NGC7538 in HaLRGB

NGC7538 is an emission and reflection nebula in the constellation Cepheus. It is approximately 9,100 light years from Earth.  It is part of a large complex known as the Cassiopeia OB2 Association, named for a grouping of young, hot O and B class stars and which also includes emission nebulae (caused by ionization of hydrogen by radiation from the hot stars), dark clouds, star clusters and reflection nebulae.  The well-known Bubble Nebula, NGC7635, is also within this complex, and there are many beautiful wide field images on Astrobin that show NGC7635, NGC7538, and other objects, providing the larger context to the magnified view of NGC7538 provided in this image.

NGC7538 is sometimes called the Northern Lagoon Nebula or the Little Lagoon Nebula.  It has the distinguishing feature of being home to the current largest known protostar, known as NGC7538S, a stellar fetus that is already over 100 solar masses and is housed in a huge accretion disk that is approximately 300 times the size of our solar system. 

Comments on adding H-alpha data:

This image is a combination of LRGB and H-alpha data.  There are a range of possible ways in which astrophotographers add H-alpha (and narrowband in general) data to LRGB data.  My H-alpha filter passes a 3 nm band of light centered at approximately 656 nm, which falls within the red region of the visible spectrum.  So, adding the H-alpha data to the red channel is a sensible approach.  But, there are arguments in favor of also adding a small proportion to the blue channel, and at least one source recommends adding a bit to both blue and green channels.  Adding H-alpha data to the luminance channel, with or without color channel augmentation, is also done, and some eschew capturing luminance data altogether and use H-alpha data as a proxy for luminance.  Aside from these considerations, there are also a range of possible techniques for incorporating the H-alpha data including the Vicent Paris “continuum subtraction” method (https://pixinsight.com/tutorials/narrowband/index.html), which is the method I used.

I composed a reference LRGB image and experimented – extensively! - with adding H-alpha.  I tried many approaches, and achieved a variety of credible results, but I ultimately preferred the approach of adding H-alpha to the red channel.  The H-alpha data were quite strong, and showed both the primary nebula and the surrounding nebulosity.  The surrounding nebulosity in the LRGB image was relatively weak, and so my goal was to primarily enhance the surrounding nebulosity while minimally affecting the main nebula.  To this end the continuum subtraction method worked well.  An excellent discussion and demonstration of the formulas I used is given by James Lamb in this video:  https://www.youtube.com/watch?v=1Wd7VxpMCH4  Adding H-alpha to the red channel alters the color balance so that red dominates the resulting image, but I felt this was a realistic representation of the ionized hydrogen emissions that accompany an OB association, within which NGC7538 resides.  And I was able to preserve the hint of pale blue in the central region, which was evident in the LRGB reference image, and which likely results from reflection.  (Reflected light is usually blue, as the shorter blue wavelength scatters more effectively than other colors.)  I tried to be a responsible custodian of the data my rig provided, and aside from the red enhancement introduced by the H-alpha addition, I did not make any hue alterations during processing.