Some time ago I put out a review on the Canon R5 and I mentioned I would do some more astrophotography tests and get back to you. I have had a few chances to take the camera out at night now and have a few thoughts on it in general.
There is a fundamental challenge when it comes to comparing noise in a camera to other cameras. First and foremost, every camera model is different. I can go back and look at my 6D, 5DmkIV, the R and the R5 and will notice differences. In comparison, the 6D has more noise and as you progress through the line up they get better except between the mkIV and the R where the mkIV did do better. The R5 seems to do better than the R and the mkIV when it comes to noise.
Now what does "do better" mean? I do not have a quantifiable way of answering that. I notice less color noise I guess.
I have gone through a few phases on my night sky photo editing. First I was very very nitpicky about noise. I fought tooth and nail to get rid of noise and learned through time that that method was a real pain in the butt and didn't produce much better details except in certain situations. Since then I have gone from fighting to accepting some noise as a reality.
At iso 6400 the ground is noisy. With a few sliders in Lightroom the noise is manageable. If you stack three images as smart objects then perform a median blend the noise might as well be gone completely. Some technical knowhow solves all of this issue.
Noise In The Stars
It is noisy like every astro image. I find now I can balance noise in the stars just fine by using the noise reduction slider in Lightroom. Click on the image above and look at the noise pattern on the image. This image has been reduced in size to 2000 pixels wide. You can see the noise in the sky just above the horizon line. It's not bad. If you are using a smart phone I doubt you will see anything. In the core noise is present but enough bright light negates most of it. Could it be cleaner? Probably. But not worth the fight for me most of the time.
The image above is not the R5. I do believe this is a 6D taken some years ago in the same spot as the top image in the springtime. I find the 6D does do a better job and picking out dust trails and collecting better details in the stars, particularly the color. Part of this could be technique but I think the 6D simply did a better job and pulling out color in the night sky. The R5 is adequate and will still produce fantastic results.
If you find yourself being quite particular about your night images and want the most detailed star images, possibly the R6 might be a better choice as it has a smaller megapixel count which allows for larger photo bin sites for collecting light.
A Quick Final Thought
As you can tell this is not a particularly in-depth review of the astrophotography potential of the R5. This is in part because I don't do that much night photography anymore. In reality, I find more photographers nitpick about small details that add little to the image, and detract small amounts at best.
Can you take night images with the R5? Yes. Will they be good? Yes. Will the R5 be as good as cameras dedicated to night photography? No. Will it be better than most other cameras? Yes, yes it will. And since the R5 is an amazing camera all around, you should have great results no matter what.