Hi guys I currently have a 70-300 VR and now wish I got the Sigma 50-500mm or something similar for birding and wildlife.

But in my look around for longer lenses I stumbled onto a package on eBay for a 500mm f8 mirror lens with a 2x TC for the price of $200. I was wondering if the extra 200mm is useful or not for wildlife even with the drop in IQ. I know all about the issues of contrast, MF, vibrations etc...

Before someone even suggests dropping 1k+ on a good super tele I'm going to go out and say this. This $200 is already at the top of the budget, if the lovely people here convince me to not get this lens I'll put the money towards probably an UWA or a short tele prime.

As alluded to earlier, this will mainly be used for wildlife, birding and perhaps the moon and some stars perhaps.

Also a 2nd question: Is there any good program to stack photos that can be easily integrated into a Lightroom workflow? Ta