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Old 17-04-2021, 11:36 AM
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PKay (Peter)
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Image Stretching Question. M83

50x5m of Luminance taken last night.

The usual background extraction, deconvolution and denoise have been applied.

The three images have different levels of stretching.

It is my intention to add RGB when conditions allow.

You hear 'black clipping' mentioned a lot. I really don't know what it means.

I need to select one of the three images to work with.

Any advice would be appreciated.
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Last edited by PKay; 17-04-2021 at 01:10 PM. Reason: Wrong file identifiers
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Old 17-04-2021, 11:46 AM
Zuts
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When you stretch you have a black, mid and white point.

My understanding is you never touch the white slider but only move the middle slider towards the curve.

When you have done this you can at the end move the black slider to just before the curve starts. Leave a little nose on the edge of the curve.

If you push the slider to far you start losing the darker parts of the image and it can look fake as the background disappears to black. This is my understanding of black clipping.

By the way lovely images and heaps of details.
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Old 17-04-2021, 11:57 AM
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multiweb (Marc)
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Clipping occurs any time you truncate your data dynamic range. Black if the left of the histogram is clipped, white if the right of your histogram is clipped. In either case you lose data. Highlights or dark neb. The histogram is different for each channel. Your first step is to set the left of each channel histogram to the same black point then you balance your color.
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Old 17-04-2021, 12:31 PM
Mickoid (Michael)
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First of all, you've gathered some great data to work with. Seeing must have been pretty good. My understanding by your file name, these might be created using your Ha filter, or are they UVIR cut filtered shots? If they're Ha filtered it means it represents the amount of hydrogen alpha present in the galaxy. Using these as luminance won't represent all the detail available in RGB that you get using a luminance filter. If they are UVIR cut filtered shots, I'd be using the middle version as it has the best dynamic range.
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Old 17-04-2021, 12:38 PM
Startrek (Martin)
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Peter,
Super M83 images !!!!
Should be in Deep Space section

Iíve never black clipped as Startools wonít let you black clip your image
It doesnít rely on levels curves and histograms by the operator like the commonly used processing software. You stretch your image by selecting a region of interest ( ROI ) and Startools automatically does the rest , brilliant !!
One of the reasons I chose Startools, I hate levels , curves and histograms
My 2 cents ....
M.
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Old 17-04-2021, 12:51 PM
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Quote:
Originally Posted by Mickoid View Post
My understanding by your file name, these might be created using your Ha filter,
Oops, I used the wrong file description.
This image comes from Luminance filter.
Fixed. Well spotted Mic

Last edited by PKay; 17-04-2021 at 01:11 PM. Reason: oops
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Old 17-04-2021, 12:54 PM
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Quote:
Originally Posted by Zuts View Post
When you stretch you have a black, mid and white point.

My understanding is you never touch the white slider but only move the middle slider towards the curve.....
I presume your talking about the little triangles in PI 'Histogram Transformation?
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Old 17-04-2021, 12:57 PM
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Quote:
Originally Posted by multiweb View Post
Clipping occurs any time you truncate your data dynamic range. Black if the left of the histogram is clipped, white if the right of your histogram is clipped. In either case you lose data. Highlights or dark neb. The histogram is different for each channel. Your first step is to set the left of each channel histogram to the same black point then you balance your color.
thanks Marc. This will take time to take on board. I'm getting a headache
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Old 17-04-2021, 01:01 PM
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PKay (Peter)
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Quote:
Originally Posted by Startrek View Post
...
One of the reasons I chose Startools, I hate levels , curves and histograms
My 2 cents ....
M.
Hey M.
I have gone to far into PI, no turning back now.
And yeh, I also at times, hate the technical crap...
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Old 17-04-2021, 01:25 PM
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PKay (Peter)
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Quote:
Originally Posted by Zuts View Post
When you stretch you have a black, mid and white point.

My understanding is you never touch the white slider but only move the middle slider towards the curve.

When you have done this you can at the end move the black slider to just before the curve starts. Leave a little nose on the edge of the curve.

If you push the slider to far you start losing the darker parts of the image and it can look fake as the background disappears to black. This is my understanding of black clipping.

By the way lovely images and heaps of details.
Eureka! You nailed it Paul.
And Marc, yeh I get it now!
Thanks you lot!

Very Happy
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Old 18-04-2021, 10:40 AM
RugbyRene (Rene)
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Personally I'd use the middle image. The first image is blown out and the third is missing some detail in the spiral arms.
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Old 18-04-2021, 11:19 AM
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Thanks Rene

After what Paul and others had to say, I used the 'Histogram Transformation' process doing it manually, instead of using the Auto 'Screen Transfer Function' which decides for you.

The first image (below) was the result of Auto, and a little over stretched.

After doing it manually, it came out same as the second image!

In future, I will always use manual mode.

All in Pixinsight by the way.
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Old 18-04-2021, 03:59 PM
etill (Elliot)
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I like the masked stretch in PI, I aim for a little short of the final stretch and then use histogram or curves transforms to finish it off.
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Old 19-04-2021, 08:14 AM
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PKay (Peter)
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Hi Elliot

Many ways of 'skinning the cat
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