Disclaimer: This article is meant to be a high level overview of the basics of narrowband astrophotography, meant to provide some insight to those curious on how these images are acquired. It is by no means an extensive guide and does not encompass some of the more subtle nuances involved in astrophotography.
Let's dive into it!
Deep space astrophotography is quite complex, entailing a number of technical challenges both in the acquisition and the processing of the images. Lets take a closer look at what is needed to obtain these kinds of images of space...
First off, what is it exactly that we're imaging? Let's take the Lambda Centauri Nebula. This nebula is a region in space some 6500 light years away and about 140 light years across, riddled with huge clouds of gas and dust. Energy from surrounding stars ionize the gases in space and makes them glow.
The object is located at such great distance from us that its light is very faint when it reaches Earth, so in order to capture it in all its glory we need to resort to a couple of things: The first one is long exposure photography.
Contrary to what's commonly believed, telescopes aren't so much magnifying glasses but light gathering instruments. The 90mm lens of my scope will gather exponentially larger amounts of light than my 3mm pupils, which is why telescopes can see these objects and naked eyes can't.
Add in a light sensor (camera) and now you have some of the tools needed to capture any Deep Space Object. Pointing the telescope straight at the nebula and taking long exposures ranging from 1 to 5 minutes will ensure that we capture enough photons to obtain a decent image.
Avoiding star trails
Here, however, we face our first challenge: As we know, the sky rotates above our heads, so just pointing a telescope and taking a long exposure won't cut it. In a matter of seconds the object will trail away from the field of view. So how do we deal with this?
An astrophotgrapher's best friend is actually not their scope or camera, but their telescope mount. Tracking mounts compensate for the Earth's rotation, so as the sky moves the mount will move the telescope at the same rate and "freeze" the object so we can take long exposures.
Additionally, a different, smaller telescope with a camera mounted on top of our primary setup will measure star movement and send pulses to the mount when it detects any drift, to ensure pinpoint accuracy and tracking. This is called autoguiding.
Acquiring our images
Once we have our object locked on and are tracking and guiding, we can start our actual shooting. Going back to our Lambda Centauri nebula, I mentioned this was a "narrowband image". This means that this was shot using a set of filters that will allow very specific wavelenghts.
The light emmited from nebulae comes from different gasses that are excited due to energy from nearby stars. Electrons in the gas atoms are struck by photons, which makes them emit light. This light glows in very specific wavelenghts. (fun fact, the same is true for plasma balls)
This means that we can use precise filters for imaging nebulae, so we can basically cut through all light pollution and let only nebula light pass. This is the magic of narrowband imaging, it allows us to capture these faint objects even from the most light polluted urban skies.
The most abundant gases are Hydrogen, Oxygen and Sulphur. A different filter is used for each one, so we take 3 separate exposures and will end up with 3 "master" images that will later be mapped to a color channel (red, green and blue) in our final image (more on that later).
Image stacking and callibrating
As we said, we need to make some pretty long exposures to capture these faint faraway objects. Unfortunately, a problem with long exposures in photography is noisy images. Here's an individual exposure, notice how grainy it looks?
To overcome noise, we resort to image stacking. Stacking is taking a large number of images and then averaging them in software, so we can increase the signal to noise ratio. By stacking we eliminate noise and we can better process the image to obtain more details. Before/after:
This is why you'll usually read statements like "X hours of exposure went into this image" in astrophotography. It's not that we took a 30 hour single photograph, it's that we took the same 3 minutes exposure over and over for X amount of hours, sometimes over several nights.
These images will get stacked for a much smoother and richer image. Next up... callibration frames! Unfortunately, taking insane amounts of frames and stacking just won't cut it. Telescopes and camera sensors are not perfectly efficient and have a bunch of defects in them...
This is why, besides our actual shots of the object (which are called "light" frames) we need to take a series of callibration frames to cancel out some of these defects... let's take a look at just some of these.
"Flat" frames are shot pointing at a source of light using a diffusion fabric, and will register the "unevenness" of how our setup captures light. These frames will be factored in when stacking our image and will eliminate any light gradients in our nebula raw shots.
When imaging, the electric current running through our sensor will generate noise in our shots. "Dark" frames are taken using the same exposure time as our light frames, but with the telescope cap on. This will record these noise patterns and then subtract them from our image.
Ok, so going back to our Lambda Centauri image, after 3 full nights of pointing and shooting at the same target, we have: - 200 x 120 second exposures in Hydrogen - 200 x 120 second exposures in Oxygen - 200 x 120 second exposures in Sulphur - 40 "flat" frames - 40 "dark" frames
Finally, let's do some post-processing!
Now we get to process our data! First up, we need to enter all our frames into dedicated software which will register, align, callibrate and stack all of them. After some intense processing time, it will deliver a master light frame for each filter.
Our master frames, however, are far from attractive. The data is there, but we need to pull it out. This entails levels adjustments, noise reduction and sharpening among others. Then, we can then combine these and map them to the Red, Green and Blue channels in a color image.
For our Lambda Centauri image, Sulphur got mapped to red, Hydrogen to green and Oxygen to blue. This choice is absolutely arbitrary, however this palette was popularized with the Hubble images and is now pretty standard (referred to as the Hubble palette).
A bit more of correcting, color adjustment and post processing, and voilá! We have acquired and processed a deep space narrowband image.
If you reached this far, thank you for reading! And if you like my work, please consider collecting or sharing!
