Widefield M51...

[tooth_dr]

I have acquired a new monitor which is completely different in colouration to my laptop.  I've processed this again to see if it helps?

 

 

[Oddsocks]

Downloading the forum image and reviewing on a calibrated screen (iMac 5K, Datacolor calibration tools, sRGB colour space) the image appears a little too red, especially the background and a little too dark, M51 has a pinkish hue.

Taking the low quality 8bit forum image and applying the PixInsight steps in order > Background Neutralisation, Colour Calibration, Star Mask, Colour Saturation (stars), Range Mask, Colour Saturation (Galaxy), TGVDenoise and Histogram Transformation produced this image, which is not that good as the source 8bit image is not sufficient as a starting point but the background looks a bit more neutral.

There are some obvious large scale calibration artefacts in the background around the brighter objects so it seems flat fielding is still not working quite right for you which may be why you clipped the black so hard?

HTH

William.

[tooth_dr]

5 hours ago, Oddsocks said:

Downloading the forum image and reviewing on a calibrated screen (iMac 5K, Datacolor calibration tools, sRGB colour space) the image appears a little too red, especially the background and a little too dark, M51 has a pinkish hue.

Taking the low quality 8bit forum image and applying the PixInsight steps in order > Background Neutralisation, Colour Calibration, Star Mask, Colour Saturation (stars), Range Mask, Colour Saturation (Galaxy), TGVDenoise and Histogram Transformation produced this image, which is not that good as the source 8bit image is not sufficient as a starting point but the background looks a bit more neutral.

There are some obvious large scale calibration artefacts in the background around the brighter objects so it seems flat fielding is still not working quite right for you which may be why you clipped the black so hard?

HTH

William.

Thanks William

I'm not sure how I clipped the black point, because to be best of my limited skill in PS, I never once lost the left hand side of the histogram during processing.  Is it possible to clip the histogram in other ways?

Those large scale defects, mostly around the stars, I'm 99% there are the result of Gradient Xterminator. 

But I can now see the pinkish hue you have mentioned!

Back to the drawing board!

 

 

[Oddsocks]

3 hours ago, tooth_dr said:

Is it possible to clip the histogram in other ways?

I have read that amongst some of the expert Photoshop imagers using the Stretch Tool they aim to keep a per-channel black background average pixel value of around 23/23/23 for each of the RGB channels (using 8bit sampling for the 'dropper tool').

Besides ensuring the black is not excessively clipped this gives some continuity from one image to the next so that when you collect all your images into one album and flick through you are not distracted by a continually changing background, it also helps you if you return to a target at some time with a longer focal length instrument and want to add detail to an existing wide field image and vice-versa. 

When using PixInsight I save my images with a slightly higher background pixel value and do the final adjustment to 23/23/23 in Photoshop, note that I did not do that with your image today but checking a moment ago I see the background is in the range 23 to 31 (using 8bit resolution for the dropper sample tool) and it could be reduced a bit more which would hide those artefacts.

If you are working at 16bit resolution, most likely, then it is worth saving a copy at 8bit and comparing 16bit to 8bit before uploading for the web since many forums will downscale the bit depth for display to save disk space on their servers. If you are setting image black level using 16bit and upload to the forums servers then the black level will almost certainly change so do a forum only version at 8bit resolution and tweak it a bit till you are happy.

Monitor calibration is a bit of a mine field, very few monitors are correctly colour balanced and need tweaking. Most video drivers have a manual calibration tool to allow you to set them up a little more accurately than the default LUT's that are installed with the monitor drivers. If you are just using the standard Windows monitor drivers it is worth checking if the monitor manufacturer has supplied better matching LUT's with their software distribution.

Before investing in the Datacolor calibration tool set, which I needed to set up good printer profiles with third party inks and different manufacturers papers. I used to use the built in by-eye calibration tools of the video drivers and check after adjustment with the free photography test cards you can find all over the web, the 50% grey card displayed full screen will quickly show any colour bias, ultimately though it is all bit subjective and using a monitor calibrator to create custom LUT's is the only way to ensure some standard calibration and continuity over a lifetime of upgrading computer hardware, just a shame the monitor/printer calibration sets are so expensive and only get used once or twice a year!

William.

[tooth_dr]

I did not realise that images were changed, nor did I know that this affected the black levels!  Everyday is a learning day William, thanks for this.