Astrophotograph of the Whirlpool Galaxy M51

This is my astrophotograph of the Whirlpool Galaxy M51 alongside its smaller companion, the Dwarf Galaxy (both about 31 million light years away). They are interacting with each other, as shown by the dusty tidal bridge connecting them, and the faint debris around the smaller galaxy. The larger galaxy has a spiral structure and dust lanes, which are clearly silhouetted against the center of Dwarf Galaxy. This demonstrates that the Dwarf Galaxy lies behind.

Although I’m fairly pleased with the result under the circumstances, I only captured 2 hr of light data despite imaging for 4 hr. Usually I don’t lose a single sub, but for this session I had to reject half of them due to poor guiding or occasional wispy cloud. After running the guiding assistant in the PHD2 guiding software and checking the night’s guiding logs, I believe there were two problems to work on: (1) The polar alignment went off, probably due to the mount settling on the grass. I need to give it time to settle first before polar aligning. (2) The Dec axis was balanced too accurately, leading to large periodic backlash clearing and then overshooting. I need to slightly imbalance the Dec axis by shifting the telescope up/down the grip. This will keep the gears engaged and the motor working in the same direction without having to clear backlash.


  • 27× 265-s light frames (Gain 900)
  • 60× dark frames
  • 50× flat frames
  • 50× dark flat frames


  • Explore Scientific ED 102 mm Apo f/7 refractor
  • Sky-Watcher EQ5 PRO SynScan GOTO equatorial mount
  • Altair Hypercam 294C PRO colour fan-cooled camera
  • Altair quad-band one-shot colour (OSC) 2″ filter
  • Altair 60mm guide scope
  • Altair GPCAM2 AR0130 mono guide camera


  • Sharpcap
  • PHD2
  • DeepSkyStacker
  • Photoshop

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