharp ones in the first few frames. I use the focus zoom function to achieve as near perfect focussing as possible, then return to the standard screen (and see the green focus indicator showing that good focus has been reached) and start the sequence. My theory is that perhaps it is lens cooling and therefore structural contraction that throws the focus out by a tiny, but important, amount. By definition, a clear sky is required, which also produces significant cooling over a few hours. Does anyone have a view on this, please?

 

[Jake Shoots Birds]

Thinking the lens is pointed upward and at 500mm most likely, and since the lens extends to zoom the front of the lens may be creeping back into the body of it the same way it will extend when the lens is pointed down and at 100mm. The tension ring will help a lot with that (I'm assuming you're using it) but may not be enough to keep it from movements small enough to make the image go soft, particularly if you're shooting in colder weather and you're noticing this over the course of hours and not minutes.

 

[Deepgreen]

Thinking the lens is pointed upward and at 500mm most likely, and since the lens extends to zoom the front of the lens may be creeping back into the body of it the same way it will extend when the lens is pointed down and at 100mm. The tension ring will help a lot with that (I'm assuming you're using it) but may not be enough to keep it from movements small enough to make the image go soft, particularly if you're shooting in colder weather and you're noticing this over the course of hours and not minutes.

Yes, I do use the tension ring in 'stiff' mode but you may be right. However, if I am doing, say, Orion at around 30 degrees elevation then the lens is pointing more horizontally than vertically and I still find the same issue arising. Leaving the camera outside to cool down before shooting would only help to a degree (no pun intended) as the night continues to cool as it progresses. I usually use a mains camera power supply so battery capacity is not an issue.