A luminous M101 with outer spiral arms

Big, beautiful spiral galaxy M101 is one of the last entries in Charles Messier’s famous catalog, but definitely not one of the least. About 170,000 light-years across, this galaxy is enormous, almost twice the size of our own Milky Way galaxy. M101 was also one of the original spiral nebulae observed by Lord Rosse’s large 19th century telescope, the Leviathan of Parsontown. M101 shares this modern telescopic field of view with more distant background galaxies, foreground stars within the Milky Way, and a companion dwarf galaxy NGC 5474 (upper left). The colors of the Milky Way stars can also be found in the starlight from the large island universe. Its core is dominated by light from cool yellowish stars. Along its grand design spiral arms are the blue colors of hotter, young stars mixed with obscuring dust lanes and pinkish star forming regions. Also known as the Pinwheel Galaxy, M101 lies within the boundaries of the northern constellation Ursa Major, about 23 million light-years away. Its companion NGC 5474 has likely been distorted by its past gravitational interactions with the dominant M101.

I chose to dump a lot of integration time with longer subframes (28 hours worth of 3 minute frames) to try to reveal some of the fainter outer arms not shown in most astrographs. I think I succeeded quite well – you can see the three fainter arms take shape on the left side of the galaxy. Compared to my last image of M101, this is quite the improvement!

Image:

Annotated Image:

Equipment:

• Nikon D800
• 600mm F4 lens
• GEM 28
• NINA
• Nikonhacker

Acquisition:

• ISO 800, F4
• Taken from a bortle 1 zone in Northwest CT
• 543 x 180s subframes (27 hours) – Taken on 3/1/2022, 3/2/2022. 3/4/2022, 3/5/2022, 3/6/2022, 3/7/2022

Processing:

Pre-processing and stacking
- All subframes were calibrated and normalized through WBPP
   - The subframes were debayered and split into monochrome channels
   - The subframes were registered with distortion correction
   - the subframes were registered based on a PSF snr estimate
   - The subframes were normalized using local normalization
- Each channel was integrated separately using the linear fit clipping algorithm
- all the channels were integrated together using the ESD clipping algorithm
Preparation of all frames:
- Stacking artifacts were cropped
- the RGB channels were combined to create an RGB image
- the RGB image was plate solved
- DBE was applied iteratively
- the RGB image was photometrically color calibrated
Deconvolution of the luminance
- a PSF was created using the dynamic PSF process
- Linear starnet was applied
- The starless image was deconvoluted
- the stars were added back in
Linear noise reduction
- A low-contrast mask was applied
- Two iterations of TGV noise reduction were applied, one targeting high-frequency noise and one targeting low-frequency noise
- A medium-contrast mask was applied
- MMT targeting all 8 scales was applied to remove large-scale noise
Stretching
- the luminance image was stretched using GHS
- the RGB image was stretched using Arcsinh stretch
Non-linear adjustments:
- LRGB combination
- HDR, LHE, and curves to increase local contrast
- Star reduction following Adam Blocks method
- Curves transformation to increase blue
   - A large scale mask was applied
   - Green and red levels were decreased
- saturation was increased
- MLT was applied targeting 5 layers of luminance
- The background level was set to .09