Superb Processing Tutorial

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Superb Processing Tutorial

Postby Dion » Wed Jan 05, 2011 8:21 pm

This is an absolutely superb, easy to follow, image processing tutorial. It was Done by a friend of mine from Astro Chat called 'Philip' (User Name Gepetto on AC) and is reproduced here with his kind permission :-


This is meant to be a simple method of producing an image that you can be proud of, using DSS and Photoshop
or an equivalent image editing tool.

I'm not going to get involved with darks or flats and I'm not even going to use dozens of subs. There are good reasons
for using darks a flats and lots of subs but I'm keeping it simple. Success at this level means you can easily move further
with processing when you feel confident enough.

Ok, first we need some subs. It's my belief that 95% of the image quality is in the unprocessed subs so it's well worth taking
your time and getting right the basics. Good focus, good tracking, reduce noise by using lower ISO's if you can and use a
light pollution filter if LP is a problem.

My simple tweaking session will be using just two 5 minute subs taken of M42 using an ED80 PRO refractor and a Canon 1000D DSLR with a cheap light pollution filter fitted. The subs are taken in RAW file format which for this Canon is .CR2 type.

PART 1: Deep Sky Stacker

First off, is a picture of one of the unprocessed images just to get an idea of what we managed to get from the imaging session.


Here are screen grabs of the various settings I use in DSS.
Not saying that there aren't better ones but these work for me and if your settings are different, you may get different results...


The two RAW images are loaded into Deep Sky Stacker (DSS) and we tell DSS that these are "light" files when asked.
Tick to check both images in the file list and click to tell DSS to Stack checked pictures.


When DSS has done it's thing, you should be looking at something like this…


You will notice that M42 looks a bit dim and the background is tinted but this isn't a problem.
First port of call is the RGB/K Levels tag. As you can see, the colour histograms aren't aligned so we pick a colour to adjust, selects it's centre
triangle and move it left or right with your arrow keys until that colour is aligned with another.


I aligned the red with the blue and then aligned the green. It's important to click apply to make your adjustment affect the image.


You should now have ended up with all three colours aligned as a single peak histogram and you'll notice that this has affected the background colour of the image to a neutral tone which is what we want…

Now, we're going to bring back the colour information that has been hidden during the stacking process.
Under the Saturation tab we slide the saturation percentage tab to in this case, about 15%.
Too much on this will just make the colour harsh and worst still more noisy. The image below shows that the colour has re appeared.


The next step is quite important. We are going to select the Luminance tab and if you are fairly new to DSS, I would strongly suggest
not touching anything apart from the tab indicated below.
The task here is to slide the tab to the left if the image is too dark or to the right if the image is too light.
Click the apply tab after each adjustment to see the effect it's had on the image.
The test image required a small tweak to the right to darken the image slightly but be careful here. If you darken too much, the image will clip to black and fine detail will be lost forever.

Once your satisfied, click apply to affect the image with your adjustments.


So here we have it, an already credible image with detail and colour present and overall, a fairly balanced neutral back ground.

Save this as a .TIFF file.
There is an option in DSS to save the file with processing steps applied to, or saved with the TIFF file. We want the processing applied option or the
tweaks will be lost

In part two, we will pass this into Photoshop for some more tweaking.
I can't over state the importance of getting things right during the DSS stage. If we pass a rubbish image to Photoshop, it will remain
a rubbish image however talented we may be at image processing during part two.


Part 2: Photoshop

The beauty of Photoshop is that there's a thousand ways of achieving the same result and there are some very sophisticated
processing methods in the various tool drop downs. My aim here is to keep it as simple as possible and to use the most common
tools. This way, there's a good chance that if you're using another image package then you may still be able to get by.

OK, onwards and upwards…

Our DSS M42 TIFF file is loaded into photoshop and before doing anything else, I've cropped for a nicer framing and rotated
the image by 90 degrees clockwise so it's the same orientation as you would see it in the sky.


Next job, is to remove some of the finer noise in the image.
My opinion on noise reduction is to do it little and often rather than flatten it all at the outset. Quite often I will apply noise reduction
maybe four or five times during the processing session. Because of this, you don't want to use aggressive settings in your noise
reduction tool.

Now, here we have a problem. The standard noise reduction tools in Photoshop aren't the best unless you really know what's going
on. There are better tools and at any one time, I might be using Noise Ninja, Neat Image or Noel's Astronomy Tools. Use whatever tool
you've got and are comfortable with but as I say, aim for a few subtle noise sweeps rather than one aggressive one. Below is our image
which has had a subtle reduction of the finer noise…


Now we're going to look at the levels so we can improve the grayscale range and a bit of care is needed here so as not to make the
image too dark or too light which would indicate a loss off the more subtle detail.

Open up levels IMAGE/ADJUSTMENTS/LEVELS and first thing, move the black slider to the right towards the beginning of the histogram.
This will make the image background a bit darker. Don't move the black pointer into the histogram as that will blacken the background and
you'll loose image data. This is commonly known as clipping and can occur at either end of the histogram. Leave a small gap as shown in the image below.
Next, grab the grey button (midpoint) and move it slightly to the left. The effect of this is to lift the midpoint or grey parts of the image and as
you move the slider, you should notice the finer wispy detail around M42 becoming more distinct.

A thing to notice now and later is that a lot of the adjustments made during processing are quite small and subtle.


Ok, onto curves IMAGE/ADJUSTMENT/CURVES… (if you're OK with using curves you can ignore the next big chunk) :)

The curves tool is a magic little beast and I use it a lot. Problem is, it's not immediately straight forward to use and I'm thinking it's not going
to be easy to explain in words but here goes..
Open curves and up pops the tool box. Here, you're looking at a box with a graph in it, a picture of your image's histogram and a diagonall
line going bottom left to top right through the graph.

The idea is that you can click your cursor on any part of that line and pull it up or down making the line into any variation of a curve.
The range of the image from black to white that you have most effect on, depends on where on the line you put your cursor. This is what the
horizontal bottom graph scale is for. Choose which part of the grey scale you want to change and put your cursor on the diagonal line
directly above that point.
Now, the more you pull that point upwards and to the left, the brighter that part of the image gets. Pulling that point down and to the right
darkens that point of the image.

Now the clever bit…
You'll notice if you try it, that if say you put your cursor half way up that line, then when you pull out a curve to the left and up (looks like a longbow), you will have had most brightening effect on the mid greys but you'll also notice that you've lifted all the other levels by a lesser amount.
We might not want this so we need to lock the parts we don't want to alter and leave the parts we do want to alter free for pulling.

OK, were going to add some limiting locks on the line.

Select the eyedropper tool from the left hand main Photoshop tool bar. With your left mouse button pressed down, just meander the eyedropper tool around our image whilst watching the curves graph. You'll notice that a small circle dances up and down the diagonal line as you pass over lighter and darker areas of the image. This is an indication of what part of the adjustment line corresponds to the brightness level on a chosen part of the image.

We don't want to alter the background so put your eyedropper on a clear part of the darker background. A circle will show down the lower end of the diagonal line. Make a mental note of where the circle is and mouse to that point followed by a left click to set a limit mark.
Next, point your eyedropper tool at a bright part of the image and again mouse to where the circle was and left click another limit mark.
(this will be higher up the diagonal line to the first one you set).

What we can do now is to grab and pull the line between these two points which effectively allows us to alter the parts of the image we want
without having much effect on the parts we want left alone.
This becomes even more of a powerful tool when you realise that not only can you do this with differing shades of black to white, you can also do any of the above to individual colours, red green or blue parts of the image..

If you look at the Channel box above the graph, it shows "default" which is adjusting the whole image, Clicking the Channel drop-down lets
you pick red, green or blue to work on separately


All the above is a long winded way of saying that we can select the red channel, set limits to the background and highlights and then eyedropper over the weak red part of the M42 image. Pulling up and left the line at that point, increases the colour brightness in the red part of the image whilst having little effect on the rest….. SIMPLES!

Time for a picture :)
Here showing is work done on the red channel curve and as you can see, it's had a magic result on bringing out the colour of the nebulosity.


The image is looking rather good but a bit blueish on the midtones.
To fix this, goto IMAGE/ADJUSTMENTS/COLOUR BALANCE. Select midtones as this is the part we want to alter.
A tweak left with the bottom slider "warms" the blue tint by adding some yellow into the mix. A tweak to the right with the top slider, "warms"
the reds by cutting back on the cyan in the mix.
Again, it's a case of subtle tweaks to makes a noticeable improvement to the image.


A quick return to curves IMAGE/ADJUSTMENTS/CURVES and this time we're using all the channels (default)
Two limit stops are set to protect the dark and bright parts and the eyedropper tool has picked a point corresponding to the fainter
grey wispy nebulosity. A subtle pull left and up from this point just gives a bit of emphasis to these areas.

Time again for a subtle wipe with your noise reduction tool


Next job is a touch of sharpening. Just enough to make the stars "pop" but not as much as would introduce unwanted artefacts.
FILTER/SHARPEN/UNSHARP MASK and set roughly as per the image below.


Nearly there, jobs are getting easier now. Just a resize at this point. IMAGE/IMAGE SIZE
I've chosen to resize using percentage because when we come to blending another core onto the image, I need to be able to
get the "core patch" at the same size/scale. Reduced to 30% gives us a manageable image for posting on the forum and such.

Another very subtle noise filter wipe.


And finally, I fancied a tad more oomph in the contrast so I returned to curves using all channels and gave a slight pull
to the curve centre. This gave the image a bit of a lift.


OK, that's job done for my liking and we've used subtlety to hugely improve the image.
More subs would make a better image but this session just shows that with a couple of quick shots and some delicate TLC, a very
nice image can be the reward..

Next session, blending a darker core to hide the over exposed part of this image….

A final comparison, none too shabby I reckon



Part 3: blending in a new M42 core

This is necessary because targets like M42 and others which have a huge dynamic range in their brightness, mean that we need to use fairly long exposures to get the fainter stuff which results in over exposure of the brighter areas.

The trick is to take some shorter exposures from which we can cut a "patch" to blend over the blown out part of the main image.
This is done using two layers in Photoshop, one for the main image and one for the short exposure.
Again, there are some quite sophisticated ways of doing this but I'm sticking to the 6" nail approach because it's simple and it works :)

I've made available two files in our Mediafire store in the usual folder TweakTips_M42 files. One is part2final.jpg which is the final image saved from part 2 of this session and the other is SXcore.jpg which is the result of a stack of two 15 second images of M42 core area. This will be our "patch" and to make things easier, I've ensured that the patch image is the same size and orientation as the main image..

Here are the two images we'll be using


Next step, open SXcore.jpg in Photoshop and select a prt of it using the lasso tool with feathering set to say 5 pixels. I've picked more than we'll need from this core image but it's best to get a few stars in the selection for using to align with the main image.


OK, EDIT/COPY this selection to the clipboard, close this image down and open the main image part2final.jpg.
Next, EDIT/PASTE to bring up our core selection in it's own layer on top of the main image….


Make sure the patch layer is selector (layer1) and set it's opacity to say 50%. This makes it easier to work on as you can see through the patch to align it's stars with the corresponding ones in the main image.
Roughly align using the mouse by dragging, then "fine tune" using the arrow keys until the overlay is perfect.


Select the erase tool from the main Photoshop toolbar (left side) and choose a brush with "fuzzy" edges size 65.
Set the brush opacity to 25%. These settings mean that we can use broad but subtle sweeps of the eraser. Again, make sure layer 1 is selected and start wiping the erasure tool into the edges of the patch. Trick here is to sweep away the outer darker areas of the patch but at the same time leaving the part over the burnt core alone.
You should start to notice that the other areas quickly become erased or blended such that it becomes very difficult to tell the patch and the main image apart…


OK, when you're satisfied with the blend then you can increase the opacity of whats left of the patch.
How much is a matter of taste but I like to keep the core of M42 bright but not over exposed. It's a dynamic part of the nebula and if it's made too dull it robs from the final image. Around the 72% mark seems about right.


A little unsharp mask of the patch layer gives the trapezium stars a "lift" then the whole thing is given a nice frame courtesy of Noel's Astro Tools and saved to admire….

Job done


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Re: Superb Processing Tutorial

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Re: Superb Processing Tutorial

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Re: Superb Processing Tutorial

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Re: Superb Processing Tutorial

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