The Swan: Hi-res Optical/Infrared Comparison

[SkyViking]

Hi All,

Here is another entry in my series of optical/infrared comparisons. I gathered the data for these images back in April (of course while Centaurus A was either too low or behind trees :P), but have not had time to process them until now.

Link to large optical image (4.8MB) (http://www.rolfolsenastrophotography.com/Astrophotography/Nebulae/25312426_jC3jWq#!i=2692859387&k=RqwSNCj&lb=1&s=O)
Link to large infrared image (4.9MB) (http://www.rolfolsenastrophotography.com/Astrophotography/Nebulae/25312426_jC3jWq#!i=2692859462&k=t2Lt6qQ&lb=1&s=O)
I can recommend opening both links in separate tabs and switching between them to really appreciate the differences!

I also reckon I'll do a video showcasing all the images in the series now, since my earlier video (http://www.rolfolsenastrophotography.com/Astrophotography/Nebulae/25312426_jC3jWq#!i=2451733928&k=zWK4znD) only showed the Carina Nebula :)

Image details:
Date: 7th, 10th and 11th April 2013
Exposure (Optical): LRGB 105:146:107:107m, total 7hrs 45mins @ -30C
Exposure (Infrared): NIR(Luminance) 240m, NIR(red) 240m, G(green) 107m, B(blue) 107m, total 7hrs 34mins @ -30C
Telescope: 10" Serrurier Truss Newtonian f/5
Camera: QSI 683wsg with Lodestar guider
Filters: Astrodon LRGB E-Series Gen 2
Taken from my observatory in Auckland, New Zealand

Optical image
This image shows the bright nebula Messier 17, also known as the Swan Nebula or the Omega Nebula.
This nebula is located about 6000 light years away in the constellation of Sagittarius. It was discovered by Swiss astronomer Philippe Loys de Chéseaux in 1745 and later catalogued by Charles Messier in 1764.
The central part of the nebula is lit up by a cluster of extremely bright young O and B type stars. It is one of the brightest and most massive star forming regions in our galaxy and appears very similar in spatial structure to the famous Orion Nebula, but the Swan is viewed more edge-on from our vantage point. The nebula is also very bright visually, actually much more so than its famous neighbour the Eagle Nebula - which traditionally is the more photographed of the two.

Visible above the nebula is a pair of very unique bright orange stars. These are LBV (Luminous Blue Variable) type stars HD 168607 and HD 168625 respectively, two rare yellow hypergiants that might have formed what is known as a pseudo-photosphere. This is a theoretical optically dense surface present due to intense stellar winds and thus not the actual surface of the star. This psudo-photophere is much cooler than the underlying surface of the star and therefore appears yellow to us. The right-most of these two stars in the image (HD 168625) also has an associated complex nebulousity surrounding. This nebula appears very similar to the double ring structure observed around Sandulaek -69° 202 which was the progenitor of Supernova 1987A in the Large Magellanic Cloud.
This would indicate that Sandulaek -69° 202 was an LBV prior to exploding as a supernova, and also that HD 168625 may suffer a similar fate in the future.
The two stars are assumed to lie at the same distance as the Swan Nebula.

Infrared image
Traditional images of emission nebulae taken in visible light primarily display the striking magenta colour from glowing Hydrogen gas, as well as large dark obscuring clouds of dust. The infrared part of the spectrum penetrates these clouds better and reveals complex details and thousands of young stars that are otherwise completely invisible. These stars shine primarily in the infrared and appear as golden red in this image. Only a minority of these are visible in traditional optical images.

To create this image I used a special infrared filter. Effectively the band observed is 700-1100nm since the silicon CCD chip is not responsive to wavelengths longer than that, and this band is called NIR (Near Infrared). One issue arising from this technique is that the NIR band does not have a colour as such, since it lies outside the visible light spectrum.
To create a colour image from the NIR data I have developed a colour stacking technique which has produced promising results so far.
I use the NIR filter together with G and B filters to create a colour composite. The resulting images are assigned to the different channels as NIR->Luminance, NIR->Red, G->Green and B->Blue.

Substituting the R channel with NIR nicely brings out the contrast between the nebula itself and all the young obscured stars that shine primarily in infrared. This colour image is then overlaid with the NIR frame used as luminousity. The result is a predominantly blue nebula, but with the areas that are bright in NIR showing as distinctly golden red which highlights the features that are visible in the NIR band and at the same time produces a visually very pleasing image.

These images are part of my series of optical/infrared comparisons of famous nebulae.
Below are links to the other images in the series:

HD Video tour of the Carina Nebula, comparing Infrared and Optical features (http://www.rolfolsenastrophotography.com/Astrophotography/Nebulae/25312426_jC3jWq#!i=2451733928&k=zWK4znD)
Image of the Carina Nebula in Infrared (http://www.rolfolsenastrophotography.com/Astrophotography/Nebulae/25312426_jC3jWq#!i=2451727602&k=bkfPw3K&lb=1&s=X3)
Image of the Lagoon Nebula in Infrared (http://www.rolfolsenastrophotography.com/Astrophotography/Nebulae/25312426_jC3jWq#!i=2491789159&k=X744Sbj&lb=1&s=X3)
Image of the Orion Nebula in Infrared (http://www.rolfolsenastrophotography.com/Astrophotography/Nebulae/25312426_jC3jWq#!i=2243508611&k=bqS4vxH&lb=1&s=X3)


I hope you enjoy, comments and critique most welcome.

Regards,
Rolf

[Rod771]

Very well done Rolf! :)

I'm Suprised you found a spare 7 odd hours while capturing Centaurus A :P... amazing.:thumbsup:

[multiweb]

Both shots look Magic, Rolf. Gorgeous colours. :thumbsup::thumbsup::thumbsup:

[pluto]

Very nice, looks great in NIR :)

[Larryp]

Both look great!

[jase]

Fantastic imaging Rolf! The result speaks for itself - stunning! The NIR certainly adds some punch with the clusters of young stars emphasised very well. I like your approach that you're using with the NIR data. Its sure does provide an aesthetic view. The traditional RGB palette rendition is my pick of the two. Top shelf work. Well done.

[RickS]

Both great images and the difference is very striking.

[strongmanmike]

Great work Rolo, interesting twists on an o'l favourite too :thumbsup:

Those stars must annoy you though, what is happening there?, perhaps some star rounding would be useful :question:

Lovely colours too

Mike

[marc4darkskies]

Enjoyed very much Rolf! Beautiful. My preference is for the RGB version. Detail and colour treatment is spot on IMO! :thumbsup:

A bit of tilt going on there? My eye detected some star elongation too.

Cheers, Marcus

[Ross G]

Two great looking photos Rolf.

I like the comparison.


Ross.

[Paul Haese]

The IR version is really great and shows an really nice transparency.

[SkyViking]

Thanks Rod, I actually got plenty of data on the side while CenA was either too low or after it had passed out of view. Since the scope was running anyway I couldn't let a single hour of clear skies slip away :lol:


Thanks very much Marc, glad you like the colours. I actually battled first with one version where I couldn't quite get the colours right, so I parked the image pair for about a month while working on some others and then only returned to it recently - and then in the end I think colour wise it turned out to be one of my best. Sometimes that's just how it is. :lol:


Thank you Hugh!


Thanks Larry :)


Thanks Jase, I'm glad you liked it, and particularly the NIR colouring approach. I've been experimenting quite a bit in the past before I settled on this scheme for NIR work.


Thank you Rick, I hope to get around to do another video that includes this image pair so the differences can really be explored fully.


Thanks Mike, yeah the guiding is not perfect, and I think there might be some astigmatism too actually. I trust it'll all be sorted once the new 12.5" is ready, I have just been too lazy to deal with it yet :lol::P


Thanks Marcus, I'm also happy with how this turned out. As above, I expect the stars to be sorted soon. :thumbsup:


Thank you Ross, these comparisons really show the differences. I hope to do more in the future.


Thanks Paul :) Yes the NIR is really quite interesting, one never knows what it'll look like for a particular nebula until the data starts ticking in. The three comparisons I have completed so far have all produced very different NIR detail.

[SkyViking]

BTW, in case anyone is interested I have recently begun producing large format posters which have become quite popular. One of these feature all the images from my Optical/NIR comparison series.

I have put an ad in the For Sale section with all the details: http://www.iceinspace.com.au/forum/showthread.php?t=111302

Regards,
Rolf