Sharpless 170 – The Small Rosette

Sharpless 170 is a diffuse H-II region about 6,000 light years away in the direction of Cassiopeia. The cloud is being ionized by an extremely hot O-type star at the centre of the image.

The visible part of the nebula is about 15 arc-minutes across, corresponding to a physical size of around 26 light years.

The dominant contribution to the nebula is from the H-α emission line at a wavelength of 656 nm, though ‘dominant’ is hardly an appropriate term given its overall faintness. The above image represents a total of 108 hours’ exposure and in fact the raw images taken through O-III and S-II filters reveal virtually nothing until they have been stacked and enhanced to increase contrast.

The chosen colour palette is that pioneered at the Canada-France-Hawaii telescope (CFHT) in which the H-α is mapped to the red channel, O-III to the green and S-II to the blue. Boosting the weaker channels to show their contribution generates a more pleasing result.

Thus in the above image, pure red corresponds to H-α alone; magenta indicates where S-II also makes a contribution; while the gold shade near the centre shows where the O-III signal is strongest. The bluish tinge in the surrounding space is due to the enhanced signal from the S-II filter.

Technical details:

Planewave CDK-14 corrected Dall-Kirkham reflector, FLI Proline P9000 cooled CCD camera and filter wheel with Astrodon LRGB and 3nm H-α, O-III and S-II narrowband filters. Total exposure times were 36 hours through each filter. All processing carried out in PixInsight.

Below are shown the separate contributions from the H-α, O-III and S-II filters respectively. Even though the O-III and S-II channels have been boosted more, the H-α signal still dominates.

S-II signal