M16 - M17 in Narrowband

May 2014 - in progress

Messier 16 and Messier 17 are two close Messier objects located on the border of Serpens, Scutum, and Sagittarius. M16 is technically an open cluster, but it is closely associated with a nebula that early observers likely did not see. Even today the nebula is tough requiring dark skies, a large aperture, and O III filter to detect visually. Photographically the cluster is lost in the nebulosity.

Messier 17 is known locally as the Swan Nebula. It is a typical summer star party object. Visually it looks like a swan. I will leave it to the reader whether it looks like a swan photographically.

Annotated Image

Both objects are located within the band of Milky Way. In this vicinity are also several Sharpless Objects (H areas). The image below shows the major objects in the vicinity of M16 and M17. The image is about 4 degrees square.

Choice of Palettes

Normally narrowband images include Hydrogen as one of the colors. In the Hubble Palette Hydrogen is assigned as Green. In others it occupies Red or Blue. Hydrogen certainly dominates this region of the sky. During my test shots I felt that using Nitrogen instead of Hydrogen produced a more interesting picture. Using my ultra narrowband filters I can separate N II from H alpha even though they are only a couple of nanometers different. So the initial images were collected using Nitrogen. As I write this I am collecting Hydrogen data and may revisit my choices once that collection is finished.

Thus what is presented here is a combination of Nitrogen, Oxygen, and Sulfur. My convention is to list the palette as the RGB subsituties.

Zoomable Image in NSO

The full size image is just under 4096x4096 and is thus quite large. The following will allow the reader to zoom into the image to explore it more closely.

Alternate Color Assignment

The NSO palette is very good as showing the structure of the clouds surrounding the two Messiers. With my color vision the details of the clouds themselves are not as apparent as if I shift the palette to OSN. This is presented below. Follow the link for a Zoomable image.

click to zoom into a full size image

M16

Messier 16 is the source of the most iconic picture of the Hubble Space Telescope - "The Pillars of Creation".

Hubble	ESO MSG LRGB Image
This work (OSN)

As I mentioned above these are challenging to see visually, but can be seen in the OSN image below. This is a full scale exerpt of area image above.

click on the image above to view the 2x magnified drizzle image

R=O G=S B=N

click on the image above to view the 2x magnified drizzle image

M17

To my eyes M17 looks dramatically different in a photo as opposed to what you see visually. I suspect this is mostly due to using narrowband filters. Processing with Pixinsight is also part of the reason since one of the major goals is to compress the dynamic range to see dimmer details. If I use a simple log on the image I get something that is much closer to what I visually observe in the 30" Challenger .

click on the image above to view the 2x magnified drizzle image

R=O G=S B=N

click on the image above to view the 2x magnified drizzle image

R=N G=S B=O

Processing Details

Data for Nitrogen, Oxygen, and Sulfur was collected between May and July 2014 at -30C and -25C.

Filter	Exposure
Nitrogen	26x900
Oxygen	27x900
Sulfur	23x900

Hydrogen	in progress

Processing

Operation	Mask
Channel Combination
Histogram Transformation
HDR Multiscale Transform -Small Scale 3	A_L
MMT	L_minusbrightstars
MT	star_mask
ACDNR
HT	~A_L
LHE	A_L
Curves (mild S)

Drizzle

I also processed the images using Drizzle. This technique allows the image to be magnified by reaching into the dithered image to recover more information.

Because my tracking is not good enough for 1.5"/pixel resolution the image required deconvolution. Otherwise it was processed normally. The drizzled images are available by clicking the images above.

Copyrights for Photos

Except as noted, all work on this site by Robert J. Hawley is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License. This permits the non commercial use of the material on this site, either in whole or in part, in other works provided that I am credited for the work.