[sjastro]

Ken (Merlin66) raised the issue of significant red light leakage reported in UV pass filters. Red light leakage would result in H-Alpha contamination of the M42 image there by making it "not strictly" a UV image.

I am currently taking RGB images of M42. The M42 R images in the nebular regions indicated my ST-X10ME imaging at about 40-50% of full well capacity.

I decided to image a red LED using red and UV pass filters. The red LED has a frequency range of 610-760 nm which conveniently includes the H-Alpha emission at 656nm.
The exposure was adjusted to aim for a 40-50% of full well capacity with the red filter. The ADU counts for both red and UV filters was recorded.
The results and LED images are in the attachment.

The conclusion is while there is red leakage in the UV pass filter, the ADU count is so low that it would be difficult to distinguish this from the statistical and readout an noise in an image, and therefore the H-Alpha contribution is probably lost in the overall noise of the UV image.

Regards

Steven