The Whirlpool Galaxy (M51) and NGC 5195

Messier 51 (NGC 5194), the Whirlpool Galaxy, is a face-on grand design spiral located in the constellation Canes Venatici, approximately 31 million light-years away¹. The galaxy is interacting with the smaller NGC 5195, currently positioned behind M51's northern arm. Gravitational interaction between the two has distorted M51's outer spiral structure and increased the density of young stars visible along the spiral arms.

NB: For wider framing and additional context around the M51 system, please check the wide field version available in the revisions section.

The spiral arms of M51a contain structured dust lanes and active H_II regions (areas of ionized hydrogen gas where ultraviolet light from young, massive stars has stripped electrons from hydrogen atoms), marking sites of ongoing star formation. NGC 5195 displays a diffuse outer halo and extended tidal features — consistent with multiple interaction epochs over hundreds of millions of years. Numerous background galaxies are present across the field, some of which are resolved well enough to show internal morphology.

Imaging and Processing

This dataset comprises 64 hours of total integration, collected over 12 nights in 2025 under Bortle 8–9 urban skies, using a 500 mm f/5.6 system with a 90 mm aperture and 3.76 µm pixels, giving a native resolution of 1.55″/px.

A 3× drizzle integration was applied to improve the sampling of this dataset. With median seeing around 1.3″, Nyquist sampling suggests a target scale of ~0.65″/px, and better results are typically achieved when aiming for 3× finer than seeing (~0.43″/px). The 3× drizzle brings the effective image scale to ~0.51″/px, falling close to this practical optimum.

Faint Background Structures

An attempt was made to isolate Integrated Flux Nebula (IFN) from the background. Although this region is known to contain galactic cirrus, photon noise and sky brightness under urban skies limited the available signal. Some subtle background texture remains visible, but no detailed IFN structures could be confidently recovered.

Note on Nyquist Sampling

The Nyquist limit defines the minimum sampling rate required to preserve all spatial detail without aliasing. In astrophotography, this implies that the pixel scale should ideally be at least 2× finer than the smallest detail (seeing-limited resolution).

For example, under 2″ seeing, a sampling rate around 1″/px or finer is recommended.

Drizzle integration, especially at 2× or 3×, can assist in reconstructing finer structure when the data is well-dithered and sufficiently sampled. In this case, 3× drizzle reduced the final sampling scale to ~0.51″/px, closely approaching the Nyquist ideal for 1.3″ seeing, and enabling improved structure reconstruction while preserving signal quality.

MTF Analysis

Modulation Transfer Function (MTF) analysis provides a quantitative measure of resolution and contrast transfer across different spatial frequencies. Using RC Astro MTF Analyzer, the graphs below show how each drizzle factor affects the signal reconstruction:

 

 

 

The MTF curves demonstrate how the 3× drizzle integration provides better preservation of high-frequency detail, with improved contrast transfer at higher spatial frequencies, resulting in the optimal resolution enhancement for this dataset.