My First Attempt At Narrowband Imaging

I’ve always shot using either a DSLR, stock and modded, or a one shot colour (OSC) camera, such as the ZWO ASI178MC. Granted, the 178MC is regarded as a planetary camera, but it’s still pretty decent for imaging the deep space objects such as nebulae, galaxies and clusters.

The inherent issue using either an OSC or DSLR is that, without filters, we’re restricted in the data we can obtain to the visual spectrum. It’s not until you start using narrowband filters, and targeting specific wavelengths, that you can open up your data acquisition. What’s more, with narrowband filters, because they target those specific wavelengths, you can almost ignore light pollution and to a large extent the moon. Obviously if you have a streetlight very close to the frame, or you’re shooting right next to a full moon, you’re still going to get some effect from those, but in the main, you can ignore these if you’re reasonably sensible about where you point your scope in relation.

The ZWO 1.25″ Duo Narrowband Filter, my first foray into narrowband imaging

As this is my first foray into narrowband (or NB) imaging, I wasn’t expecting amazing results right out of the gate. I used the ZWO 1.25′ Duo Narrowband Filter, which isolates the hydrogen alpha (656nm) and Oiii (500nm) wavelengths specifically. This sat in front of the ZWO ASI178MC and I used the SkyWatcher 72ED mounted on the Star Adventurer. Guiding was handled using a generic 9×50 finder and the ZWO ASI120MM via PHD2, whilst images were captured via AstroPhotograpy Tool (APT.)

Granted, this is a little heavy for the mount being used, but I can often push 2 minute subs with it, and even if I need to dial back to 1 minute, so long as I capture enough frames, it should be sufficient. As it turned out I was only able to get 30 second subs out of it due to being west heavy but not being set up for it properly. Lesson learned!

One of the good things about this, and similar, filters is that they’re a relatively inexpensive way of accessing NB imaging without having to invest in a monochrome camera, separate filters for Ha, Oiii and Sii, and filter wheel. They can help to improve the contrast and visibility of emission nebulae when imaging with an OSC. And with regards to this specific filter, it’s also fully compatible with the range of ZWO OSC cameras.

Although not as specific a bandwidth as, say the 12 or 6nm dedicated Ha filter, it’s still a good entry point in allowing through an 80% transmission rate at 15nm in Ha and 35nm in Oiii, although I anticipate that fairly soon one is going to want to tie those bandwidths down even tighter. But down that road lays monochrome NB imaging and decent skies and weather are few and far between in the UK, so if I end up going that route I may just have to accept that projects are going to take much longer to complete. And I guess that’s another advantage of dual NB in that you can capture those wavelengths at the same time and not have to spread it out across multiple nights possibly.

With a clear forecast I was lucky for first light with this filter. Decided to stay with the target I’ve been shooting most recently from my home location so, considering how drawn I’ve felt towards them, I’m finding it quite fitting that first light with this filter should be the Pillars of Creation. I obviously didn’t expect anything near Hubble quality, but it turned out fairly decent considering I only had an hour’s worth of usable data.

Stacking And Processing

The stacking I carried out in AstroPixel Processor (APP), and broke it down into three sections.

First off I did a normal stack for the RGB data, which I then saved without further processing it. I then renormalised the data and extracted the Ha wavelength. The stacking of that provided a monochrome image, which I then saved, again not doing any processing. I then repeated that for the Oiii wavelength. I may well redo the stacking but this time running it through the star reduction tool that has just been released in the beta version of APP.

Opening Photoshop, I brought in all three images that I’d saved from APP. Using the “copy all” I pasted the Ha into the red channel of the RGB image, and the Oiii into both green and blue channels. From there I ran it through a partial normal process of cropping for the stacking artifacts on the edges, and getting rid of as much noise as possible, whilst boosting each channel slightly with levels and curves adjustment layers. I then flattened and saved the image before bringing into Lightroom for some minor tweaks before the final save.

What I’ve taken from this is that narrowband imaging isn’t actually that difficult even with an OSC camera. In fact, to my mind, it seems easier to achieve with an OSC than it is with monochrome and separate filters for each wavelength of Ha, Oiii and Sii. Granted, my own filter only allows through Ha and Oiii, but it’s certainly enough at this point. And I definitely noticed that I didn’t need to run it through any light pollution removal algorithms due to the specific band passes the filter allows through. So for me, at this point, narrowband is the way to go with certain DSO’s and I’m looking forward to spending a lot more time imaging in this way.

The final image isn’t much to look at, it’s noisy due to the lack of integration time, but just that hour looks better to my eyes than the more than double amount of time I have in broadband. I’ll probably try and integrate that data in with any future narrowband I do, but for now, I’m happy with the way this has gone.

I’ll leave you with the final image from my first light with this fantastic and inexpensive filter. Thank you for taking the time to read, and clear skies all!

The Pillars of Creation in Ha and Oiii. 120 x 30 seconds at 270 gain, 340 offset. Taken with the ASI178MC and 72ED with a ZWO 1.25″ Duo Narrowband Filter. Mounted on the SkyWatcher Star Adventurer

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