Grand Design Spirals

A grand design spiral galaxy has prominent and well-defined spiral arms, extending around the galaxy through hundreds of degrees and observable over a large fraction of the galaxy's radius. As of 2002, approximately 10 percent of all currently known spiral galaxies are classified as grand design type spirals, including M81, or Bode’s Galaxy.

Density wave theory is the preferred explanation for the well-defined structure of grand design spirals, first suggested by Lin and Shu in 1964. The term "grand design" was not used in this work but appeared in the 1966 continuation paper [2], and Lin is usually credited with coining of the term. [1]

That said, M81 is such a beautiful galaxy because of its symmetry and well-defined spiral arms. Pink, HII regions decorate the arms with its characteristic pink-magenta color. The blue arms reveal young, massive stars burning their fuel quickly, while the yellow core and dust lanes suggest older stars, consuming their fuel more slowly and appearing reddish yellow as a result.

Acquisition

This image was the result of an opportunity I had to use the Schulman 32-in telescope at the University of Arizona’s Mt. Lemmon Sky Center. One of the larger, more modern telescopes intended for public outreach anywhere, visitors to the center can peer through an eyepiece on the Schulmann during their evening programs. For those who are interested in astrophotography, remote access imaging is available.

Processing Notes

This image consists entirely of LRGB data, with no H-Alpha data taken. Wanting to emphasize the HII regions more, I had to do a bit of a cheat, taking the red image and isolating those regions with intensity adjustments, coupled with masks and some PI pixelmath to isolate just the knots in the spiral arms. I then colorized the frame to the color of HII using the formula of Red*0.93 + Blue*0.07 and then added that to the LRGB image, again using pixelmath.

I also have to tip my hat to Adam Block for his Horizons tutorial on M51. I struggled to get the right balance of the yellow and blue, often feeling like I was doing surgery with oven mitts on. GHS, Color saturation, and curves transformation weren’t giving the result I wanted very easily. Adam’s trick of using curves transformation in the Lab color space made it much easier.

Raw Images

• Images were 600 sec (L) or 300 sec (RGB)
• Calibrations with WBPP

RGB Images

• Linear Fit to normalize histograms
• Linear fit of three channels using Red channel (lowest signal) as the reference
• RGB combine
• Dynamic Background Extraction
• Spectrophotometric Color Calibration
• Blur Exterminator (default settings)
• Noise Exterminator (default settings)
• Generalized Hyperbolic Stretch, including linear stretch to raise the black point

Luminance

• Dynamic Background Extraction
• Blur Exterminator (default settings)
• Noise Exterminator (default settings)
• Multiple, gentle applications of Generalized Hyperbolic Stretch, a including linear stretch to raise the black point a bit

LRGB Combination

• RGB Working space to equalize the color channels in the RGB Image
• Channel Extraction to isolate the L from the RGB Image
• Linear Fit to set the extracted L image to the same level as the actual luminance data
• Channel Combination to restore extracted L to RGB
• LRGB combination to create a new RGB image with the RGB + L data

"Pseudo" H-Alpha

• Starting with the Red channel data, start wutg StarXterminator to remove stars
• GHS linear stretch to raise the black point and eliminate unwanted faint detail
• Apply a inverted core mask created with Hartmutt Bornemann’s GAME script, and set the core to 0 using Pixel Math
• Repeated application of pixelmath relation Red – med(Red) to suppress background, until just HII knots and some areas of the spiral arms were left.
• Final adjustments with Pixelmath using iif(Red<factor, 0, Red) to get rid of faint, unwanted bits. Experiment with the value of factor.
• Convert frame to RGB
• Scale the frame so that the red values in the knots are approximately equal to the areas to be enhanced using pixelmath.
• Fold HA into the LRGB image with pixelmath equation LRGB + factor*HA. Experiment with factor to get the right blend.

(It would be a lot easier to just have several HA frames to add to the image and the rendering would be more representative of the truth.)

Final Adjustments

• HDR Multiscale transform to increase contrast and bring out fine detail in the core and the arms
• Touch up core and arms using a core mask and an inverted core mask to isolate stretching to selected areas
• Local histogram equalization
• Curves transformation using b channel of Lab color space to enhance blue in spiral arms and yellow/brown in core area

Footnotes

(1) https://en.wikipedia.org/wiki/Grand_design_spiral_galaxy
(2) https://www.pnas.org/doi/abs/10.1073/pnas.55.2.229