ASRAS ADVANCED ASTROPHOTO PROCESSING WORKSHOP

Feb/Mar 2012 Presented by Joel Schmid

INTRODUCTION

• Have you ever taken what you think will be a GREAT astrophoto, but have no idea what to do with it to make it like the photo’s in the popular astro magazines and websites ?

• Have you tried your hand at processing an astro-image yourself, but the results were not exactly “stellar” ?

• Just getting started in astro-imaging and looking for some good advice on processing ?

• Well then you have come to the right place !

• In this lesson I will attempt to break down the workflow of stacking, basic processing, advanced processing, and final polish of a more difficult subject. M42 (Orion Nebula) is a high dynamic range astro-image requiring more advanced processing. To show how its done, I will break it down into an easy to follow step-by-step process.

Required Software for Astro-image Processing

The following software is required for processing an astro-image. Many of these applications are available on the internet for free. After emptying your wallet on a scope and camera gear, this is a good thing !

Stacking software - • Deep Sky Stacker – http://deepskystacker.free.fr/english/download.htm (Free shareware)

Processing software - • Gimp image processing – http://GIMP.org (Free shareware)• Adobe Photoshop CS2, CS3, CS4, CS5 – Not free, but much more powerful than GIMP. Allows more

sophisticated image manipulation.

• IRIS : stacking and processing http://www.astrosurf.org/buil/us/iris/iris.htm (free !)

• OPTIONAL PROCESSING GOODIESThe following are add-ons to the main processing software and are relatively in-expensive. They perform many useful functions to improve an astro-image even further.

• ProDigital Software - http://www.prodigitalsoftware.com/Products.html (home of Astronomy tools and Star Spikes Pro) ProDigitals Astronomy Tools is an almost essential set of photoshop actions that have many extremely useful tools for noise reduction, star color enhancement, light pollution removal, etc.

Price = 24.99 (very reasonable)

• Star Spikes Pro – Also from ProDigital, Star Spikes Pro is a very nice add-on feature for astro-images. It creates brilliant, beautiful artificial diffraction spikes in an image and really makes the final result sparkle.

Price = 49.99 (a little steep, but the effect is VERY nice)

• Gradient Exterminator – Very effective at removing sky gradients caused by light pollution. Price = 29.99 (and worth every penny)

Lets get started !

Things you will need :

• Light Frames – Light frames are your actual photos that you took of the object. Typically you want at LEAST 12 good frames of your object for stacking. The more frames you have for stacking, the better the results – up to a point. Typically having MORE than 30 frames reaches the point of diminishing returns in the stacking software. Using many frames in the stacking software allows the software to reduce background noise and remove hot or cold pixels. For an object that has high dynamic range like M42(bright core and dim details outside) you will want at least 12 long exposures to capture the dim stuff (5 minutes or longer) and 12 short exposures to capture the really bright core. (30 secs or less) For a Galaxy shot like M31, you would need at least 12 long exposures for the outside edge, and 12 short exposures for the core.

•Dark Frames – Dark frames are shots taken with the camera and the lens cap on, or with the telescope objective covered. Dark Frames are very important for subtracting amp glow and hot pixels. Every camera has what is called HOT PIXELS. A “hot pixel” is one little (or sometimes a few) pixels in the camera CCD array that does not play nice. It is way brighter than the rest of the pixels, and usually one of the primary colors (red, green,blue) If these were not removed by subtracting the dark frames during stacking, the final stacked image would have little speckles of bright red, green, or blue. These would make silly looking stars, so we want to remove them ! Some older cameras also have amp glow caused by heating of the CCD amplifier in the camera during long exposures. This results in the lights having a purple or bluish stain in one corner or along one side of the image. Taking dark frames at the same exposure length as the lights allows the stacking software to remove these.

•Flat Frames – When you point a camera down the telescope tube and take a long exposure, the resulting picture will end up having “tunnel vision” or a shadow circle of the telescope tube in the background of the picture. This is called vignetting. The best way to remove vignetting is to take some FLAT FRAMES. Flat frames are shots taken with the camera attached to the telescope and focused exactly the same as the lights. The telescope is pointed at a neutral light source like a grey cloudy sky or with a clean white t-shirt stretched over the objective of the scope. The camera should be set on the AV mode so that exposures are automatically adjusted as you shoot the flat frames.

Here is a typical dark frame. This is a 5 minute exposure with the cameras lens cap on and the temperature outside was close to the same as the night I shot my lights.

Hot pixels

Here is a typical flat frame. Notice how it looks like a foggy tunnel. This is what the camera picks up looking down the telescope tube on a long exposure.

Flat Frame

Step 1 – stack the lights, darks, and flats in DSS

Our object for this lesson is a surprisingly difficult one – M42, Orions Nebula. M42 has a very broad dynamic range, with faint, gaseous outer regions and a very bright core.

It will be challenging to capture all this beautiful region of space has to offer. To accommodate the broad range of brightness, I took three sets of exposures –

•12 ea 5 minutes to catch all the faint stuff around the nebula

•12 ea 2 min exposures to catch the brighter parts of the core

•12 ea 30 sec exposures at reduced ISO (lower sensor gain in camera) to capture the trapezium area without burning it out.

I will then stack these exposure sets separately and then combine them later in photoshop.

Start up Deep Sky Stacker and open your lights, darks, and flats.

If this is the first time you have used DSS, lets take a look at some basic settings first :

In DSS, click on the Raw/FITS DDP settings at the bottom of the REGESTERING AND STACKING menu as shown here. The RAW/FITS menu settings window should open. Set the Bayer matrix Transformation to AHD Interpolation. Set the White balance to USE CAMERA WHITE BALANCE. Now click APPLY to save the changes. This will tell DSS to use the best color interpretation of your cameras raw files and the best white balance while stacking.

Open your lights in DSS. Click on the red “open picture files” at the top of the REGISTERING AND STACKING MENU.

Open your darks with matching exposure times to the lights

Open your flats taken through the same scope at same focus as lights

Don’t worry about adding dark flat fames or offset bias, these are usually used by experts on more difficult images and would not help our object M42 for this lesson. We can learn more about these in a future lesson.

The next step after opening your lights, darks, and flats is tell DSS to “compute offsets” This will cause DSS to look at each frame and determine its quality, how round the stars are, and create a rating system so you can sort the frames from best to worst. This allows you to select the best frame to use as a reference for stacking. First click the check all in the menu to select all your lights, darks, and flats. Then click on the compute offsets. This will probably take a few minutes.

Now click on the top of the Score column to sort all the lights from best rated to worst. Click on the one with highest score and then RIGHT CLICK and select Use as reference frame. This will tell DSS to use the image with the best quality stars as the reference for stacking all the other lights. This will ensure you get the best stacking results.

Now select register checked pictures. Check all three boxes. In the box for Select the best % pictures enter a value of 90. This will tell DSS to keep the best 90% of the pictures and throw the worst 10 % away. Then click the ADVANCED tab and set the star detect slider = 7%

If all your lights have the same field of view and the object you photographed are all basically centered, select standard mode. If your lights have an object that has shifted around during your exposures, select “mosiac” mode.

There are many different modes available for stacking. You can read through the HELP files in DSS to see how each one works. I normally use the Median Kappa-Sigma clipping mode for most of my images. Select the options as shown below if you want to use this method.

Once you have the stacking modes for your lights, darks, and flats set up, click OK. DSS shows you a summary of the actions it will perform.

Here is our stacked results. DSS shows us what the stacked image looks like BEFORE its processed in photoshop or whatever you will be using for image processing. Don’t worry if it doesn’t have much color or looks washed out. Now click on Save picture to file. It is VERY important to check the box in the save window for “Embed adjustments in image but do not apply” This prevents DSS from changing the stacked results before you open it in your dedicated processing software. You want to do all the detailed work in the software. You don’t want DSS to do it for you. Save the picture as a 16 bit TIFF file. DSS will also allow you to save the file as a FITS format file if you have processing software that works with FITS files.

Repeat the stacking procedure for your other exposures. Make sure you clear out the long exposures in DSS first to make room for the shorts. For example, for M42, I would now select all the long exposure lights in DSS, then right click and select REMOVE FROM LIST to make room for the short exposures. Then click on OPEN PICTURE FILES again but this time pick the SHORT exposures for the core. Click on the CHECK ALL in the menu again and do another COMPUTE OFFSETS. When that is done, select the highest score image and use it as reference. Tell DSS to REGISTER CHECKED PICTURES and wait for DSS to stack the short exposures. When its done, SAVE the short exposure stack in the same place you saved the long exposures. Give it an easy name to remember like M42 short 30 sec. Now your done with stacking, so close up DSS.

Now open the LONG exposure saved stacked file in the processing software. I always start with the long exposure first, so I can layer mask the short exposures later. In this screenshot I have opened the file in photoshop. The first thing to look at in any image is the histogram. The histogram shows you the relative range of pixel brightness in the image. Having the histogram view open during processing is very helpful in making sure that you don’t lose any data while stretching the image. To see the histogram click on the WINDOW menu and select Histogram.

Notice how all the pixel data in the stacked image is jammed up all the way on the left side. What we need to do is draw all the hidden colors and details in the image out so we can see it. This is called “stretching” the image.

The best way to stretch an image in photoshop is with curves. Click on Image, then Adjustments, then Curves.

A good setting for the first curve is to place a point on the line and then set its input = 12 and output = 32. Place a second point on the line and set its input = 128 and its output = 191.

After the first set of curves have been applied, it is a good idea to make all three color channels equal brightness. In photoshop, this is done with levels. Click on Image, then Adjustments, then Levels. From the channels dropdown list select the red channel and move its black slider in the levels window so that it falls directly on top of the blue channel in the histogram. Then repeat for the green channel. Always slide the RED channel first. The red channel is usually the strongest in an image as a result of light pollution. Images taken from a really dark site have much better color balance right out of the camera.

While we stretch the image, we need to protect the bright stars from getting burned out and bloated. As we brighten and enhance the missing details, the stars are getting brighter at the same time. They will quickly lose their individual colors and become just pure white and too large for the scale of the image. To prevent this, we will create a star mask. A star mask is a carefully blended selection of just the bright stars in the image. We can then select the opposite, or inverse of the stars to keep stretching the rest of the hidden detail to bring it out while keeping the stars close to original size and color. The first step to creating a star mask is to open the color range selection window. Set the fuzziness slider to 100. Then click on any part of the background in the image that does NOT have a star or nebula gas in it. Now adjust the fuzziness slider so that you can see just the stars and bright core of M42 in the window.

Now select the inverse of the color range selection you just made. This will grab just the stars. Use the magic wand tool to deselect the bright core of M42 so that it doesn’t become part of the star mask. Hold the Alt key and click the magic wand around inside the bright core to remove it from the stars selection. You want to subtract ALL the background nebula out of the selection so that it doesn’t get left out of the stretching. Set the tolerance of the wand to 20.

The next step is very important. To blend the star selections smoothly into the image, we want to feather the stars. First I will enlarge the selection by just 0ne pixel, then feather it. To do that click on Select, then Modify, then enlarge. Enter a value of 1. This helps to ensure I have selected the stars and some of its glow. Then click on Select, then Modify, then Feather. A value of 4 or 5 is a good setting to blend the stars into the background.

Invert the selection again so that just the stars have the little “marching ants”. This makes sure you have stars selected and not background. Now save the star mask by clicking on Select and Save selection. Name the selection “bright stars” You will use this selection many times during the rest of the processing.

Now select the inverse of your star mask. This will cause photoshop to select everything else BUT the stars for further enhancement.

Now its time for a second set of curves to further stretch the image. Use the same curve as the first time. Place two dots on the RGB line. Set the first to input=12, output=32, and the second one at input=128, output=191.

After the second set of curves is applied, the image is now too bright. We need to apply another levels adjustment. This time we will adjust all three channels at once. Move the black slider closer to the spike in the histogram to lower the image brightness. Be very careful NOT to move the black pointer too close. Doing this will clip off the dark, shadowy parts of the image and make it look unnaturally dark in the final result.

Now we apply another curve just like the first two to further stretch the data.

Uh Oh – a few problems in this image have now been revealed. There are a couple big dust spots on the left side, and an uneven background color bar across the bottom. The bar is created when one of the frames in the stack did not have the image centered the same as the other frames. Have no fear ! I will show you how to fix these.

Use the healing brush tool in photoshop to blend over the dust spots in the image. It’s the tool that looks like a band-aid. Set the brush size to 100.

After fixing the dust spots, use the levels adjustment to darken the image again after the third curve adjustment. Again, be careful NOT to go to dark with the black slider.

Before we fix the uneven seam at the bottom we will use the crop tool to trim off the uneven edges resulting from stacking. Uneven edges almost always show up after stacking, because each light frame shifts a little during the night as you take each exposure. The stars are all lined up, but the edges of each picture shift as the sky rotates overhead.

We should also reduce some of the noise in the background using the deep space noise reduction in Noels actions. Its always best to use DEEP SPACE for a nebula or galaxy image because the regular SPACE NOISE function is too aggressive and will muddy up the details in a nebula or galaxy.

Reload your star mask again and make a curves adjustment like the one shown here to brighten up the stars a little. NOTE: You may need to INVERT the star mask selection to select JUST stars, depending on what state your star mask was when you saved it. Just look to make sure the little selection “ants” are around the stars, and NOT the outside of the image. This helps avoid “panda eyes” – those little black rings around the stars located inside the nebula.

Now is also a good time to SAVE your progress so that all your work is not lost if the computer crashes or the power goes out.

Now we will use the magnetic lasso tool to carefully select the brighter bottom seam in preparation for repairing it. Magnetic lasso allows you to draw a line around objects in an image, and photoshop sort of grabs onto the edges of the object to help you select it. Dragging the selection line past the edges of the image causes photoshop to select the edge of the image.

Now that we have the oddly colored bottom edge selected, we need to smoothly blend the selection into the rest of the image. Click on select, then modify, then feather. Set the feather amount to 100. This will create a very gradual blend of the changes we will make to the selection to get it to match the rest of the image

Here I have made a levels adjustment to the separate red, green, and blue channels to try and match the bottom piece as closely as possible to the rest of the image.

To do that I adjust the red, green, and blue sliders while watching the bottom edge so that I get a close a match as possible to the upper 2/3 of image.

Note that the edges of the selection are NOT visible in photoshop – but its still selected. When a selection has a large feather, photoshop cannot show the edges of selection.

After adjusting the brightness of red, green, blue channels in the lower section of our image, we need to complete the repair by erasing the thin seam created by the feathered selection box. A good way to do this in photoshop is with the clone stamp tool. The clone stamp tool allows you to select a nearby part of the image and blend it smoothly under the mouse pointer brush as you hold down the left mouse button. Since I have selected a point above the seam, the mouse brush blends the background above the seam and erases the ugly line as I go

Now our lower edge is all fixed ! You can barely tell it had a mismatched background brightness and colors. We were able to keep all the stars in that lower section instead of having to crop them off.

Since this image was shot through an achromatic refractor, we have some color aberrations on the bright stars. Noels actions has a tool to fix these, but I don’t want them to be completely removed. Instead, I want to apply the removal at a reduced amount, so that I get to keep a little of the intense blue halo around the bright stars. This gives them the feel of being big and hot as I imagine them to be. To do this I am going to add a duplicate layer on top of my original so that I can use the opacity and fill sliders to change how much of the blue halos I can see.

Now we see that after having run the “reduce small blue halos” around the bright stars, I am using the opacity slider in the layers window to fade that effect out a little as it lays on top of my original layer. This allows a little of the blue halos to shine through from below. This gives the illusion that these massive stars are super hot and glowing bluish/white- instead of just plain white.

At this point we have stretched the image fairly hard, and the background noise is creeping in. To reduce it, I will use the reduce deep space noise for a second time in Noels actions list.

And here we have the results after noise reduction. The graininess and color blotching have been reduced, creating a smooth background. I did not use the more aggressive space noise reduction as this action tends to blur and muddy up the wispy gas in an image. That action works better when you have just stars, like in a globular cluster.

Since all of our stretching has excluded the stars so far to prevent them from burning out, I will use a nice action in Noels list to boost the star colors and bring them out a little. Running this action twice makes the hidden colors of the stars pop out. Reds, yellows, and blue stars are now more visible.

NOTE – sometimes an image will show some green stars. There is no such thing as a “green” star in the universe. If this happens, try using the magic wand to select the green stars, feather it by 3 pixel, then use a curves adjustment to JUST green channel to bring the green level back down.

To further enhance the stars colors and brightness, I will use the select color range tool to create a new star mask. Since we have stretched the image three times already, many more stars that were hidden in the background are now visible. I want my new star mask to include some of these. Once again after setting the fuzziness slider, I will use the magic wand and hold down the ALT key to subtract parts of the image I don’t want in the star mask. Then I will feather the selection with a value of 4 or 5, and save it as star mask 2

Now that I have more of the bright stars selected, I click on select, then inverse to change the selection to all the background around the stars. Then I apply another curves adjustment to each of the red, green, blue channels separately to try and bring more of the hidden color and detail of the image out. We can see that the background is beginning to look a little brownish red from all the Ha dust and gas that lies in M42.

The running man is also picking up details and colors.

Once I get the background color cranked up as high as I can go before it starts to get noisy again, I invert my star mask selection again to select just the stars. Since I have been stretching just the background and not the stars, I need to brighten up the stars just a bit so they don’t get black rings around them. To do this I apply a curves adjustment to them like the one shown here.

During all of our stretching some of the big stars will get too big and bloated. We need to put them on a diet ! A good way to do this is to use the make stars smaller in Noels actions. Run this action twice to shrink up the bloated stars.

Once I have all the background details and colors showing in the image, its time to use some layer masking to see details in the core of M42. Since M42 has a high dynamic range of faint areas and a very bright core, I will use layers in photoshop to blend in shorter exposures of the core to allow us to see the trapizeum area. To do this I will open the stack of shorter 2 minute exposures that I have previously processed to show detail in the outside portions of the core. I will use another stack of even shorter 30 sec exposures to layer in so we can see the actual trapezium. After opening my outer core exposure image. I click select, then select all to grab the entire image. I then click edit, then copy to transfer it to the clipboard.

Next I click back on my original image and select edit, then paste.

The shorter exposures are now laying on TOP of the long exposures underneath.

Now I need to get the outer core copy to lay directly on top of my original stretched image so that I can use a layer mask to reveal just the core of the nebula. The easiest way to do this is to switch the layer blend mode to difference. The difference mode shows you the two layers in a contrasted view so you can see whats under the first layer. Now use the keyboard arrows to move the top layer up or down and left or right until its lined up perfectly with the layer under it. You can tell when its perfectly aligned when the stars turn black.

Once I have the outer core layer aligned, I open another file of the stack I made with very short 30 sec exposures for the inner core and trapezium area. I do a very small curves adjustment to this stack so that ONLY the trapezium can be seen. This will be layer masked on top of the outer core layer for a composite that has the best of all M42 has to offer. Lots of brown dust, all the whispy parts of the outer nebula, running man, and field stars, and a nicely resolved inner core and trap area with all the trap stars visible and the “rainbow wall”.

Once you have all the layers in place, you can use the little eye icons next to layers to turn them on or off so you can see whats underneath. Switch the layer blend mode back to NORMAL for all three layers.

The first step in creating the composite is to turn off the top inner trap layer with the eyeball icon. Then we want to add a layer mask. A layer mask is a way to either HIDE or REVEAL the layer that you have selected, then use the brush tool to “paint” over the layer to reveal the hidden parts of the image, or hide them away so that the layer under it shines through. For this mask I will select HIDE ALL.

Once we have added the HIDE ALL layer mask, we select the brush tool. Since we want to reveal the parts of the core layer where we paint with the brush, we need to switch the foreground color to be OPPISITE of the layer mask color. Looking at the layers window we see that the hide all mask is BLACK. So we click on the “switch foreground and background colors” to switch the brush color to WHITE. Now when we paint on the image with the brush, the CORE layer will appear where our brush goes.

Here is the result of painting on the layer mask. The better exposed core area is now visible. We have replaced the burned out portion of the nebula with the painted in section from the shorter exposure layer. But there is an obvious problem. The transition between the painted area and the rest of the nebula is very ugly. We will fix this by blending the painted on area with a gauss blur

Click on filter, then blur, then Gaussian blur.

Now slide the blur radius up the scale until the layer mask painted area gets nice and fuzzy. You can see the effects on the image behind the gauss blur window. Adjust the slider till you get a nice smooth blending of the painted core area and the rest of the image.

Once we get the outer core area nicely blended, we go back to layers and click on merge down. This merges the outer core layer into the bottom layer which has all our detailed background.

Now I repeat the layer mask, paint, and gauss blur process on the trapizeum area of the nebula.

Once I have a blended in trap area, I notice it’s a little under exposed and washed out. To stretch out just the core and make it match the rest of the colors in the nebula, I select just the trap layer and run a curves adjustment on it.

Once I am satisfied with the inner core I merge it down in the layers menu. Now I have a nice composite of the nebula. The trap and core are well resolved with lots of vivid color and detail.

Now I will use a neat technique I learned from some guys on the Cloudy Nights website. If you switch the image mode to lab mode you can apply curves adjustments to the A and B color channels separately. This acts like an HDR compression of the color channels and really enhances the colors of the image. This works much better than just cranking up the saturation slider.

Switch the layers window to the channels window and then click on the A channel. Open the image adjustments, curves and apply a curve to the A channel just like this one.

Then select the B channel and apply the same curve to the B channel.

Click back on the LAB channel to see the results. The image now has a nice rich color range in it. The purples, reds, and blues have all been enhanced.

Now load your star mask 2 with all the stars in it.

With just the stars selected, open the image adjustments hue/saturation window and crank up the saturation of the stars by + 20. This will bring the intense star colors out.

After all the noise reduction we performed during the stretching process, the fine details in the image tend to get a little fuzzy. A great technique I read about from imager Tony Hallas is to use a high pass filter in a layer whose blending mode is set to OVERLAY. This works even better than a unsharp mask. Go to layers and click add duplicate layer. Name this layer HIPASS. Now select Filter, then Other, then High pass.

Set the radius of the high pass between 1.0 and 2.5 at most. Experiment with values to see what looks best.

Then click OK.

Flatten the high pass layer down from the layers menu.

Sometimes the high pass filter will make the background of the image and around the whispy parts noisy or speckled. Noels actions can fix this with the space noise reduction. Here I have run that action and cleaned up the speckles.

At this point the image is pretty much finished.

Something I like to do in almost all of my images is add some artificial diffraction spikes. ProDigital makes a REALLY nice photoshop plug in called “StarSpikesPro” This plug-in is a highly customized special effect that really makes the stars in an image “pop” with color and sparkle.

The final step in the process is to darken the background a little with a curves adjustment that has one data point on in with the OUTPUT set to 0, and the input set to 12.

And here is our final result. We have all the nice brown dusty background, a beautiful nebula and running man, and a nicely resolved trap with a colorful “rainbow wall” of shocked gas to the left of the trap stars.

A real wall hanger !