Image Acquisition and Processing Notes

The moon was below the horizon and we had clear skies, so I wanted to try imaging a reflection nebula with the old Meade LX200 SCT.  I saw this section of the sky featured in a video on Lukomatico’s YouTube channel and made note of it.  I later located NGC6914 in Stellarium and Telescopius, copied the coordinate metadata and pasted it into ASIAir’s Plan tool, and rotated the camera to frame the subject and the guide camera to have several stars on each side of the meridian.   

I collected 93 images those first two nights, yet only 9 were useable.  Almost every one was ruined by high, thin clouds or satellites.   On the the third night, however, luck was on my side and I was able to gather 56 good frames.  I used PixInsight's SubFrameSelector tool to cull out sub-frames whose median FWHM exceeded 3.8 and eccentricity exceeded 0.48.

I was left with a stack of only 59 light frames, each with an exposure of 240s each and gain of 120 (6hrs 36 minutes total integration time).  All were taken with an ASI294MC Pro with UV-IR Cut filter in the drawer.   The AM5 mount was guided using an ASI294MM mini camera in a Celestron off-axis guider.  On the third night, when seeing conditions were the best, guiding averaged 0.81” or 0.81 pixels total RMS error.    It deteriorated considerably as the target sunk lower in the sky, where glow was more evident and the transparency diminished.  

Here is what the image looked like in its linear state, right out of Pixinsight's Weighted Batch Pre Processor (WBPP).  No gradient removal (I later used GraXpert's kriging algorithm) or color calibration (I used PixInsight's Spectro-Photometric Color Calibration or SPCC).  After having read Russel Corman's essays on signal processing, I now begin by using PixInsight's Geometry/IntegerResample tool to decrease the 2X drizzled image for processing.  After gradient removal and color calibration, this image was refined using BlurXTerminator, StarXTerminator, Generalized Hyperbolic Stretch, NoiseXTerminator, Histogram Transfer (only to adjust black point), and finally Curves to adjust the brightness saturation of reds and blues.



Description

NGC6914 is a reflection nebula located approximately 6,000 light-years away in the constellation of Cygnus.  Most of the region is glowing red as young stars in the Cygnus OB2 association ionize the surrounding hydrogen clouds.  However, it is two bright blue reflection nebulas that are the center of attention in this image: vdB 132 is in the center right and vdB 131 is immediately above it.   These two gas clouds are being ionized by ultraviolet radiation from stars in the Cygnus OB2 association that pierce through holes in the dark dust nebula that paints a black, dentritic drainage-like pattern across the field of view.  It’s like rivers of dust flowing through a glowing red landscape.

There is a third, purplish-blue reflection nebula visible at top center, which glows as light from powerful, hot blue stars reflecting off dark, dense cosmic clouds with some red from ionizing hydrogen mixed in.

NGC6914 was discovered by French astronomer Édouard Stephan on August 29, 1881.  A brilliant astronomer, early on in his career, when he was director of the Marseille Observatory, he used the observatory's 80cm telescope to make the first measurements of the angular diameter of stars.  He was creating a reference system of fixed objects.  He also discovered Stephan’s Quintet a beautiful cluster of five spiral galaxies in the constellation Pegasus, near the Deer Lick Group.  A month after collecting the data for this image NGC6914 (shown above), I swung my LX200 over to to Stephan's Quintet and captured this image: