I found the light at the prime focus of my C5 is pretty hot even with the Cheapy-ERF in place.
It could burn the skin within 5s I believe. Sounds bad?
However, I have just gone through an imaging session of more than 20 minutes, but the PST etalon is still in good sharp and still maintain a stable temperature, otherwise, the bandwidth cannot be kept to show all those detail.
Of course, I believe that I will still want to find way to reduce the temperature further, in order to optimize the performance, and be safer at the same time.