Disclaimer: This post is a deep dive into astrophotography so after we get past the nice pics it’s gonna get deep. I’ve given the ‘jargon police’ the night off so there will be some things in here you’re probably not familiar with. If you want to know how the process of taking these photos works from the inside then you might like the whole post.
Target: The Lagoon Nebula
The Lagoon is a famous object. It’s great visually in the telescope as well as a photographic target. It’s in the Messier Catalog as number 8 (M8) and hanging out right next to it is another M object, M20 the ‘Trifid’ Nebula. These are both objects I’ve imaged a number of times before, and had good results but they are in Sagittarius, a constellation that never gets all that high above the horizon from the US, even if you live in the south.
But here in the Southern Hemisphere it’s directly overhead at the zenith! Objects at the zenith have the least amount of the Earth’s atmosphere between you and them so you get better clarity. Also, at home I’m shooting from sea level–you can’t be looking through more air than that! Here in Namibia I’m at 2960 ft elevation which isn’t a huge thing but it is enough to help.
Oh the struggle
So on the bucket list for this trip was the classic “everybody’s gotta shoot it if you get the chance” Eta Carina Nebula; a true Southern Hemisphere magnificent target, probably number one on everybody’s docket and I’m no exception. However, this time of year, it’s fairly low on the horizon as night falls and even though I start imaging as soon as I can after dark, I only have so much time before it looks bad and the telescope has a hard time tracking it.
Typically you don’t image things once they drop below 30° elevation above the horizon because the Earth’s atmosphere makes them so blurry but here, I’m violating that rule, especially for Eta Carina, because the skies here are so clear, and the seeing is very steady in this cooler time of year. The quality of each individual exposure doesn’t vary all that much so I’m following Eta Carina down to 15° or so, or until the scope tracking just deteriorates so much the individual frames are compromised too much.
What Now?
So that means that by 9:30 or 10 I was done with Eta Carina and needed something new since I intend to shoot until the Moon (or the Sun) come up. As I write this, I’m four nights of shooting in on Eta Carina and still am only 3/4 of the way done getting my data.
So I needed a quick fix; something I know I can get, that would be a good update to my portfolio (last time I shot the Lagoon was 2020) and the Lagoon is a great target so I cued it up!
I thought I could do a good job in two nights so I decided to take 30 4-minute exposures through the L-Extreme dual passband (Hydrogen Alpha and Oxygen III); and 20 3-minute exposures each of red, green, and blue for a total of 90 subs. Eventually these will be gone through and combined into master shots of the three colors and Hydrogen. Then the RGB shots will be combined into their appropriate color channels in a digital color image and the master Hydrogen image will be prepared to act as a Luminosity enhancer by strengthening color and detail “from behind” as it were. Total exposure time was five hours.
Finally the image!
Ok, here is the Lagoon Nebula (right) with a photobomb by the Trifid Nebula (left)
The Trifid and Lagoon Nebulas in Sagittarius, July 28/29, Namibia Africa
There is a lot of really good detail here and you can click on the image to get a larger version but how about some closeups!
I love showing this object to people at stargazes because it’s a “three in one” object: an emission nebula, a dark nebula, and an open star cluster (bonus!).
The brightest patch here is the location of some really bright stars who’s incredible radiation output goes slamming into the floating hydrogen gas and causes the whole large nebula to glow; the color is more or less accurate.
The Dark Nebula and the reason for it’s name is the meandering dark lane just to the left of the bright patch–the actual “Lagoon”.
Just on the other side of the Lagoon is an open star cluster of about 15 stars and the rest of the nebula has many interesting dark clouds of opaque dust and darker material.
Trifid Fun
Also in the vicinity, and often overlooked but a very interesting object in it’s own right is M20, the Trifid Nebula. I recently shot this with my scope at the observatory in Hilton Head but in this shot I got a lot more data in the blue area of the nebula I think because it is so dark here in Namibia that there’s a lot more contrast between the nebula and the background sky.
The Trifid Nebula, M20
In all of these sky photos, when you see pink or red, that’s Hydrogen clouds. When you see blue, that’s Oxygen and the Trifid is a really great combination of both, and observing good social distancing! Note that the Hubble Space Telescope uses a different color palette, using Hydrogen, Oxygen III, and Sulfur II filters which is why Hubble photos have that distinctive look. I really like the pseudo 3-D look that this photo has!
Process this!
These are the steps you go through to make an image look like something. I have recently started using a processing program called Pixinsight which is the most complex, scientific grade, incomprehensible batch of tools I’ve ever run across. This thing makes Photoshop look like MacPaint back in the 1980s! I still use photoshop to finish images off, but the stuff they’ve got going on in Pixinsight is like ‘voodoo magic fairy dust’ to astrophotographers. It’s literally the same software they use on Hubble and now James Webb Space Telescope photos.
Honestly, the majority of the tools here are still a complete mystery to me and many of the parameter choices of the tools I do use are unbelievably geeky, (‘Windsorized Clipping”???) but after buying some training programs from people who know this stuff cold I’ve gotten to a place where I’ve have a workflow that I can understand and gives me good results.
Step One: Batch Preprocessing
You essentially take all 90 of the images and load them into this preprocessing window along with some shots that you’ve taken with the lens cap on. These are ‘dark frames’ and are essentially a recording of the camera’s self-noise that will be subtracted from the individual frames before combining them.
The Batch Script goes through and does about 8 different things before combining them into a cleaned, calibrated, location in the sky known, master shot for each of Red, Green, Blue, and Hydrogen (Luminance). It took about 1.5 hours to do this on my laptop.
Step Two: Getting Real
The first task is to combine the master Red Green and Blue images into an actual color image. So you call up the LRGB Combination tool and create a color photo which you can save.
Now you would think that after all this, you’d have something that looks pretty decent on your screen but wrong moosebreath, the image looks almost totally black, and you need to use software to brighten it up because most of the image data is lurking down there in the super dark grays just above the blacks.
So there’s a tool for that too, and it’s the Screen Transfer Function tool. Basically you push one button and it stretches the hell out of the data and you finally see what the image you’ve got actually looks like. Always a suspenseful moment followed by joy and satisfaction, or abject horror and the realization you don’t know jack!
Assuming you aren’t starting over, you realize that the STF you just used is a bit overzealous and has a tendency to overcook all the bright stuff so you make an adjustment or two to get back to reality and then call up the Histogram Transformation tool. There’s a way to forward the STF settings in there so you can actually apply them to your color image for real, and save it. While you’re in there you can make some more specific tweaks. You’re not trying to ‘finish’ the image here, just get it ready for the luminance layer to be added in.
Step 3: Starry Night? No!
Here’s where it gets weird; The software maneuvers you use in order to make the nebula look good, generally don’t operate well on stars so that always placed limits on what you could do until: StarXTerminator!
As you might expect, this is a little AI software routine that looks into your image, identifies all the stars and then extracts them into a separate color image you can save as the “stars only” version, leaving behind a starless version of the nebula you can also save, after you have properly processed it.
Typically the first thing that I do is a layer of noise removal. You see, when you have to jack up all the data from the dark grays into something you can see, you are also jacking up all the remaining digital noise as well, in spite of your best efforts in pre-processing. So I give it an effective but not too aggressive dose of NoiseXTerminator, with a touch of detail enhancement as well. This gets rid of the ‘granular’ look of the cloudy stuff making it all nice and smooth like butter! Time for a Save!
Step 4: Sharp Dressed Neb
The last major step I do in Pixinsight is to run BlurXTerminator, which is an AI driven version of the standard ‘deconvolution’ based sharpening routines you’ve seen in Photoshop for decades, but this is soooooo much better! The author of BlurX has done extensive training of the AI engine to understand what’s ok to work on and what to leave alone specifically in astrophotos and I can assure you it is the single biggest thing that makes my photos look more like NASA and not a hobbyist with a $10k setup!
Essentially there’s a ‘More’ slider to go sharper and sharper and I slide this up until I see a really good level of crispness creep in without it looking fake or pushing the algorithm so hard it starts making things up and creating little structures that aren’t actually in the real photo.
As a bonus it can also shrink bloated stars and reduce big halos around bright stars so there’s two different sliders for that as well as a routine to make some of those ‘not so round’ stars at the edge of the image more round.
Step 5: How Illuminating
Then I pull up the Hydrogen image which is going to be edited to be used as a Luminance layer in Photoshop later, so I do all the same things to it I did to the RGB pic taking care to not make the brights too bright because that will wash out the color in the RGB image instead of enhancing it.
A Luminance layer essentially lurks in the background and “props up” the RGB image by enhancing color and detail ‘from behind’
After this I save the Nebula Only, Stars Only, and processed Luminance images as 16-bit TIFF files so Photoshop can read them.
Step 6: Moving Forward
In Photoshop I’ll load all three images as separate files, take the RGB image and Save As… a Photoshop document with all it’s layering capability. If the RGB image needs a color tweak or looks a little too bright I’ll add a couple Adjustment Layers to take care of that.
Next I switch over to the Luminance layer see if it’s got any bright hot spots and take care of them before copying the entire image over and pasting it as a new layer in the RGB image. Set the Blend Mode popup to Luminance and you’re going to be getting your first real look at how your image is going to turn out. You can do some tweaks to the luminance layer if you need to because this is the first time you’ve seen Luminance and RGB working together.
Time for stars. I’ll go into the stars-only photo and generally you want the stars to be a little more colorful than they actually turned out so I hit them with the Saturation tool until you can easily tell the difference between the red stars, yellow and blue. If I see green stars (which don’t actually exist) I hit the image with the Levels tool, set for green channel only and take it down so far it’s a non issue. Copy the whole star image and paste it into the main pic document as the top layer over the Luminance layer and set the Blend Mode to “Screen” which overlays the stars onto the nebula image.
Sometimes the stars are so powerful and plentiful you have to do a couple things to minimize them, especially if your target is in a rich star field like this one was. If they’re just too bright, to the point where they are overrunning the nebula which is supposed to be the focus of the photo, then you can adjust the opacity slider for this layer down 20 or 30% and they will fade out enough that they haven’t taken over but you still get the idea that this object is in a busy area of the Milky Way and all those stars are impressive in their own right, aren’t they?
But sometimes, it’s still too much so you can hit the stars-only layer with Photoshop’s “Dust and Scratches” filter and by setting Radius and Levels to low values like 2 you can just wipe out all the super dim, tiny stars to help feature the nebula more without giving up the view that “this is a busy area of the Milky Way”.
Next you can combine all of these layers into a composite, single image by creating a “Stamp Layer” above all the RGB/Luminosity/Stars Layers. You do this because if you just “Flatten” the image, you ruin the ability to go back and make additional tweaks.
Now you can do things to the image like doing a final crop to get rid of those wacky stars at the edges, and to really compose the way your final image will look in the frame.
These files are freaking huge and nobody wants you to send them a 117 megabyte full-res photo so I make a couple of JPG versions, one at full resolution (keeping it under 20MB) and one crunched down that you can send around.
For this particular image I did some various crops to create closeups of the two separate nebulae, and that detail view.
Well that’s about how I do it. Each image has it’s own issues to deal with and goals you want to achieve but this workflow has worked really well for me and I share it here so that it might be of use to someone else.
Carpe Noctem!
Bill Gwynne
aka Bill the Sky Guy
Image Stats
Hydrogen/Oxygen for Luminance: 30 4-minute exposures
Red: 20 3-minute exposures
Green: 20 3-minute exposures
Blue: 20 3-minute exposures
Total Integration time: 5 hours
Processing time: 3.5 hours