Askar 151phq; AP Mach2 GTO ASI6200MM, - Chroma 5nm Narrowband Filters
H,S,O: (46,28,81 x 720s, Bin 1, Gain 100); Total integration time = 31 hrs (July 17, & 19 to 23, 2024) Maple Bay, BC

Ou4 (The Squid Nebula) represents a nice challenge to astrophotography and I find myself going back to it almost annually to try another technique and see if my skill and available processing tools have improved. As part of my new M.O. of trying to explain what it is that we are looking at, here is the part where I would normally dive into an explanatory narrative. However, I have no idea what the Squid is, so I can’t really do that. For this information, there are a few plausible theories on the interweb that I recommend you read, in addition to the story of its discovery in 2011 that is ultra cool. Instead, I am going to describe a little bit about what narrowband emissions are and perhaps this can be integrated into our understanding of the physical processes that are at play, not only in this image, but also whenever we employ narrowband imaging.

Condensed (solids, liquids & highly supercritical gases) materials can emit/reflect/adsorb a broad spectrum of light energy by jiggling the inter and intra electromagnetic bonds at a wide variety of frequencies. The wavelengths cover a large range representing the degrees of freedom of movement in the bonds limited by the quantization of photon energy levels. The peak wavelength of emission is largely determined by the nature of the intermolecular bonds and the temperature (amount of jiggling).

Gases, say at standard temperature and pressure (STP = 1bar, 15C) generally have no inter-molecular attraction (except when they collide!), so cannot emit generally light by jiggling inter molecular electromagnetic bonds. Instead, they are restricted to emitting or adsorbing light frequencies corresponding relatively precisely to any degrees of freedom they have in either their intra-molecular (atomic) bonds or electron quantum numbers or quantum energy levels. As a result, if we want our images to selectively look at the emissions given off by gases, we use filters that exclude most of the light wavelength/frequencies other than those narrow bands of frequencies given off by the gases. Hence narrowband imaging is our way of photographing the gases themselves, although we do capture broadband starlight too due to overlap with our narrow bands.

There are a multitude of gaseous ions (charged) and neutral molecules and atoms that we could choose to focus in on, and from these we can only select a few to avoid going broke purchasing filters. I would love to say that we picked Ha, [OIII], and [SII] because we were particularly, scientifically interested in these atoms and ions, but that wouldn’t be true. Instead, we picked the filters/narrowband wavelengths for more practical reasons:

1) We wanted the narrow bands to be in the visible range, although this is not a requirement for general astrophysical purposes
2) We wanted the signal to be strong from a wide variety of deep sky targets
3) We didn’t want to different peaks to essentially duplicate the same signal, either by overlapping wavelength peaks (eg. NII & Ha), or more fundamentally – different peaks generated by the same gas in the same conditions (Ha & Hb). In other words, we don't want the same picture of the same thing with no additional information.

Ha (hydrogen alpha -= 656.3nm) would likely be the top pick wavelength by most astrophotographers restricted to only 1. It is both important and has a strong signal because hydrogen is the most comment element out there. In a molecular cloud it exists as a the simplest of molecules (hence the name of the cloud) and has very limited degrees of movement freedom as a gas. The process of Ha emission is when a Hydrogen atom is hit by an ultraviolet photon (likely from a star), its lone electron breaks away from the nucleus (a proton) entirely (also breaking its bond from its molecular partner). But this is not what causes the Ha emission – a Ha positive ion is just proton and has no electrons to emit any photons.   Rather, it is after the H atom has recaptured an electron in an excited quantum orbital and is transitioning to its ground state that the Ha photon is emitted. Specifically, it is the transition of the electron from orbital quantum number 3 to number 2 that yields the photon of more specific Ha wavelength. Of course other narrowband wavelengths are given off by the electron transition through its quantum energy levels, but most of these are outside the visible range.

As an aside, the actual recapture of the electron by a proton is very interesting and another photon is given off during this process, called breaking (or Bremsstrahlung in German) radiation. If a fast moving electron recognizes a proton with which it desires a relationship, it has to slam on its brakes suddenly in order to get to know one another. This often requires a release of a lot of energy in the form of an X-ray photon and it is this type of radiation that is used in a dentist office to X-ray your teeth. If the circumstances are correct deep sky objects can give off X-rays when fast moving electrons encounter positive ions.

What makes Ha filters so vital is that its brightness is proportional both to the density of hydrogen and to the intensity (not so much to the frequency) of the UV light that hits them. In addition, the Ha atoms and molecules are so light, that they are subject to inertial forces brought on by the UV photon momentum. It is typical to see sharp hydrogen concentration filamental or planar fronts in a Ha signal, caused by either supersonic or subsonic flow, and well as diffuse signals formed by diffuse clouds. Hydrogen atoms and molecules that exist in the interstellar medium are too rarefied to give a signal, while dust prevents the UV light from travelling deep into the molecular clouds, or the resulting Ha emissions from escaping. Diffuse Ha signal forms much of the background (Sh2-129) of our Squid image.

A second choice for a filter is most often the [OIII] (actually 495.9 and 500.7nm depending on how you spin it) signal, and to describe what is going on in this case is a little trickier.   First of all, the OIII (which is code for an O+2 ion) which is code for an oxygen atom that has been ionized by the removal of 2 of its original 8 photons. Now, the first thing to say is that an oxygen atom really, really likes its electrons. It actually desires 10 in total to fill its lowest 2 orbitals to reach Neon Nirvana. So to take two electrons away requires high frequency UV light and a lot of it to create a significant amount of OIII ions to make a signal. This is good for astrophotography since now our signal from our filter will allow us to “see” a different signal from different parts of the same nebula.

Another characteristic of the [OIII] descriptor are the square brackets (not bras and kets) surrounding the anion name. They make me chuckle because they stand for “forbidden”, reflecting the poor parenting skills of early astrophysicists (as in drinking is "forbidden"). This particular signal from a OIII electron quantum transition could not be produced down here on earth and was therefor labelled as forbidden. So much so, that initially the [OIII] signal was thought to be created by an as yet unknown element called “nebulium” - it couldn't be oxygen as that is forbidden.   As it turns out, forbidden doesn’t have its normal binary meaning, and only means that oxygen can’t emit this light at home, while out and about in space it can do it all it wants. It seems that whenever we tried to get an oxygen ion to emit at this frequency it would interact with another ion/atom/molecule and take a different path towards its ground state and the [OIII] emission would not occur. The half life of [OIII] emission is on the order of minutes, and in that time a humongous number of interactions will have taken place, even in our strongest of vacuums so the emission was deemed forbidden.   The ultra-low density needed to avoid bumping into other entities only exists in space and cannot be reproduced in by us. This makes [OIII] signal difficult to interpret, because it is conditional on a “goldilocks” concentration of oxygen – too much or too little and the signal is lost. It is a much heavier atom than hydrogen however, and is therefore less subject to inertial forces. As a result it is generally found closer to star clusters in stellar nurseries (where the UV light is strongest) and generally only forms sharp fronts where there is supersonic flow. In our Squid image, I don’t really know, but its composition in almost entirely made of [OIII] emissions.

Next, and arguably a weak third filter selection is [SII] (672.4 nm) which is code for a forbidden S+ anion emission. Sulphur also likes its electrons, but it is far more flexible that Oxygen it how it shares them around. It only takes weak UV to ionize and excite sulphur ions, and it only has to give up one electron for our cause.   Like oxygen however, it commonly shares them with hydrogen (H2S), but more importantly, with organic compounds. Once bonded as part of an organic, however, it freezes to the surface of dust in the molecular cloud at cloud prevailing temperatures – taking this sulfur out of the gas phase and available for emission. Note that this process is very slow, but inexorably leads to a different signal (originating from a different place) than Ha even though both signals are associated with the molecular cloud itself and more modest UV bombardment. The sulphur (or sulfur) signal is usually further away than the other two filter signals from a star cluster in a molecular cloud, but in our Squid image, doesn’t really add much additional information to the Ha signal from the molecular cloud.

As a final aside, the emission of infrared (15 micron) photons by [[CO2]] molecules is doubly forbidden.   Firstly, because at STP, excited CO2 will yield its energy thermally to air molecules bumping into it billions of times before it has its 0.5 seconds half-life to re-emit its photon. It is doubly forbidden because it is nowadays it is forbidden to mention that this emission is forbidden in the first place.

It turns out that forbidden things are sometimes allowed, when conditions are not under our control.