# Dates: 

23-24, 26-28 April 2025

# Location: 

Washington D.C.

# Equipment:

ASI 2600MM Pro (monochrome) camera

Chroma 36mm LRGB Filter Set

WO Fluorostar 91mm f/5.9 triplet APO refractor

iOptron GEM28-EC mount

# Data and exposure times:

Data was acquired as LRGB images with the following exposure times:

14.11 hours (242x210s subs) with Luminance filter (L).

3.50 hours (60x210s subs) with Red filter (R).

3.56 hours (61x210s subs) with Green filter (G).

3.44 hours (59x210s subs) with Blue filter (B).

# Atmospheric conditions:

The shown image was developed from data acquired in a Bortle Class 8 area (i.e. in an environment experiencing a degree of light pollution typical of a city) where the sky quality during observation was such that both transparency (i.e. the level of atmospheric clarity) and seeing (i.e. the level of atmospheric turbulence) varied from average to below average.

# Preprocessing notes:

Created LRGB "masters" by Calibration, Cosmetic Correction, Weighted Subframes, Star Alignment, and Integration. 

# Postprocessing notes:

a. Dynamic Cropping of LRGB masters each to the same dimensions having a 3:2 aspect ratio.

b. Applied a Screen Transfer Function to view the resulting images.

c. For the L master: Applied a Dynamic Background Extractor and saved the settings to be used later when applying a DBE on the RGB masters.

d. Applied BlurXT and NoiseXT.

e. Applied a Histogram Transformation. This step generated a nonlinear image which was saved as a postprocessed L image.

f. "Built" a color image from the R, G and B masters by using LRGB Combination and applied a DBE to the color image using the same DBE settings as used for the L master.

g. Since a color image is involved, this necessitated the application of Background Neutralization and Color Calibration to the result from step f above.

h. Applied BlurXT, NoiseXT and a Histogram Transformation. Saved the nonlinear result as a postprocessed RGB image.

i. Used LRGB Combination to "apply" an instance from the postprocessed L image to the postprocessed RGB image. 

j. Applied StarXterminator to create starless (i.e. containing the target image - in this case M51) and stars-only images.

k. Processed the starless image, after applying a range selection mask to protect the background area, using Local Histogram Equalization, Curves Transformation and Color Saturation. Curves Transformation was used only to boost the saturation whereas Color Saturation was used to enhance specific color hues.

l. Applied SCNR (Subtractive Chromatic Noise Reduction). Removed mask and used an expression in Pixel Math to combine the result from step k above with the stars-only image from step j.

m. As a final step, after protecting the target image with a Star Mask, applied a (star reduction) Morphological Transformation to the result from step l above.



M51 (or NGC 5194 or the Whirlpool Galaxy)