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Old 17-04-2021, 01:34 PM
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ChrisD is offline
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How to stretch OSC images without losing colour (pixinsight)

I recently posted an image on Images > Deep Space.


There were some comments about the quality of the colour of the image, especially since it was from a OSC camera. I thought I might pass on something I used to improve the colours.

I don't know how widely this is know, but I've found that the Histogram Transformation function is stretching the luminance values but not the colour so much. The ArcsinhStretch function worked way better with RGB values during the stretch.

In the image below the same image was stretched with the HistogramTransformation function on the left and ArcsinhStretch on the right. You can see the difference.

If you want to maintain colour in an RGB stretch, I would definitely recommend using ArchsinhStretch.

I hope someone finds this handy.

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Old 18-04-2021, 09:37 AM
RugbyRene (Rene)
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I've never been able to get ArcSinh Stretch to work for me. I tend to use Masked Stretch which works for 90% of my images.
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Old 18-04-2021, 06:15 PM
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All I could find was a youtube video in German, but I figured it out eventually.

In ArcsinhStretch, open up the "realtime preview" window.
Slide the Stretch factor slider until you're happy with the contrast in the image.
Press the "estimate black point" button. This will set the black point.
Use the lower black point slider to remove the colour speckles from the preview.
Then apply.

You can play with the other checkboxes, but the above is all I basically do.

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Old 19-04-2021, 03:55 PM
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Hi Chris,

I tried this but got really red stars.

I usually do a normal histogram transformation, then use starnet to remove the stars and then boost the saturation on the nebula/galaxy. The colour is still there it just needs to be boosted.

Once I am happy with the processing I put the stars back in.

second pic is arcsine stretch
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Click for full-size image (arcsine.jpg)
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Last edited by Zuts; 19-04-2021 at 04:06 PM.
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Old 19-04-2021, 09:20 PM
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ChrisV (Chris)
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With OSC processing, I first DBE, colour calibrate, noise reduce, convolve (sometimes) and then stretch (mix of histogram, arcsin etc). Images aren't very colourful at this stage.

After that I work on boosting colour etc. Then they can get pretty gaudy!

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Old 25-04-2021, 07:10 AM
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Geoff45 (Geoff)
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I often do a preliminary stretch with arcsinh stretch and then finish it off with histogram stretch.

Last edited by Geoff45; 25-04-2021 at 09:09 AM.
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