Realm of the Galaxies - Part 2

In the last issue we explored methods of creating an LRGB image of the magnificent Whirlpool Galaxy (M51) and applied deconvolution to both the luminance frame and the RGB colour image. Now, Nik Szymanek shows how to extract the fine detail from within the core of the galaxy.

Nik Szymanek

In attempting to extract the faintest details from spiral galaxies, the aggressive processing needed can often hide other details. This is not surprising as the most exciting features are the spiral arms and all the interesting structure found there. However, optimising the spiral arms usually entails ‘burning out’ the central core of the galaxy, which renders the detail invisible beneath a bright white smear.

In the case of our M51 image, the nucleus in the 300-second sub-exposures is clearly visible so we can attempt to recover these details and get the best of both worlds. Note that longer exposure times will also burn out the core but in those circumstances the details will be unrecoverable because the detector itself has been overwhelmed. So keep that in mind when setting the sub-exposure duration.

In the previous Masterclass series ( Astronomy Now Dec 25–March 26), we addressed how to maintain structural detail in the Orion Nebula (M42) by carefully blending multiple images together. Doing this allowed us to retain detail in the Trapezium stars as well as extended nebulosity on the outskirts of M42 and M43. I will call on the same technique here as it works splendidly with bright galaxy cores.

Basically, we can create three versions of the same image that have varying degrees of brightness and blend them together. I will use PixInsight, but other programs such as Astro Pixel Processor and SIRIL will be perfectly suitable. I started by opening the 32-bit FITS RGB file that had previously been deconvolved with Blur Xterminator and, using the Screen Transfer Function (STF) tool, clicked onthe Auto Stretch button to get a starting point. I then moved the sliders to the right to darken the sky background but maintained a good level of detail in the spiral arms. Save the image in this state as a 16-bit TIFF file because we will next be combining the scaled images in Affinity Photo . To do this I used PixInsight’s Histogram Transformation tool.

Next, we will darken the image a bit more to give us a ‘mid’-level image by moving the sliders farther towards the right. By now the sky background will probably be too dark and some of the spiral arms will have lost their sparkle. Not a problem – we are only really interested in the area around the galactic core. Once again the image must be scaled and saved. Finally, darken the image one more time, concentrating on the core itself, and save the image. Notice that in this dark view M51 has a bright stellar centre but around that are interesting dust lanes and other features. Step one shows the three scaled versions used along with the STF settings applied.

MASTERCLASS

The perfect blend

Now we will blend the three versions together. Open the dark image first and in the Layers menu rename the single layer to ‘Dark’. Open the intermediate version and copy and paste it onto the dark version. Rename the second layer ‘Mid’. Finally, open the bright image and copy and paste it onto the other two images, renaming the top layer ‘Bright’. Labelling the three layers helps to keep track of everything during processing.

Hide the top Bright layer by clicking on the layer button at right. Click on the Mid layer and choose the Erase Brush Tool. From the top line of options change the Opacity value to around 25% with a Hardness value of 0% to keep the brush soft-edged. Using the keyboard’s bracket keys, choose a brush size that encompasses the core out to where the inner spiral arms start to unwind. Mouse clicking will create a void in the Mid layer, so the lower Dark layer becomes visible. Now reduce the brush size further to concentrate on just the inner core. Step two shows the cross-hatched void in the Mid layer mask, with the base layer hidden. Note how the cross-hatching gradually diminishes away from the core due to the soft-edged brush used. Click on the Bright layer button to make the layer visible. Before blending further, I darkened the sky background using the Shadows/Highlights tool. To blend the top layer, it is necessary to increase the brush size somewhat. If the brush size is too small there will be a definite cut-off between the layers, which we do not want. You may have to experiment with the brush size and opacity strength but with practice this kind of blending can be done in just a few seconds. It may sound clunky and certainly does not have the elegance of working with multiple masks in PixInsight but it is very easy and quick to use.

By this stage we have a blended three- layer image so I recommend saving it using Affinity Photo’s .afphoto file structure to retain the three layers, should we need to come back at any point. Once done, I flattened the image into a single layer and saved it before nipping back to PixInsight for the next step: the famous star removal using Star Xterminator . There is just one box to tick, ‘Generate Star Image’, and a few seconds later the stars are temporarily removed to another image, which must be saved. I also took the opportunity to run Noise Xterminator on the starless image, which worked a treat and smoothed the image of errant pixel noise. Both these tools are available within Affinity Photo as plug-ins but it is quicker to apply them in PixInsight .

With the stars gone, I used my usual routine of applying Tone Mapping to the galaxy to boost low-contrast detail in the spiral arms. I set the Tone Compression slider to 0% and the Local Contrast slider to around 8%. Be careful not to go too far at this stage as it is possible to start to overcook the core detail ( Step three ).

Continuing, I used the Vibrance adjustment tool to globally boost the colour in the galaxy. There are two sliders, Vibrance and Saturation, and a mixture of both works well but, as always, be careful not to push either too far. We can enhance the galaxy further by using the Colour Balance tool to make a small adjustment. With the default Mid tones option selected, I moved both the Cyan/Red and Yellow/Blue sliders a small amount to the right to boost the blue of the spiral arms and the red from the H II regions. Note that the star removal process lifts a lot of the smaller H II nebulae onto the star layer, but these will be returned shortly.

The last procedure is to use the Selective Colour tool to boost the blue and cyan components in the spiral arms further. To finish, I brought back the stars and boosted the saturation, which included any H II regions that had jumped ship onto the star layer ( Step four ).

We have succeeded in our aims: to show the core details, the spread of ejected stars from NGC 5195, the galactic H II regions and even the extended arm drawn out from M51, which is now visible beyond NGC 5195.

Nik Szymanek is an astro-imager based in north Essex and is the author of Shooting Stars and Shooting Stars II , which are Astronomy Now books about modern astrophotography techniques.