Here's my second observation of Mars this apparition, I think I've figured out the dispersion correction with the Kellner eyepiece. Mars is getting pretty big now!
This dispersion correction will be very important when objects go below 60 degrees altitude. I've found a dedicated dispersion corrector on the internet for a couple of hundred dollars on the internet, but they add another 2 or 4 elements to the optical train. Also they work only below 40 degrees altitude and to adjust them you have to touch the telescope.
I've got only three lenses (Kellner design), and to adjust the dispersion I just move the planet to another part of the chip.
For anything below 30 degrees I can't compensate for the dispersion anymore with this method without putting the camera even more off-axis, in that case a dedicated dispersion corrector is better. Althoug you have to be pretty desperate to image below that altitude, even for Mercury I image higher.
That's an amazing image! Interesting to see that as reported on other web sites what appears to be a disturbance over the NPC is hanging around. Will be interesting to see how that develops.
