Focus problem with Opteka 500mm mirror lens

Your focus isn't changing. It's blur caused by vibration, possibly of the mirror and/or shutter. If you're not using a wired or wireless remote, it's also due to pressing the shutter button.

Beyond that, where I was located there was quite a bit of upper atmosphere turbulence shortly after sunset. With the planets low in the west, they're in the area of the sky most affected by the temperature differential along the terminator.

_{EOS 7D Mark II + Kenko 2.0X Teleplus PRO 300 DGX + EF 70-200mm f/2.8 L IS II. ISO 800, f/5.6, 1/1000. Wired remote with mirror lockup.}

This shot of the Moon¹ taken during the conjunction, shows a lot of atmospheric turbulence. Parts of the Moon are much sharper than other parts that are all the same distance from the shooting position.

Or your exposure time may be too long and you're seeing blur due to the motion of the planets during the exposure.

_{¹ Postprocessed using a CT of 2500K (even when finishing in monochrome, CT will affect relative contrast of the different areas of the Moon), Monochrome, red filter, and adjustments to contrast, highlights, and shadows. Moderate NR and sharpening.}

Your ISO should be no higher than 100 or 200. Shutter speed should be approx. 1/200th or faster on a sturdy tripod with the mirror locked up. Your high ISO is destroying detail.

BUT honestly, the Opteka is not a good performer. I don't think you will get the sharp pic you desire from it even after you minimize vibration and lower noise. Sigma and Quantaray mirrors are much better and sell for $50 regularly. Tamron's are excellent.