Narrowband imaging is all about the filters you use to block out every bit of light except one narrow section of the electromagnetic spectrum. The photons landing at your telescope's front door with the correct wavelength (around 656nm for hydrogen-alpha) are allowed to pass. Everyone else is kicked out. And sometimes with narrowband imaging you just want to go...narrower. Over the last ten years of astronomy and astrophotography I have used 12 nanometer, 7nm, and 6nm filters. (These numbers represent the gap in the filter, allowing light of a specific range of wavelengths through). I bought an Antlia 3nm Narrowband H-alpha Pro Filter a month ago, but haven't had a chance to test it out until now--mostly due to poor weather. The Antlia 3nm Ha filter is only $269 USD, Agena Astro; compare this price with high-end filters Chroma $575, Astrodon $564.
Wednesday, Oct. 6th was my first night out with the 3nm Ha in the EFW, along with my Astronomik 6nm Ha, OIII, and SII filters. At some point I may do a side-by-side comparison with the Astronomik 6nm, but I'm already working out different strategies for using both, depending on the target. I assumed I would double exposure times or number of exposures going from 6nm to 3, and went with a little of both--67 x 300 second subs for the Rosette Nebula, which is very bright. One noticeable difference during capture was lower contrast in the individual subs--or the appearance of lower contrast because I think there's now hydrogen data where I was expecting empty space, and this gives the entire frame a brighter, more uniform appearance? I was expecting more Ha data--that's the reason to go narrower, to cut out everything except hydrogen-alpha (looking at you NII--nitrogen 2).
So, I was a little worried, watching five minute exposures appear in Ekos, that seemed flatter, with less detail, but after capturing 70 and stacking 67, the result was better than expected. The other difference I hadn't really anticipated--or even thought about--is longer autofocus times because I've bumped up the exposure time to 30 seconds at 1x1 binning. For the ZWO EAF, I went with linear/SEP in Ekos and it takes 4 - 5 minutes for the full autofocus process, most of that in exposure time. I usually see about 2 with other narrowband filters, and something like a minute with the clear filter (taking 1 - 4 second exposures with the Clear). The Astronomik filters are close to parfocal, but the Antlia is pretty far off, to the point where I will probably use a starting offset for the Ha 3nm when jumping between different filters. Okay, so longer exposure times and focusing routine, what about the imaging? Well, I'm impressed. Unfortunately, I don't have experience with other 3nm Ha filters, but I can tell you there is a noticeable difference between the 3nm and my 6nm--and I like what I'm seeing. Like any new tool or process, it takes some getting used to, but once you're there, this Antlia 3nm Ha filter seems to work well, does exactly what it's designed to do, and so far--after three nights of imaging--is well worth the price. I'm really happy with the results so far.
Here it is in hydrogen-alpha, just the one narrowband slice of the spectrum. Andromeda Galaxy, Messier 31, is probably the first target for northern hemisphere astrophotographers--well, it's either M31 or M42 Orion Nebula. Both are intensely bright deep sky objects you can see without a telescope, given reasonably clear and dark skies. I have captured M31 more times than I can count. The galaxy stands out in the night sky when it swings around every year, making it an easy target. Galaxies are also great full spectrum targets for color cameras, DSLRs and mirrorless cameras, but that doesn't mean you can't pull some awesome data out of Andromeda in narrowband or infrared. I captured M31 in near-infrared (longpass 850nm) a couple years ago, and last night I captured our galactic neighbor in very narrow hydrogen-alpha (3nm Ha), and this stacked set of 42 sub-exposures highlights massive HII regions, mostly emission nebulae, along the outer spiral arms, with dense bands of dust and debris ringing the galactic core. This will make a great red channel in an RGB image, or I may go with HaRGB.
I started another narrowband imaging run of the California Nebula (NGC 1499), this time in 3nm. Here's my first pass of 100 x 240-second sub-exposures in hydrogen-alpha (Antlia 3nm Ha filter) with my ZWO ASI1600MM-Pro monochrome camera and William Optics SpaceCat 51 Refractor (250mm FL @ f/4.9).
Some notes on the image below:
NGC 1499, the California Nebula, is a large HII region and star-forming cloud in the Constellation Perseus. Its 100 lightyear length is roughly shaped like the State of California—I mean, you can see the San Joaquin Valley, the Sierras and where the land angles in for southern California and the L.A. Basin. So, the name fits the shape. NGC 1499 is east of the Perseus Molecular Cloud, a large star-forming region surrounding the star clusters, IC 348 and NGC 1333, but also includes a bunch of hot O-type stars on their own. The HI supershell in Perseus, a broader backdrop of atomic hydrogen and helium, encompasses the molecular cloud as well as the California Nebula. Almost every nebula I have ever imaged is within our galaxy, the Milky Way, but what's interesting with NGC 1499 is that our solar system and the California Nebula are both in the Orion Arm of the Milky Way, about 1200 lightyears apart. It's just up the galactic street from us. We're practically neighbors!
The O-type blue-giant star Menkib (the Bayer designation is ξ Persei) is 12,700 times brighter than our star, the Sun, and it's 30 times the mass. It's also seriously hot with a surface temperature around 35000 degrees kelvin. Compare that with 5778 k surface temp of the Sun. There's evidence that Menkib is a "runaway star" based on it's abnormally high radial velocity, which means it is moving away from its stellar association—where it originally formed—in a calculable direction and at an unusually high speed, as if it was ejected by some stellar event or disruption (e.g. gravitational interactions with other stars, supernova explosions, usually something violent and large-scale). The intense radiation from Menkib has carved out the hollow on one side of the California Nebula while lighting up (ionizing) the roughly 900 trillion kilometers of dense clouds of molecular hydrogen. Menkib is continuously bathing all of this hydrogen in radiation, and this is where ionization steps in to help us with our imaging. Electrons get a bit excited with all this loose energy about, and individual ions will lose electrons in the process. In this short-lived energized state a hydrogen molecule is unstable, and when an electron inevitably drops back into a stable position, its ground state, this transition energy causes a photon, a particle of light, to be released in the corresponding wavelength. That's the ionization part, but it's these photons, traveling 1200 lightyears, that we pick up with the digital sensors in our cameras. The simplest definition of astrophotography is catching photons from distant objects, usually either generated or reflected by them.
The elliptical galaxy IC 2005 is just a tiny smear of light in the East Bay area of the California Nebula. It's 279 million lightyears away us, but it's still bright enough to shine through the dense clouds of ionized hydrogen that make up NGC 1499. IC 2005 is 30,000 light years in diameter and it's moving away from us at just over 5800 kilometers per second, about 1.9% of the speed of light.
IC 2027 is an elliptical galaxy in the Constellation Perseus, about 287 million lightyears from our galaxy.
IC 2003 is a planetary nebula in Perseus, about a light year in diameter and 12,000 light years away, roughly halfway between us and the edge of our Galaxy.
Here's my astro setup for the last couple nights, my wide-field narrowband rig, with the William Optics SpaceCat 51 APO refractor and the ZWO ASI1600MM-Pro monochrome camera, ZWO electronic filter wheel and autofocus. Sky-Watcher EQ6-R Pro equatorial mount. Controller: Raspberry Pi 4 4GB running INDI/Ekos/KStars.
It was nice to finally have the new moon line up with some clear night skies! I'm shooting the Rosette Nebula in Monoceros, right next door to Orion, which I love seeing in the sky.
Narrowband North America Nebula (NGC 7000) in the Constellation Cygnus, with the "Cygnus Wall" across the lower third of the image. You're looking at about 130 trillion miles of dense clouds of ionized hydrogen, with a bit of oxygen thrown in. NGC 7000 is around 1600 lightyears aways, and the "Cygnus Wall" is about 20 light years long. Imaging notes: William Optics GT81 Apochromatic refractor, Astronomik 6-nanometer hydrogen-alpha (Ha), Oxygen 3 (OIII), sulfur 2 (SII) filters, ZWO ASI1600MM-Pro monochrome camera, QHY 5iii178 guide camera, Sky-Watcher EQ6-R Pro Mount.
First narrowband test with the new Apertura Newtonian scope, although it was barely a test—31 sub-exposures in Ha before the clouds started rolling in, and with a very bright full moon in the sky. IC 1396 is a large (3 degrees in diameter) emission nebula and star-forming region in the constellation Cepheus, about 2400 lightyears away. Within IC 1396 is the Elephant's Trunk Nebula catalogued as IC 1396A, pictured here in hydrogen-alpha (narrowband). This structure is well over a hundred trillion miles long (roughly 20 lightyears), so if you want to get from the top to the bottom you'd better have a pretty fast vehicle.
Notes: 31 x 240 second subs in Ha. Apertura 8" Newtonian 880mm FL at f/4.4, ZWO ASI1600MM-Pro monochrome camera, Astronomik 6nm Hydrogen-alpha filter, SkyWatcher EQ6-R Pro mount, controller: INDI/Ekos/KStars. I managed to capture 31 subs of IC1396A, but no calibration frames—that's only half the subs I would normally want for this one in Hydrogen-alpha. I was not dithering here either, which is something I normally have on, and the result is some "walking noise" which is pattern noise in one direction--top-right to bottom left, in this case, introduced either by polar alignment drift or differential flexure between the guide scope and the OTA (this is the most likely suspect here, but not absolutely sure). Guiding total RMS averaged around 1 arcsecond the whole time, not the best, but I attributed it to the full moon, lack of contrast, but who knows. I didn't see any walking noise with the color rig, but even so, I'm going to go with off-axis guiding next time th eskies clear and see what happens!
Clouds swept in around midnight, and then it rained through the night, so I took what I could get and did some mild processing. There's serious coma around the edges. I have the stars pretty well dialed in with the color train, but not here. I have some measuring and caliper work to do!
The area around the constellation Cepheus is an astrophotography buffet, and because the constellation is circumpolar, it's in the night sky for at least half the year—if you're anywhere near the north. Both the Iris Nebula and the Fireworks Galaxy, two amazing deep sky objects I captured last week, are here, along with the Elephant's Trunk nebula, Cave and Wizard nebulas—and a lot more! Some of these targets span the border with Cassiopeia at the bottom.
BiColor version of the central region of the Elephant's Trunk Nebula--Hydrogen-alpha and Oxygen 3:
There's a beautiful moon out there tonight! Nikon D750, AstroTech AT6RC 1350mm f/9, 1/2500 sec exposure.
I selected IC 5146, the Cocoon Nebula as a nice broadband color target for my second clear night with the Newt, and at 800mm focal length, the Cocoon should fit nicely in the frame, with room for the long trail of dust flowing away from it--if I can manage it.
Mechanics: the stars only look slightly better tonight, still wacky around the edges. They were a coma-induced mess last night, but I rearranged some of the imaging train and...it looks, well, not much better. But I'll get there. I am not the type to spend all night twiddling with spacers and digital calipers to calculate the correct back-distance and eliminate every last trace of coma. I'd rather image, and make small corrections along the way, even if it means the stars look like crap before I get things properly dialed in.
So, I was also looking for a nice target to shoot while the Cocoon climbed a bit higher in the sky, more than 30°, and I went for NGC 6946, Caldwell 12, the Fireworks Galaxy in Cepheus. I don't normally go for galaxies, unless they're relatively close by, but there are a few like M101 (Pinwheel Galaxy) and M81 (Bode's Galaxy) which are great targets if you're running with less than a 1000mm--other than the obvious ones like M31, M33, LMC, etc.
The Fireworks Galaxy is 22 million lightyears away. To put that distance in perspective, one lightyear is almost 6 trillion miles. I'll leave the multiplying by 22 million up to you. Fireworks is a medium-sized galaxy, about half the size of the Milky Way at about 40,000 lightyears across. It's a spiral galaxy, and it has the advantage, from our perspective, of being face-on, or is that top-down? Notes: 31 x 120 second subs, no filters.
NGC 6946, Caldwell 12, the Fireworks Galaxy in Cepheus:
The Cocoon Nebula (IC 5146, Caldwell 19, Sh 2-125) is a roiling mix of new stars, ionized gases emitting light, mostly in wavelengths out at the hydrogen end of the spectrum, with additional layers of clouds of dust and gas lit up by those same stars. Actually, most of the ionizing weight appears to be carried by the central pre-main-sequence B type star, BD+46°3474. The whole circular structure labeled IC 5146 is a reflection nebula and an H ii region that forms a "cocoon" around BD+46°3474 (that bright star in the center), which is probably less than 10 million years old (compare that to our star, the Sun, at 4.6 billion years old). This central star's intense radiation and stellar winds have hollowed out the dense molecular clouds, and the radiation flowing off it is ionizing the material and lighting it up. There's a long dark band of gas trailing off frame at the bottom of this image that leads to a separately identified dark nebula, Barnard 168 (B168). The Cocoon is around 2500 lightyears away in the constellation Cygnus. Imaging Notes: There's so much going on here across the visual spectrum right on out to hydrogen-alpha and near-infrared. Most color astro cameras, DSLRs, and mirrorless cameras will be able to pick it all up. If you can, light pollution allowing, try to catch the Cocoon Nebula without any extra filters. My imaging run: 67 x 240 second subs, no filters. Apertura 8 inch f/4 800m FL Imaging Newtonian, ZWO ASI071MC color camera, GSO Coma Corrector, SkyWatcher EQ6-R Pro mount, Stellarmate/Raspberry Pi 4 running INDI/Ekos/KStars.
IC 5146, the Cocoon Nebula in Cygnus:
NGC 6820 (Sh2-86) in hydrogen-alpha narrowband. This emission nebula in the constellation Vulpecula is made up of one vast pillar and several not-so-vast pillars of dark molecular dust and gases that have been eroded by the driving stellar winds and intense radiation of the star cluster NGC 6823 (above NGC 6820 in this rotation). Did someone say, "Bok globules"? Yeah, we got those, small dense chunks of dust and gas (hydrogen, carbon, helium, silicates, etc.) that lead to star formation. Of course, small is relative in this case. You can probably take a dozen copies of our entire solar system, including the Kuiper Belt and Oort Cloud and easily fit them all inside most. I gave this monochrome image a hint of vintage tint, a bit or warmth, to counter the core temperatures of Bok globules, which are thought to be some of the coldest objects in the universe. It also makes the image appear a bit brighter.