[meade expert]

I encountered a strange issue again with triangular stars in my image after having collimated (using 4mm nagler on star).

In and out of focus the stars look round and concentric but approaching focus they go triangular??

I am at a loss what might be causing this?

I made sure the corrector plate retaining ring was loose , the corrected plate was shimmed with some thin cardstock where the cork pads are to stop the corrector sliding (missing paper compression gasket?).

I did find by accident last night that when I moved my dew heater from just behind the cell that holds the corrector plate (around white tube) to around the corrector plate cell itself the stars started to become rounder rather than triangles.?

Any thoughts welcome