I discovered a Schleiren imaging technique by chance...well... some 45 years ago...when eyepeice projecting an image of the sun with a Royal 2.5" refractor (heady stuff, I know
)
After removing the projection screen, the image randomly projected onto a white painted wall (in shadow) , some 3-4 metres away. Moving closer to the wall I noticed apart from my shadow, thermal shadows coming off my hand.
The rather bright, and very point light-like, light from the eyepiece was the key!
Simple, fun and it works
