M51 – The Whirlpool Galaxy – Take 2
The above image is an attempt I made at M51 – The Whirlpool Galaxy just under a year ago. It is a combination of 29 x 120s LIGHT frames and the requisite BIAS/FLATS and DARKS. It was stacked in DSS and then processed in Photoshop.
I have been waiting to have another go at this target as it is a nice looking galaxy and I was quite chuffed with my original effort. It is easy to find, floating just off the “handle” of the Plough/Ursa Major, roughly opposite to M101, the subject of another recent post.
Star Chart from FreeStarCharts.com
The Plough itself is nice and easy to find in the northern skies, revolving around the Pole Star or Polaris. By the time the night is dark enough at this time of year, the handle is high in the sky and, hence, so is M51, hopefully cutting down on the amount of atmosphere that needs to be “seen” through to image the target.
So, on Thursday last week, I set up the scope and equipment and aimed for M51, firing off an 8 minute (480 sec) test sub first to check for trailing. Luckily, the image came back free of trails, suggesting the polar-alignment and guiding were working nicely together. I thought I would give 8 minute subs a go partly because I hadn’t gone that long before and wanted to test the mount but, also, because the Moon is rising late and waning at the moment, so was out of the way for this session.
About 2 a.m. I finished up with 3 hours worth of 480 second LIGHT frames plus BIAS and FLAT Frames. I packed up and got warm again, before starting another run for the DARK subs with the equipment inside.
Then the fun started.
Over the next couple of days I used PIXINSIGHT to process the images before a final tweak in Photoshop. I may get around to writing up some tutorials for Pixinsight sometime in the future but, at the moment, I am still a complete novice, lapping up other imagers’ tutorials, so will stick to the basic workflow for now – but feel free to ask any questions as you read it.
- BATCH PREPROCESSING Script to register and calibrate the LIGHT/DARK/BIAS/FLAT Frames
- SCREEN TRANSFER FUNCTION to apply an automatic stretch to “see” data
- IMAGE INTEGRATION tool to align/stack the calibrated frames
- DYNAMIC CROP
- DYNAMIC BACKGROUND EXTRACTION
- COLOUR CALIBRATION Tool
- SCNR to reduce green tinge
- Clear STF Screen Stretch to return to LINEAR DATA
- HISTOGRAM TRANSFORMATION Tool (applied twice)
- HDR MULTISCALE TRANSFORMATION – to bring out details
- ACDNR – noise reduction
- MASKED SATURATION BOOST – boost colour
- Saved as 16bit TIFF files and opened in Photoshop.
- Simple LEVELS and CURVES adjustments in Photoshop to finish image and a final tweak of the colour saturation.
I suspect I have overdone some of the tools and adjustments, but I’m still learning, so I have an excuse. Noise is also still a bit of a problem, but an expected side-effect of un-cooled DSLR imaging. I was dithering this plan which does seem to have helped keep the noise in the image down, and I will have a go at processing the image again without the DARK frames in the stack. Many, more experienced, imagers suggest that DSLR DARK Frames can introduce more noise than they eliminate, so it will be interesting to go through the Workflow again and see how it compares without the darks.
One of the other drawbacks of the DSLR is the size of the sensor. Compared to many dedicated CCD cameras, the DSLR sensor is quite large. This is fine for widefield imaging of large galaxies (M31) or large areas of nebulosity but, for smaller targets like M51 it means a lot of cropping of the original frame is required. This increases the size of the pixels and can degrade the quality of the final image. This was the case for my M51 as it was very small on the original LIGHT frames – as you can see below. Cropping to that extent also helps show up any failings in focusing, as in my final image below – but practice makes perfect.
Anyway, after a few days of tweaking, cropping and tweaking some more, I finally got an image I was happy with. Hope you like it and you can see the clear difference that having longer subs on a dark night can make – and an extra year’s practice in processing! You can also see it in the Gallery!
Any questions or thoughts, let me know below!
Thanks for reading!