Samyang 135mm pair of unexpected diffraction spikes

[Stefan73]

Hi,

Does anyone know what I'm doing wrong here?  Attached is a single 90s sub (coloured and stretched in siril) from a Samyang 135mm at F2 attached to an ASI2600 at 100 gain 24 offset.

All of the stars appear to have two diffraction spikes which I wasn't expecting.  The spikes stay in the same orientation through 5 hours of imaging that night (and are very pronounced after stacking) so it isn't an accidental tree or washing line.  I promise I did not leave my focusing mask on and there aren't any cables nearby which could have plausibly fallen across the lens.

Coming from the world of 130PDS I'm expecting some sort of obvious obstruction across the lens to have caused the spikes, which I can't see so I'm a bit stumped.

Thanks for any ideas!

[jfrijhoff]

I was going to say if you left the focusing mask on 😃, but I read you say it wasn’t. Those spikes with the different colors sure do look like them. I wouldn’t know what else it would be.

[Stefan73]

Well somewhat embarrassingly I found this hand print on the rear lens on closer inspection.  Which does look a bit like  a focusing mask I guess!  Not managed to test after cleaning but hopefully it can be filed under "stupid things I've done to ruin a night's imagining".

[Justin richens]

we have all been there!

[jfrijhoff]

Hope it's solved! Similarly, I (and others) were sure I was experiencing walking noise. Dithering or whatsoever didn't work. Turned out that after cleaning my Samyang, I hadn't cleaned it properly and there were streaks from cleaning on the front lens, barely visible with the naked eye, but very noticeable after stacking 🙂.

[Adam J]

The other classic one is spider silk threads. 

Adam