Hello again John! I'm not sure what you mean with changing the number of chunks in the .yt/config file? I did change the number of chunks and lowered the exposition time to get one photon list, but is that list enough? Don't I need to merge it with another observation? Or you meant changing the photon_per_chunk parameter somewhere else in the yt code? Or perhaps you meant I should try the first option (run it in a machine with more RAM / parallel)? I'll try that right now anyway! Thanks for everything, I'll keep an eye on the patch! Barbara ------------------------------------------------------------------------ Hi Barbara, Any chance you could try the option I mentioned where you change number of chunks in the .yt/config file? That way you might be able to avoid doing merging altogether. This is a bug I fixed yesterday?when I have some way of getting the patch to you I will let you know. This happens when you choose parameters for project_photons that would result in a higher number of photons being drawn than exist in the original set that you generated. I?m going to change this error message so it?s more informative. Best, John