My latest encounters capturing narrowband images with a color camera have once again opened my eyes to a new level of astrophotography possibilities. In this post, I’ll share my results using a color CMOS sensor camera.
In the past, I found that by collecting narrowband data such as H-Alpha with a DSLR, you can complement your broadband (true color RGB) astrophotography. This is great news for DSLR astrophotographers that want to amplify their deep sky imaging.
Narrowband Astrophotography Filters
The ongoing learning curve involved with astrophotography continues to provide me with endless moments of victories and defeat. This time, I’ve experimented using an affordable one shot color camera: the Altair Hypercam 183C – with a set of 2″ round mounted 12nm Astronomik filters.
So what is a narrow band filter anyway? These are filters that allow a specific “narrow band width” of light to pass through them. Traditionally, the 3 different narrowband filters associated with astrophotography are:
- Ha (Hydrogen Alpha)
- OIII (Oxygen III)
- SII (Sulfur II)
My goal was straightforward, to capture a deep-sky image from my light polluted backyard with more interstellar details and nebulosity than ever before. The intense city glow is largely ignored, and the light from the deep-sky target can shine through. My target: NGC 6960 – The Western Veil Nebula.
This time of year offers up an endless amount of narrow band imaging targets. The one I have selected for this project is a supernova remnant in Cygnus – The Western Veil Nebula. NGC 6960 (often referred to as “The Witch’s Broom”), is an exemplary target for narrow band imaging with a DSLR or color camera due to its composition of hydrogen, oxygen, and sulfur.
I was given a great deal to buy these filters used from Sean Molony at www.redstickastro.com. You may recognize his name from the work he has done with the Astro Imaging Channel. Thank you, Sean, and keep up the fantastic work.
Narrowband Images with a Color Camera
Is narrowband imaging with a DSLR or color camera a waste of time?
The astrophotography community made it quite clear that a monochrome camera such as the ZWO ASI1600MM-Cool is the way to go when imaging with narrowband filters. The bayer matrix design of a color CMOS camera sensor cuts the light gathering power by 1/4, meaning less detail is recorded than with a Mono CCD.
These warnings have not gone ignored, and every bit of that statement is true. However, the financial position I, and so many other backyard astrophotographers are in, mean that purchasing a new mono camera for astrophotography in the $2K range is not something that can be done quickly.
Utilizing the gear I have to push the limits of whats possible is perhaps my favorite aspect of this enchanting hobby. The Altair Hypercam 183C utilizes an internal cooling fan and open design to reduce noise from the 20MP CMOS sensor. The highly sensitive sensor also benefits from CCD-level controls such as Gain adjustment and produces high-resolution 12bit RAW .FIT images.
Benefits from narrowband using a DSLR
Here is where it gets exciting. If you own a DSLR camera – You too can start reaping the benefits of filtered deep-sky astrophotography from almost any city location. The added contrast and details provided by using narrowband filters with your camera can improve your existing deep-sky photos, and create new imaging possibilities. Sure, a dedicated monochrome camera will perform better in this scenario, but you can start employing narrow band benefits to your DSLR now.
2″ round mounted filters like the Astronomik versions above can be threaded directly to your field flattener/reducer or 2″ nosepiece. They can be used with a DSLR camera or CCD when you upgrade.
The Veil Nebula in SII, Ha, OIII and RGB
Adding Ha to your images can add a lot of additional details, particularly to emission nebulae. I have recently discovered first-hand at how OII can make your planetary and supernovae remnant images “pop”. I can, of course, continue to use the Astronomik filters when I eventually do decide to upgrade to a Mono CCD camera.
The diagram below showcases the different wavelengths of light I recorded through each filter using the Altair Hypercam 183C:
The H-alpha portion of the data acquisition process yielded impressive results. The amount of hydrogen gas in the Veil Nebula was evident in my 300-second exposures using Astro Photography Tool. The data would have been better had I taken the time to capture proper bias and flat frames during the acquisition stage.
I’ll blame this one on my lack of experience using the Hypercam 183C. I have since found a suitable target ADU for the 183C by using the flats aid tool in APT. (2200 ADU seems to be working well)
The OIII image was my hands-down favorite, as the composition of this supernova remnant contained lots of delicate Oxygen light wavelengths. This was a pleasant surprise and exceeded my expectations for narrowband images using a color camera.
This time around, I completed the data set with darks, flats, and bias support files. The difference in the image quality was evident right away.
The Veil Nebula in OIII
The SII stage of the image was the least impressive. The Sulfur II content in the Western Veil Nebula appears to be much less than Oxygen III and H-Alpha. However, my SII data suffered from a lack of integrated exposure time and poor seeing conditions.
Processing Narrowband Images
Like many aspects of the astrophotography image processing realm, there are a number of ways to go about it. My preferred method is to copy and paste each of the narrowband greyscale images into the corresponding R-G-B channels in Photoshop for a full color (yet false color) image.
Bob Franke has provided a fantastic tutorial for Hubble Palette Color Map images in Photoshop. This technique creates a false color image by mapping SII, Ha and OIII data as to the R, G and B channels respectively (SHO). This method can produce some stunningly detailed and colorful results.
Others prefer the robust features of PixInsight to complete the task as seen in the false-color image in Ha, SII and OII below. The true power of this method is fully deployed when used with a cooled Mono camera. For example, Jay Bolt of DSO Imaging recently completed a narrowband project on the Wizard Nebula using a QHY163m.
Cooled Monochrome Camera Example
My personal tastes lean towards a hybrid RGB image that showcases the true colors of the deep-sky object as opposed to the Hubble Palette. That may change as I progress further into narrowband imaging, and crave the intricate details a full SHO image provides.
NGC 6960 – The Western Veil Nebula
My final photo of the Veil Nebula combines a standard broadband RGB image with narrowband Ha and OIII details. I was not overly impressed with the HST palette version of my image that mapped the NB data to the RGB channels. (R=SII, G=Ha, B=OIII), but that is just a matter of personal taste.
The RGB data was shot using the Altair Hypercam 183C camera with a Baader UHC-S Filter to block the city glow. The true color portion of the image was used to give the photo a more natural look, with accurate star colors.
Camera: Altair Hypercam 183C
Filters: Astronomik 12nm Ha, OIII
Ha: 47 x 300s Bin 2×2
OII: 64 x 180s Bin 2×2
Camera: Altair Hypercam 183C
Filter: Baader UHC-S Filter
RGB: 40 x 120s Bin 2×2
Stacking Ha, OIII and SII in DSS
I am often pushed towards the PixInsight software by my colleagues and friends. My future in deep-sky astrophotography will undoubtedly include PI at a later date. I feel no rush to make this transition, as I would like to reach the full potential of my beloved DeepSkyStacker and Photoshop workflow first.
The great news is that the aging (free) registering and stacking software known as DeepSkyStacker works brilliantly at combining this narrowband data. DSS creates smooth, high resolution .TIF images from your collection of individual image frames.
When using the Altair Hypercam 183C with DeepSkyStacker, one of the main adjustments I needed to make was the star detection threshold setting. Because of the narrow wavelength of light collected on the CMOS sensor, the RAW .fit images come out of the camera very dark. So dark, in fact, that DSS does not recognize a single star in the field at the default 10% star detection threshold setting. I have found setting this slider from 3-5% effective.
You’ll want to include darks, flats and bias frames in your stack, just as you would with a regular color image. My image exposures using the Hypercam 183C in H-Alpha and OII were 300-seconds and 180-seconds, respectively. I shot data in SII as well, but the data collected was of little value due to poor imaging conditions and lack of overall exposure time.
Image Processing in Adobe Photoshop
Full disclosure, I am new to narrowband imaging and there may be far superior methods for combining your data using Photoshop. The workflow used to create my composite image of the Western Veil Nebula is as follows:
1. Process each narrowband image separately
My processing steps included the traditional operations of level adjustments, curves stretch, sharpening and noise reduction. The biggest difference this time, as that the data is grayscale and not full color.
I first isolate the color channel that is the most dominant for each filter. For example, the H-Alpha records almost exclusively as red in a CMOS sensor. I then copy and paste the red channel only to a new document before processing. Video – Processing H-Alpha
My OIII image contained the most data in the blue channel. This is the channel I extracted for processing in grayscale.
2. Create a new RGB image using Channels
Create a new document, and open the Channels tab. With your pre-processed narrowband images open, copy and paste the main image layer in the corresponding channel for each color.
For my RGB+NB composite image, the Ha was used to boost the red channel, OII for the green channel, and a 50/50 combination of Ha/OII for the blue channel. Once the grayscale narrowband images have all been slotted into each channel, the new false color RGB image will reveal itself.
You can adjust the color levels independently by making curve adjustments to each channel. When the color balance is to your liking, final processing can be applied to the final RGB image as a whole.
3. Finishing touches
I enjoy the Astronomy Tools Action Set and continue to utilize the useful 1-click actions when processing in Photoshop. Some of the actions applied were “Increase Star Color” and “Make Stars Smaller”. The StarSpikes Pro filter was used to create a synthetic diffraction spike on the star 52 Cygni.
I also created and applied a master luminance layer using the Ha and OII grayscale images for an increase in contrast and structure. This is a powerful image processing technique that can be enjoyed after collecting narrowband images with a color camera.
Master Luminance Layer in Photoshop
If you’re looking to retain the natural colors in your galaxy or nebula, adding the narrowband data as a luminance layer is the answer. I absolutely love the look of natural color stars and appreciate the idea of seeing space as it truly exists to our eyes.
This is common practice when shooting deep-sky objects with a Mono CCD in LRGB format. The L stands for luminance as its only job is to collect as much light as possible. The same concept can be used when shooting narrowband images with a color camera, it just takes much more time.
The additional detail gathered by using a narrowband filter such as Ha and OIII make for a detailed master luminance layer in your final image. Meaning, the only the highlights, and contrast from the data can be harnessed without affecting the natural color of your deep sky photo.
The legendary Doug Hubble of the Astrophotography Tutorials YouTube Channel and myastroimages.com shares a useful tutorial on creating a master luminance layer from your narrowband data in Photoshop. This process involves first removing the stars in your astrophoto and then applying the image using the screen blending mode to your existing color image.
As always, it is my pleasure to share my journey through each stage of backyard astrophotography with you all. I really think that adding a set of narrowband filters to your kit can improve your images from the city, no matter which kind of camera you are currently using.
I’ll continue using the Altair Hypercam 183C for the remainder of the summer and share my results. Next up, I’ll review some new CLS/UHC filters from Baader and Explore Scientific for collecting color/luminance data with your DSLR or CCD camera. To stay up-to-date, please subscribe to the AstroBackyard Newsletter.
A special thank you goes out to Jay Bolt, Sean Molony, and Steve Malia for making this post possible. Until next time, clear skies!
Ron Brecher is a leader in PixInsight image processing. Many of the principals shared in this tutorial can also be applied to taking narrowband images with a color camera.
Sara Wager is an accomplished astrophotographer with images that have graced the face of APOD multiple times.