Here in the southern hemisphere we are fortunate at the moment to be able to image the planets at high elevation angles. Jupiter and Saturn are peaking at around 73 degrees for me right now, and I thought I'd take the advantage of imaging one of these planets at a range of angles to measure (if possible) the effect of elevation angle upon colour balance, something I've investigated previously from a theoretical perspective in this thread.
I had looked at two different methods of determining colour shift, one based upon Solar Irradiance measurements, the other based upon measurements of G2V stars reported in KellySky. But what does this really mean for the planets? So I took images of Saturn at the highest point I could (73.3 degrees) and then at every 5 degrees elevation thereafter as Saturn headed west. I don't normally like imaging to the west, as the planets move over the city of Melbourne (where I live) and I end up pointing right over my hot roof so the seeing deteriorates. However, there was no way to avoid this.
The first image below shows an animated gif of the stacked, processed and sharpened videos (5 mins duration @ 100fps, 5000 frames stacked except for 30 and 35 degrees where just 2000 frames were stacked to improve final image quality). Obviously as the elevation reduces the image quality drops, but I was also interested in measuring any colour shift. So I selected the planet's disc and rings in Photoshop and recorded the median red, green and blue values for each of the stacked (but otherwise unprocessed) images. I then plotted the ratios of red/green and blue/green for each image and compared them to the results from the Irradiance and G2V star analysis. I took additional care to make sure I was selecting the same image size (including darker surroundings) for all the images, however there is some error probable with this method, but I tried to reduce it as much as I could.
An image showing the process to capture the median red/green/blue values is below, along with the graph showing the measured values compared to the models I discussed earlier. Note that the graph shows the measured ratios compared to the 73.3° elevation angle rather than the 90° value for the models (which I couldn't measure). What the measurements show is that at 30 degrees elevation a shift of around 5% in both red and blue is required to get the colours back to the "true" value, which is slightly less than what both the Irradiance and G2V star analyses suggest.
Celestron Evolution 9.25" SCT @ f21 with ASI 224MC, stacked in AS!3 with 3x drizzle, sharpened in Registax, final touches in Photoshop Elements, final image 50% larger than captured.
Edited by Tulloch, 08 September 2020 - 04:25 PM.