The Witch’s Broom

NGC6960, The Witch's Broom
NGC6960, The Witch’s Broom Nebula (part of the Veil Nebula

This image is from 18 5-minute exposures. With luck, I’ll get some more tonight and sharpen it up a bit.

Edit: As promised, here’s the same image with additional exposures:

NGC6960, The Witch's Broom
…with 65 sub-exposures

The single exposures aren’t bad on their own. However, the noise in the image is terrible:

NGC6960 raw exposure
Single exposure with all defects exposed

(The electronics in the image sensor caused the odd artifact along the upper right edge.)

For your convenience, here are a couple of 100% crops of the single vs. combined images.

Single exposure:

100% crop
Single exposure, 100% crop

Note the many tiny bright blue dots. These are defects in the image; pixels that fired inappropriately. There are many other fine grain flaws, but the blue dots stand out.

…with 65 sub-exposures:

100% crrop
Combined multiple exposures, 100%crop

The blue dots are gone.

When I started this hobby I thought the purpose of multiple exposures was to make dim objects brighter. But it isn’t — the goal, the magic sauce, is noise reduction. With many exposures, the errors cancel out, and a clearer view of the underlying reality emerges. We hope.

In the single exposure above, the inaccuracies of the individual sensor elements give a gritty character, as if the image was a sand painting made with somewhat impure sand. The image that results from combining the 65 sub-exposures has much less of that gritty appearance.

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