About the Target

Messier 16, also known as the NGC 6611, and is associated with the popular names of: "The Eagle Nebula", "The Star Queen Nebula", "The Spire", and lets not forget its most famous feature "The Pillars of Creation".  It is a diffuse emission nebula associated with a star cluster in the constellation of Serpens.  It is located about 5700 light-years away. 

Wikipedia tells us the following about the Pillars of Creation:
"Images produced by Jeff Hester and Paul Scowen using the Hubble Space Telescope in 1995 greatly improved scientific understanding of processes inside the nebula. One of these became famous as the "Pillars of Creation", depicting a large region of star formation. Its small dark pockets are believed to be protostars (Bok globules). The pillar structure resembles that of a much larger instance in the Soul Nebula of Cassiopeia, imaged with the Spitzer Space Telescope in 2005[ equally characterized as "Pillars of Star Creation". or "Pillars of Star Formation". These columns – which resemble stalagmites protruding from the floor of a cavern – are composed of interstellar hydrogen gas and dust, which act as incubators for new stars. Inside the columns and on their surface astronomers have found knots or globules of denser gas, called EGGs ("Evaporating Gaseous Globules). Stars are being formed inside some of these.

X-ray images from the Chandra observatory compared with Hubble's "Pillars" image have shown that X-ray sources (from young stars) do not coincide with the pillars, but rather randomly dot the nebula. Any protostars in the pillars' EGGs are not yet hot enough to emit X-rays.
Evidence from the Spitzer Telescope originally suggested that the pillars in M16 may be threatened by a "past supernova". Hot gas observed by Spitzer in 2007 suggested they were already – likely – being disturbed by a supernova that exploded 8,000 to 9,000 years ago. Due to the distance the main blast of light would have reached Earth for a brief time 1,000 to 2,000 years ago. A more slowly moving, theorized, shock wave would have taken a few thousand years to move through the nebula and would have blown away the delicate pillars. However, in 2014 the pillars were imaged a second time by Hubble, in both visible light and infrared light. The images being 20 years later provided a new, detailed account of the rate of evaporation occurring within the pillars. No supernova is evidenced within them, and it is estimated in some form they still exist – and will appear for at least 100,000 more years."
About the Project

I don't know about you, but Messier 16 - The Eagle Nebula is one of my Favorite objects in the sky.  I think it started when I first saw the "Pillars of Creation" shot by the Hubble Space Telescope back in 1995.

https://www.nasa.gov/image-feature/the-pillars-of-creation

That incredible image impressed me for a long time, so when I first started my astrophotographic journey in 2019, it was one of the first targets I went after.  It was a pretty crude image but I could see the "eagle" so I was pretty happy with it.  Last summer I revisited  M16, capturing images over two nights (a first for me at the time) and I processed it with Pixinsight  - which I was just learning - and got results that were a distinct improvement. 

At the time - a One-Shot-Color Camera was what I had, so no Hubble Palette for me.  I upped my game by gathering more subs and experimented with more sophisticated processing techniques.This Year's CaptureBut this year - I was really psyched to try m16 for the third time.  This time I had a telescope platform with a slightly longer focal length and I had a brand new ASI 2600MM-Pro Mono setup - so for the first time, I would be able to shoot with Narrowband and use the Hubble Palette to render the image in a way similar to seen in that first Hubble image.

Then we hit Rochester's Monsoon Season of 2021……

I began this project back on the Fourth of July and took 2.5 hours of subs before having to shut down. Given my site's tree line,  I only have about a 3-hour window where I can shoot any given target.  With some luck,  3 or 4 more nights should do it and I would have - I hoped - a wonderful image. Since then I have set my gear up twice to try and capture more subs, only to be shut down by the clouds.   Finally - on Wednesday the 14th, ClearOutside suggested I had an opening.   Gary Optiz - who is not only an accomplished astrophotographer but also a real weather wiz, warned me that we would be in the smoke plume from the forest fires out west and that would certainly impact imaging.  The Moon was also up as well,  but not too bad yet.  This would be about my last opportunity to shoot for this project this year, so I had to go for it. 

The captures seemed to go well but the sky transparency was very bad due to the smoke and because of the high humidity (dew was forming on the scope tubes and running off in streams!)The Resulting DataWhen I looked at the data the next day - I could see that I would have some processing challenges. First I had way fewer frames than I wanted (the story of my life!). 

When I scanned all of my frames with Blink I found about 25% of them had some evidence of very thin clouds passing through the frame. This would normally cause me to cull these images before stacking - but I had so few frames this time - I was very reluctant to do so. I really fought hard for those frames and I wanted to get some value from them. 

The Processing

The problem here is that the impacted images had very different gradients depending on when they were taken.  This would cause what would look like a higher variance for a given pixel when sampled across the subs.  This would raise havoc with the rejection algorithms and would cause the final pixel values to be off. 

I have been hearing about a new script in Pixinsight called NormaliseScaleGradients that does something kind of cool.  After calibration and registration, you pick a reference frame and each sub is normalized to the gradient seen in THAT reference frame.   This effectively removes arbitrary differences due to thin clouds, stray lights, and such.  Now that all of the subs look more similar -  the rejection algorithms can work more effectively and the image integration can now use more subs - weighted by their noise profiles.  This suggests that even pretty marginal frames can add some useful data to the final integrated results.  So I keep most of the questionable images in the mix, they had their gradients normalized, and they were included in the final master image, weighted by noise in comparison to other images. 

Anything that lets you keep hard-fought subs into the mix is my friend! 

I also had a problem where the size of the stars was a function of when I took the images.  Narrowband stars always look a little funky anyways- and this was certainly not going to help. So processing took a lot of time.  I was very careful to maximize what I could with deconvolution and I was also able to shrink some of the star sizes using the deconvolution function by taking the point spread function model created by PFSImage and trimming the outer edges a bit before application.  I picked up this type from a video by Adam Bock and it did help shrink the images.  But I still had mismatched filter channels so I spent a lot of time with star masks trying to help things out.  And I did help things a bit - they were much better than when I started, but having said that - don't look too carefully at the stars in this image as they are still a mess!

Capture Information

Lights Frames

• 18 x 300 seconds, bin 1x1 @ -15C, Gain 100.0, Astronmiks 6nm Ha Filter
• 14 x 300 seconds, bin 1x1 @ -15C, Gain 100.0, Astronmiks 6nm OIII Filter
• 15 x 300 seconds, bin 1x1 @ -15C, Gain 100.0, Astronomik 6nm SII Filter
• Total of 3.9 hours

Cal Frames

• 30 Darks at 300 seconds, bin 1x1, -15C, gain 100
• 25 Dark Flats at Flat exposure times, bin 1x1, -15C, gain 100
• Flats done separately for each evening to account for camera rotator variances:
• 25 Ha Flats
• 25 OIII Flats
• 25 SII Flats

Capture Hardware

• Scope: Astrophysics 130mm Starfire F/8.35 APO refractor
• Guide Scope: Televue 76mm Doublet
• Camera: ZWO AS2600mm-pro with ZWO 7x36 Filter wheel with ZWO LRGB filter set,
• and Astronomiks 6nm Narrowband filter set
• Guide Camera: ZWO ASI290Mini
• Focus Motor: Pegasus Astro Focus Cube 2
• Camera Rotator: Pegasus Astro Falcon
• Mount: Ioptron CEM60
• Polar Alignment: Polemaster camera

[b] [/b]SoftwareCapture Software: PHD2 Guider, Sequence Generator Pro controller, 
Image Processing: Pixinsight, Photoshop - assisted by Coffee, extensive processing[b] [/b]indecision and second-guessing, editor regret and much swearing…..