So I got a new camera and filter wheel. I finally got a monochrome camera. Why a monochrome camera? Well because for astronomy different objects produce light at different wavelengths, some not easily visible by normal color cameras. Having a monochrome or “mono” camera adds a bit of complexity to capturing images, however because these cameras lack a color matrix over the sensor, they are more sensitive to light. There is also the benefit to being able to capture only the light that you want. This helps eliminate problems with light pollution or even a bright Moon that would otherwise washout the target.
This camera is a cooled CMOS camera with a 3/4th size sensor. It’s sensor is a bit smaller than an entry level DSLR, the camera is cooled with a fan and a heat skink, providing a cleaner signal without thermal noise. In front of the camera is a filter wheel that houses 7 filters which I will explain.
Luminance filter: This is basically a clear filter that blocks infrared light. This lets all other light photons to hit the sensor. For a normal “broadband” image, this is the foundation of the image.
Red, Green, Blue: Like the name suggests, the basically allow light to pass that fits into the colors respective bandwidth. *Below image taken from Baader’s website*
As you can see, the RGB filters span about 100 nanometers. This is considered broad band. Buried in those wide bands are narrow bands. For me, narrow band is the whole end game for getting a monochrome camera. While RGB spans 100 or so nanometers each, narrow band filters pass only single digit nanometers or light, usually. Some pass up to 12nm of light.
So why, if these objects are so far away, and so faint would someone want to cut down the amount of light getting to the camera? Because these objects in space emit at very specific wavelengths. By blocking everything else, you are only getting the object you want. Now, not all objects in the night sky are ideal for narrow band imaging. Galaxies are one example. Galaxies are mostly collections of stars with some neblosity within them. Stars emit light in the broad band. Now, if you look closely at a very well done picture of a galaxy, you might see red blotches within the galaxy. Those are areas of hydrogen emission nebula not unlike M42, the Orion Nebula and countless other local nebulas. Those pictures usually are made up of LRGB and an extra set of hydrogen-alpha narrow band data.
Hydrogen-alpha (H-a): YES! Hydrogen. The most common element in the galaxy, no the UNIVERSE! So, what is Hydrogen-alpha? Well, in short, its a very specific wavelength of light emitted by Hydrogen. In visible light, it’s in the red band, however with many narrowband images, including those taken by the Hubble Space Telescope, it’s made green in processing. The famous pillars of creation, that stunning blueish green background? It would appear red in the visible light. Hydrogen-alpha is also used for solar scopes to view the Sun in amazing detail. These are very different filters looking at the same band of light. You can not mix these two however!
Oxygen III (Oiii): Oxygen. Wait, I thought people can’t breath in space. Well, they can’t! But, oxygen is made in stars. Oxygen, carbon and other heavy elements weren’t made during the big bang, they are made in stars. And when stars explode, those elements fly through space, condense and along with several other heavy metals form planets and life. Oiii is oxygen that has been doubly ironized, meaning the electrons are dropping two energy levels. It is found in planetary nebula, which isn’t surprising as oxygen is formed in stars, and planetary nebula are made from stars throwing off part of their gas into the surrounding space. It’s also found in diffuse nebula. This is where stars are normally born, and some die. The oxygen is from older stars that have come and gone. This wavelength is found in the blue/blue green part of the visible spectrum.
Sulfur II (Sii): Sulfur II is, you guessed it, singularly ironized Sulfur. It’s far into the red spectrum and it’s much fainter than it’s nearest companion H-a.
So, now we have discussed the three main filters used in narrow band astrophotography, let’s see what each of those look like alone, and then combined together.
33 stacked Hydrogen-alpha frames, 300 seconds each.
48 stacked Sulfur II subs, 300 seconds each.
33 Oxygen III subs, 300 seconds each.
Here is what all three put together looks like. The RGB color mapping is SHO. So Red = Sii, Green = H-a, Blue = Oiii. I am still waiting to get more data before I call this image finished. But I am pretty happy with how its turning out so far and I wanted to share some info about narrow band astrophotography. Thanks for reading! Check out a Full Size image here!