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Observation by Graham Roberts: Messier 101

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Graham Roberts


2019 Mar 29 - 22:00


2019 Jun 08 - 19:58



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Ursa Major

Field centre

RA: 14h03m
Dec: +54°20'
Position angle: -104°03'

Field size

0°43' × 0°41'

  • William Optics GT81 & 50mm guidescope
  • ZWO1600MM-Cool mono CMOS sensor

L 31 x 180 sec HaRGB each 16×180 sec


Fairvale Observatory – Redhill – Surrey – UK

Target name

Messier 101


Messier 101

About this image

Graham comments that: With nearly 5-hours of subs, processing of M101 would be a challenge in order to bring out the galaxy’s colours and perhaps highlight the stellar nursery areas that are found along the spiral arms and are rich in Ha-light.  To do this I first changed the RGB image stack to Lab Colour in Photoshop, increased the saturation in the (a) and (b) channels before returning to RGB mode.  This had the desired effect of successfully enhancing the colours, which can otherwise look washed-out after stacking, stretching and combining.  After this I split the RGB channels and pasted the Ha-stack into the Red channel, before re-combining again into an RGB image, at which point the H II regions along the spiral arms just lit up!  These were both new processing techniques for me that greatly improved the final image and hold great promise for processing HaLRGB objects in the future.

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