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Processing with PS CS2: Curves or Levels first?

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#1 Traveler

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Posted 13 September 2017 - 11:10 PM

Yesterday i gave a try on several astrophoto's i made. Normaly one of the first steps with a stacked DSS output (autosafe.tif) is stretching the picture with the Levels function of Photoshop CS 2. Reading a tutorial from Starizona ("LRGB with Photoshop") however, i read that "stretching" is done with the Curves function of Photoshop. Levels is used after several other processing, almost at the end of the mentioned method.

 

I wonder what method is best:

 

1] how do you stretch your picture: curves or levels?

 

2] what is the reason for that?

 

 

Hope to read your experiences. (I am not using/own PixInsight).



#2 Calypte

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Posted 14 September 2017 - 12:37 AM

The short and unsatisfying answer is, "It depends!"  

 

You may find exactly what you want by looking for demos on Youtube.  You might also obtain the book Lessons from the Masters, edited by Robert Gendler and available from Springer Verlag.  Many topics are covered, but there are chapters by Adam Block, Johannes Schedler, and Steve Mazlin, each describing their respective workflows -- which are all different.  Pixinsight is not covered in this book.  It didn't have the popularity that it has now when the book was published. 

 

Also, back in 2010 through 2013, Astronomy magazine ran a monthly column by Tony Hallas where he described his procedures.  He yanks up on the curve, saves, reloads, yanks up again, etc., with occasional forays into Levels to adjust the black point and bring down the white point.  He continues to do this until the image is stretched to his liking.  Luminance and RGB are processed separately until he's satisfied and it's time to combine them as layers.  Tony also put out some DVDs to show his procedures (EZ CCD DVD, vols. 1, 2, & 3).  Tony's Astronomy column was followed by a series by Adam Block, which ran until his situation at Mt. Lemmon changed.  There also comes a point when they run out of "basic" topics that can be fitted into a short magazine column.  Both Tony's and Adam's articles assumed Photoshop.

 

I took Adam Block's workshop several years ago, and I follow his procedures.  Adam uses CCDStack for calibration and stacking.  Then the files are taken into Photoshop as 16-bit tiffs.  The luminance is already scaled when it comes out of CCDStack, although at that point its appearance is somewhat subdued.  The RGB is not scaled in CCDStack.  When it's brought into Photoshop, Adam's first step is to go into Levels and bring down the white point arrow until it's just above the data, which is all crowded into a tall black line on the left side of the Levels window.  Then he expands a bit with curves, increases the saturation, and then he uses Shadows-Highlights (under Adjustments) to dramatically increase the color.  Sometimes more than one iteration of these steps is required.  Adam does this while the luminance layer is on top of the RGB layer (blended in "luminosity" mode), so he can monitor the effect. 

 

This is greatly simplified, and there's much, much more to it than I can take time to explain here.  Other people here will have their own preferred procedures.  Please look at the Lessons book mentioned above.  You might also invest in one of Adam's video tutorials (they come on thumb drives), which include his actual data taken with a 32-inch scope, so you can try his procedures for yourself on his data.  


Edited by Calypte, 14 September 2017 - 12:44 AM.

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#3 terry59

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Posted 14 September 2017 - 07:26 AM

Yesterday i gave a try on several astrophoto's i made. Normaly one of the first steps with a stacked DSS output (autosafe.tif) is stretching the picture with the Levels function of Photoshop CS 2. Reading a tutorial from Starizona ("LRGB with Photoshop") however, i read that "stretching" is done with the Curves function of Photoshop. Levels is used after several other processing, almost at the end of the mentioned method.

 

I wonder what method is best:

 

1] how do you stretch your picture: curves or levels?

 

2] what is the reason for that?

 

 

Hope to read your experiences. (I am not using/own PixInsight).

With levels all you can do is adjust three sliders. With curves you can define and adjust as many points as you want, giving you much more control. They are different tools. Also, multiple small stretches are far better than less large ones.

 

This is not a "you have to have PI" pitch, but there is a difference between stretching  linear data and non linear data.

 

I am a user of both PI and PS


Edited by terry59, 14 September 2017 - 07:27 AM.


#4 Whuppy

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Posted 14 September 2017 - 09:51 AM

The first thing I was told was to crop the image before doing any streching.

#5 Chucke

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Posted 14 September 2017 - 11:42 AM

Cropping is usually the last thing I do.  Don't know why you would want to crop before stacking since until then you don't really know where the image borders are.  Everyone has their own preferences. 

 

I start by stretching in curves until the image starts being significantly light.  Then I adjust the black point.  Rinse and repeat until it looks the way I want or the data starts picket fencing. 

 

Sometimes I just use the scaled version from CCDStack if it is better than what I can do manually.

 

I got a lot of my technique from Tony.  His videos were really good.  I found they will not play correctly using the supplied player under Win10 which is why he stopped selling them.  However, the player Adam supplied works with them for me.

 

I am always looking for better ways to do things.  Someday I hope to take one of Adam's classes.


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#6 terry59

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Posted 14 September 2017 - 12:03 PM

Always crop edges first. It makes a very large difference. Do this for each filter and again when they are assembled

#7 Calypte

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Posted 14 September 2017 - 03:02 PM

I'm with Chucke on cropping.  Cropping is one of my last steps, although I usually do some additional tweaking after cropping.



#8 Stelios

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Posted 14 September 2017 - 06:21 PM

The tutorial on how to stretch in Deep Sky Imaging Primer is worth the price of the book. 

 

Actually every page is worth the price of that book...

 

The answer is curves. Much more powerful. But you need to learn how to do it right.


Edited by Stelios, 14 September 2017 - 06:21 PM.

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