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Messier 16 – The Eagle Nebula in hydrogen alpha light

Messier 16, also known as the Eagle Nebula, is a favorite target of astrophotographers.  Over the years I have frequently turned my telescopes toward Messier 16, always attempting to tease out more detail in this wonderful object.

The image below was taken on the night of June 9, 2018 at the San Diego Astronomy Association site at Tierra del Sol, Sand Diego County, California.  The telescope I used was the Stellarvue SV70T using the dedicated 0.8x reducer/flattener.  The camera was the ZWO ASI1600MM-Cooled with gain & offset set to 200 and 50 respectively.  I used a Baader 7nm hydrogen-alpha filter.  I took 52 separate subexposures of 270 seconds each.  The image acquisition program I used was Sequence Generator Pro.  I also use PHD2 to run my guiding camera.  The subframes were calibrated with PixInsight by using temperature and time matching dark frames.  Flat field frames were also used in the image calibration.  The subframes were also aligned and stacked in PixInsight.  I used Pixinsight’s LocalNormalization tool and DrizzleIntegration tool to optimize the stacked final image.  Other than histogram stretching using the HistogramTransformation tool, no noise reduction, sharpening tool or other post calibration processing was performed on the image.  The stretched and unstretched images were saved as 16 bit TIFF format images for export into Photoshop CC.

I have posted four different versions of the original image below.  The first three have been processed differently  – different amounts of histogram “stretch” – to highlight the different features found in the image.  The fourth image is a crop of the unstretched central portion of Messier 16 which nicely shows off the detail of the gas pillars and the open cluster.  If you would like to see versions of the images below at the camera’s native resolution – 4656 x 3520 pixels – the full resolution versions of each are posted on my Astrobin page.

The first image below has been stretched to allow us to see the extent of the softly glowing hydrogen gas in the space surrounding the Messier 16 nebula & cluster.  As a result of the stretch, the nebula core has been brightened up to the point that it is difficult to make out some of the details of the gas pillars and the cluster stars are somewhat bloated.   Click directly on the images to see a larger view.

The next image below has been stretched less and does not show much of the large areas of softly glowing hydrogen gas.  However, the Messier 16 core is not as washed out and more detail can be seen.  The cluster stars at the core are also less bloated.

The next image below has only had a very light histogram stretch.   The areas of extended nebulosity seen in the images above are not visible.  However,  the central core has very nice detail and very dim structures that were washed out in the above images are visible here.

Lastly, the image below is an unstretched crop of the dithered original image.  The Stellarvue SV70T with the 0.8x reducer has a focal length of 336 mm.  Paired with the 3.8 um pixels of the ZWO ASI1600MM-C this telescope-camera combination produces a 2.33 arc second per pixel image scale, which is slightly undersampled and will show blocky stars.  However, PixInsight’s fantastic DrizzleIntegration tool will take a slightly undersampled stack of subframes and turn them into an image with beautiful round tiny stars.  If you use PixInsight, I highly recommend that you incorporate this tool into your image calibration and integration processes.

Posted in Bright Nebula, Processing Notes

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