By Dan Kusz | Astrophoto Review column
Astrophotography has an exceptionally steep learning curve.
It includes dealing with a number of difficulties, from the technical interactions between equipment and software to battling mother nature, and then working through the challenge of post-processing all the data into a beautiful image of the heavens.
In this column, we are going to look at all of these challenges that Paul Dopson had to overcome to produce his image of NGC 281: the Pacman Nebula.
Paul used his Explore Scientific ED127 F7.5 Triplet refractor matched with a ZWO ASI1600MM-Pro dedicated astronomy camera. Using a 4.5nm narrowband filter, he acquired 360 Frames at 180 seconds each equally across all three narrowband filters. All the sub frames were taken from his tree-filled, Bortle 7 backyard. The image was processed in Pixinsight, so I will use techniques found in this editing program for recommendations.
The framing of NGC281 is well done, filling the frame edge-to-edge, creating that inquisitive exploration of all the features within the nebula. The nebula is rendered in the classic Hubble palette, which maps Sulfur II(SII) to Red, hydrogen alpha(Ha) to Green, and Oxygen III(OIII) to Blue. The saturation of the colour is well presented and not overdone. Pushing the colour saturation can diminish the detail, so using a light touch really benefits the image.
There are a couple recommendations regarding colour in this image that I would like to present — the first being the removal of the slight green cast (Fig.1) that Is found around the dark portion of the nebula in the centre.
Fig. 1 – Green Cast
Running selective colour noise reduction (SCNR) in Pixinsight on the green will remove this green cast in the image. The background has a magenta cast (fig.2), a common artifact of Hubble palette combinations. This has a simple fix in Pixinsight. First, simply invert the colour of the image and the magenta will appear green. Then run another instance of SCNR on the inverted image, and then invert the image back and the magenta will be gone.
Fig. 2 – Magenta cast in background
The stars in the image could benefit from taking separate images using RGB filters for natural star colour (fig.3). You don’t need much, as stars are bright in the sky. A quick run of 30 second exposures and capturing 30 for each RGB filter will provide you with some nice, tight, and colourful stars to add into your image using the pixel math function in Pixinsight.
Fig. 3 – No RGB star colour
It is also worth noting that the overall sharpness of the image is a bit soft (fig.4). This could be a result (assuming focus is correct) of some non-optimal sub frames making their way into the stack, resulting in some degradation in the final stacked image.
Fig. 4 – Soft details in nebula
I recommend culling your sub-optimal subframes base on a full width half maximum (FWHM) using the sub frame selector function in Pixinsight. Stacking less but higher quality sub frames will result in a sharper image after integration. Using a round of deconvolution can bring back some of the sharpness in the image, but it needs to be used carefully. Going too far will create unwanted artifacts in the image.
Overall, this is a really good image.
With a few minor adjustments, Paul can easily bring this image to the next level. Imaging in a Bortle 7 zone (with weather cooperating) and adding more integration time will really help the overall ease of processing and quality of the final image. I am a big proponent of large integration in images!
Astronomy is a slow hobby. Imaging something that is not going anywhere for the span of our lifetimes doesn’t need to be rushed. Take the time needed and collect as many photons as possible to create an image that will last a lifetime.
Clear skies,
Dan Kusz
