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Deep Sky Image Processing in Photoshop

Well, this is it.  In this deep sky image processing tutorial, I’ll be combining all of the data I was able to collect on the Orion Nebula this winter.  As we transition into Spring, a new array of deep-sky imaging targets will present themselves.  The winter astrophotography targets in the Orion constellation will have to wait another year to get photographed.

Deep Sky Image Processing

Processing Walkthrough – Orion Nebula with a DSLR

Canon DSLR for astrophotography

The total amount of detail I was able to capture on M42 this winter was 3 Hours and 8 minutes of color RGB data.  I will be incorporating 2 hours and 40 minutes of Ha data into the final image using the HaRGB processing technique.  In this post, I’ll show you exactly how I process my image of the Orion Nebula using Adobe Photoshop.  I’ll start with the Autosave.tif file produced by DSS.

Some of the images used in my final photo were shot during the AstroBackyard YouTube video: Let’s Photograph the Orion Nebula.

DeepSkyStacker

The screenshot below shows the results of registering and stacking 4 nights worth of imaging from my backyard.  This winter has been plagued with numerous cloudy nights, so I had to capture photons here and there, under varying sky conditions.

Yes, it is very white!  That’s light pollution for you.

DeepSkyStacker

Orion Nebula stacked .TIF file in DeepSkyStacker

The photo sets from each imaging session were loaded into the group tabs of DeepSkyStacker.  My modified Canon T3i camera was set to ISO 800 for each imaging session, but I bumped the exposure time up to 3.5 minutes for the fourth and final set.

Using the group tabs in DSS

  • Dec 22, 2016 – 23 frames – 180″ @ ISO 800
  • Feb 2, 2017 – 24 frames – 180″ @ ISO 800
  • Feb 3, 2017 – 10 frames – 180″ @ ISO 800
  • Feb 27, 2017 – 11 frames – 210″ @ ISO 800

Image sets 1-3 were stacked using darks, bias and flat calibration/support frames. The final and fourth set did not use flat frames as I was not able to shoot them the morning after the imaging session.

I do not make any adjustments to the stacked image in DeepSkyStacker.  I bring the 32-bit Autosave.tif file into Adobe Photoshop for all post-processing.

Processing in Adobe Photoshop

I use two Photoshop Plugins in this tutorial, Astronomy Tools Action Set, and Gradient Xterminator.  See all of the astrophotography software I use here.

Cropping/Rotating the file in Photoshop

The first thing I like to do is to rotate and crop the image.  A temporary levels adjustment was made to get a better look at the edges of the frame.  As you can see, my frames rotated and shifted slightly between the imaging sessions.  This creates an unusable sky at the edges of the image, so I will crop the image to about 85%.  In the future, I plan to incorporate a plate-solving software such as AstroTortilla to help line up my images over multiple nights.

deep sky image processing

Rotating and cropping the image in Photoshop

To save some of the outer regions around the nebula, I will have to repair some of the outer background sky using the healing brush, and the content-aware fill tool in Photoshop.  Ideally, you would want to keep as much of your original frame as possible.  Once I have cropped the image, I will adjust the black point of the image.

Levels Adjustment / Setting Blackpoint

As you can see in the image below, the histogram shows that the majority of the image data in contained in the mid-level tones.  I will move the slider to the left of the histogram over until it touches the information contained within the image.  This will darken the background sky and increase the contrast of the original image.

Levels adjustment

the first levels adjustment creates much more contrast in the image

The slider to the right of the data was moved inwards as well.  Is is important that you do not clip the data and lose any pixel information.  You may notice that the core of the Orion Nebula is completely white and “blown out”, I will correct this issue later on.




Before setting the initial black-point, I will give the image a semi-aggressive curve stretch to reveal more of the outer nebulosity.  This will also discern where the nebula ends and the background sky begins. Before Photoshop will let us make this adjustment, we will need to convert the image from a 32-bit file to a 16-bit file.

Image > Mode > 16 Bits/Channel

An HDR Toning window will open up.  Avoid choosing the tempting default preset of Local Adaptation, and instead, select Exposure and Gamma from the Method selection area. Leave the default exposure and gamma settings.  As this tutorial moves on, we will be creating our own HDR (High Dynamic Range) version of the Orion Nebula using very specific actions and settings.

At this point, you can adjust the levels once more, as there is likely empty space to the left of the data in the histogram again.  You may also choose to create a copy of your original layer, or create a new adjustment layer to work from.  Having snapshots of your image at each stage of the processing workflow will help you go back and fine-tune your edits.  Personally, I like to use a mixture of new layer copies using the History feature of Adobe Photoshop.

Here is what my initial curve stretch looks like:

curves adjustment in Photoshop

 

The curves stretch I applied brought forward the fainter details of the outer nebulosity.

Here is a little trick I like to use: With the curves window open, hold down CTRL, and click an area of the nebula you want to bring forward.  This will plot a point on the histogram you can pull from to stretch that particular tonal range.  You can also plot an additional point of a neutral area of background sky, and know that you are pulling data forward from only the nebula itself, and not the space around it.

Levels Once the curve stretch has been applied there are two ways to set the black point of the image.  The Set Gray Point eyedropper in the levels window is great for a quick overall adjustment.  Although some astrophotographers will argue that this method results in a loss of overall range of data.   You can also manually set the color of your background sky by plotting a Color Sampler eye dropper in a neutral area of space.

Using the Info window, adjust the left-hand slider on each RGB level until the values are balanced.  A background sky with Red/Green/Blue values of about 30/30/30 is a good starting point.

Creating a star mask

If you don’t want to risk the chance of brightening the stars in your image and blowing them out, try using a layer mask to protect them from growing in size and intensity.  The art of stretching the deep-sky object, but not the stars is a constant challenge when processing astrophotography images.

You can create this mask by using the Color Range tool.  Select > Color Range.

Select Color RangeThen, use the eyedropper to select a medium-sized star within the frame.  Adjusting the Fuzziness slider will affect how much of the color range (and stars) will be selected.

You will have to experiment with the fuzziness slider to select your intended amount of stars.  In my example, I used a value of 140.  After the stars have been selected, I suggest softening the selection for a more natural blend in the mask.  To do this:

Select > Modify > Expand (2 Pixels)

Select > Modify > Feather (3 Pixels)

Again, these values will vary based on your image scale. If you are shooting wide field through a Canon T3i or similar model, these settings should work well.

Like many tasks in Photoshop, there are numerous ways to accomplish a layer mask adjustment.  For this step, I prefer to invert the selection of stars (Select > Inverse) and make my curve adjustment to all areas of the image except the star mask I created.

Here is what my image of the Orion Nebula looks like at this stage:

Image Processing - Orion Nebula

I cropped the image in a little more and used Gradient Xterminator around the edges of the DSO to balance the background sky.  Again, the core is still blown out at this stage.  I will add 2 additional stacks of 15 and 30-second images of the bright core to reveal the full range of detail in the Orion Nebula.

Astronomy Tools Action Set

At this stage of my image processing workflow, I will use my first action from Noel Carboni’s action set.   The action is called Local Contrast Enhancement.

This action does a great job at sharpening details and increasing the contrast of the deep-sky object.  It is wise to create a new layer with this action applied, so you can toggle the effect on and off.  For my image, I am going to apply a layer with this action at 75% opacity.  I have also created a mask on this layer so that it does not affect the areas of space where I do not want to increase the contrast.

Directly after this action, I prefer to run Enhance DSO and Reduce Stars.  This action can takes up to a minute or more to complete, depending on your image and the computer you are using.  Again, a new layer using this action is recommended, as this action can dramatically change the look of your image.

Here is a before/after look at my image after running Local Contrast Enhancement and Enhance DSO and Reduce Stars:

Photoshop actions before - after

Before and After applying actions in Photoshop

To make a new adjustment layer with all previous actions and adjustments made, use the keyboard shortcut: CTRL + ALT + SHIFT + N + E.  This is a very helpful technique to use as your continue to add adjustment layers to your image.

Applying the “Tamed Core” Layer

At this stage, I will apply a pre-processed stack of shorter exposures to the image.  To capture these images I shot a series of 15-second and 30-second exposures with the goal of collecting detail in the brightest areas of the Orion Nebula.  A good indicator of this dynamic range in values is the ability to discern the individual stars in the Trapezium.

The short exposures were stacked in DeepSkyStacker using dark, bias and flat frames just as the primary image was.

Orion Nebula Core ExposureThese layers were processed in the exact same fashion as the primary image.  This means that similar adjustments were made to the levels, curves, and actions – but in an isolated area.

Blending the two images will be a lot easier if they have been pre-processed in the same manner.  Some may argue that combining the core should have taken place much earlier in the process.  However, this timing of this workflow works best for my personal taste.  With so many opinions about how to properly process a deep sky image, I prefer to lean towards the workflow that I enjoy most.  This way, I can enjoy the hobby for years to come.

Here’s where it gets fun

Select the image of the detailed core, and paste it onto your original image as a new layer.  Rather than using a traditional mask method, I like to use a feathered eraser brush at an opacity of 15%.  This allows me to subtly remove the unwanted data on the top layer (the core), one brush stroke at a time.




When I need to see the faint details of the edges of the core layer, I simply create a 100% white layer and place it as the layer below.  The amount of brightness of the core is a matter of taste.  This aspect of the image has varying points of view as to how an HDR Orion Nebula is “supposed to look”.

I personally think that the Orion Nebula should have a bright core!  With the right amount of blending it is possible to show the full range of detail and keep the core as the brightest area of the image.  Flattening core to a lower brightness than the outer nebulosity can give the nebula a plastic look.

Blending the core

Layering in the core can take a long time if you are particular about the overall look of your image.  I used several copies of both stacks of shorter exposures to gradually work the new core into my existing image.

Final Processing Steps

With the full dynamic range captured in the image (depending on who you ask), I can now go ahead and make my final image processing steps to further increase the color and detail of the image.

Color sampler toolAt this point, I like to double check the levels of color in the background sky.  Using the Color Sampler Tool in 2 areas of the background sky indicates that the image is rather well balanced at the moment.

Increase Vibrance and Saturation

To increase the saturation of the Nebula without bringing noise and unwanted color from the background sky, I’ll use the Select Color Range tool again.  This time, use the eyedropper to select the color from the nebula you wish to intensify.  I choose the mid-level pink areas of Orion.

You may also want to run some actions on your image such as Increase Star Color, and Make Stars Smaller.  As always, apply these actions to a new layer so that you can control the amount of the adjustment using the opacity slider.  I will often use both of these actions, in small amounts.

Adding a layer of H-Alpha

This is where the image really starts to “pop”.  I shot over 2 hours worth of data through a 12nm Astronomik clip filter with my Canon T3i.  I will combine this data with the RGB image we just processed using the HaRGB processing technique outlined in this tutorial:

Deep Sky Image Processing in HaRGB – Tutorial

Orion nebula in Ha

The Orion Nebula in Ha

The image above is 32 X 5-minute subs @ ISO 1600

If you are interested learning how to shoot H-Alpha with your DSLR camera, read my post on how a DSLR Ha Filter can improve your astrophotography.

Without explaining every detail in the HaRGB tutorial I linked above, the premise is basically to add the Ha as a luminosity layer at about 75% over your original color image.

Because the data in the core of the H-Alpha version of Orion was blown out, it is important to note that I removed this area of the Ha luminosity layer, so that I did not lose any detail in the final composite image.  By turning the Ha layer off and on, you can determine which areas of the nebula are being improved, and which areas are losing detail and/or color.  I prefer to create another layer mask using the Ha layer, leaving only the key improvement areas at the full 75% opacity.

Below is my final image of the Orion Nebula using the processing methods outlined above:

Orion Nebula - AstroBackyard

Final Image Details:

Hardware:

Mount: Sky-Watcher HEQ5 Pro Synscan
Telescope: Explore Scientific ED102 CF
Imaging Camera: Canon T3i (600D) Modified
Filters: Hutech IDAS LPS, Astronomik 12nm Ha
Flattener/Reducer: William Optics FF III
Guide Scope: Orion Mini 50mm, Starwave 50mm
Guide Camera: Meade DSI, Altair Astro GPCAM2 AR0130

Software:

Image Aquisition: BackyardEOS
Autoguiding: PHD2 Guiding
Registering/Stacking: DeepSkyStacker
Image Processing: Adobe Photoshop CC

Exposure Details:

RGB: 3 Hours, 8 Minutes (55 frames)
Ha: 2 Hours, 40 Minutes (32 frames)
Total Integrated Exposure: 5 Hours, 48 Minutes

I am always looking to improve my deep sky image processing techniques.  For a video presentation of these techniques in action, please visit the AstroBackyard YouTube Channel.  If you like to see more of my deep sky astrophotography images, please have a look at the Photo Gallery.

AstroBackyardThis winter was a memorable one for me.  By sharing my experiences in the backyard on this blog and on YouTube, I was able to connect with fellow backyard astronomers on a deeper level.  There may not have been many clear nights, but the ones that were felt extra special.  Until next time, clear skies!

Deep Sky Image Processing Help:

Settings for DeepSkyStacker

Video Tutorial: Deep Sky Image Processing in Photoshop

 

 

AstroBackyard on Facebook

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Forgotten Light Frames

|Nebulae|0 Comments

While digging though some old folders on Adobe Bridge, I stumbled across some unprocessed, 300 second light frames of the Flaming Star Nebula from November 2013!  When you are desperate to get out and image a new target, this is like hitting gold.  I was originally looking for my raw files of the Pacman Nebula, which I feel is in desperate need a new process. (Those stars look pretty rough)  I found a folder labelled “Flaming Star – 5 Min Lights”.  I never processed this image!  The Flaming Star Nebula is a colorful collection of glowing gas and dust lit up by the bright star AE Aurigae.  The tough part about this process will be the limited exposure time.  1 hour of data is really not ideal for a quality astrophotography image.  I find that out the hard way below:

IC 405 – The Flaming Star Nebula

IC 405 - Flaming Star Nebula

 

Photo Details

Photographed on: November 29, 2013

Telescope: Explore Scientific ED80 with WO Flat III 0.8x FR/FF
Mount: Skywatcher HEQ5 Pro Synscan
Guiding: Meade DSI Pro II and PHD Guiding
Guide Scope: Orion Mini 50mm
Camera: Canon EOS 450D (Stock)
ISO: 1600
Exposure: 1 hour (12 x 300s)
Processing Software: Deep Sky Stacker, Photoshop CC
Support Files: 15 darks

Guided with PHD Guiding
Stacked in Deep Sky Stacker
Processed in Adobe Photoshop CC

This image was acquired using Canon EOS Utilities, and not BackyardEOS as I use now.  This was photo was also shot before I modified my Canon 450D for astrophotography.

Now you might be thinking “how could you spend hours imaging a nebula and forget to process it?”  It’s simple – life is busy!  I likely had a busy week following the the imaging session, and began I new session before I even looked at the precious data collected on that cold November night.  I don’t see any dark frames to support the image.  This may have been another reason I held off.  I bet that I wanted to take 5 minute darks of the same temperature before stacking, but never got around to it.  This could be a problem.

 

But first, let’s get this cleared up

This is budget Astrophotography.  Most of my gear was purchased used from online forums and astronomy classifieds.  The total value of the equipment used to photograph this nebula was purchased for under $3,000.  It’s not top-of-the line gear by any stretch of the imagination.  My astrophotography image processing skills were self-taught.   I am no scientist, that’s for sure.  Just like you, I have a strong desire to capture beautiful images of the night sky.  I always appreciate constructive criticism, and enjoy helping others learn through my mistakes.

 

Stacking without Dark Frames?

First of all, I’ll have to use dark frames from a different night to stack with the Flaming Star light frames.  This means that it is very important to match the temperature of my light frames from that night of imaging.  I have done a poor job of creating a master dark library, so finding matching dark’s may be tough.  I usually try to record the temperature of my dark frames in the file folder, for this very situation.  There are external software applications available that can help create a dark frame library, such as Dark Library.  I remember using this years ago, but their website appears to be down right now.  I will use the 5 minute dark frames from my Pacman Nebula image taken earlier that month, labelled 4 degrees.  Another option is to just stack the light frames without any dark’s.  I’ll try both and compare the two.

 

Here is the version stacked with no dark frames:

Deep Sky Stacker with No Darks

Here is the version using dark frames from a previous night:

Deep Sky Stacker with Darks

 

As you can see, stacking with the dark frames produced a better result.  Even though the temperature of dark frames did not match perfectly, the dark frames removed some of the dead pixels and noise from the image.  Notice the red streak of dead pixels on the “no-darks” version.  All of these imperfections would become intensified after processing!  I performed a few basic edits to the examples above to have a better look at the differences. (Levels, Gradient Xterminator, and Curves)  Now that we have registered and stacked our 1 hour’s worth of data, let’s start stretching the data in Photoshop.

 

How to take proper Dark Frames for Deep Sky Stacker

The answer to this and more in the FAQ section

Processing the Image in Photoshop

If you have followed any of my astrophotography tutorials on my website, or video tutorials on YouTube, you already know the basics of my processing workflow.  This process has evolved over the years as I learn new tricks.  However, processing the Flaming Star Nebula was particularly tough because of the limited exposure time on the subject.  Add in the fact that this nebula is quite faint, with many bright stars surrounding it, and you’ve got an astrophotography challenge for even the most experienced astrophotographer.

 

Quick Astrophotography Tip

Try to frame your deep-sky object in an interesting way.  Include nearby star clusters, nebulae or galaxies.  For inspiration, search for your target on APOD, and see how the professionals have framed the object.  This may spark your creativity to photograph an existing target in a different way.

HLVG – Green Noise Remover

The entire image had a noticeable green cast over it, perhaps because of the extreme amount of noise, or the miss-matched dark frames.  I ran Deep Sky Colors HLVG on medium, which helped a lot.  HLVG was created by Rogelio Bernal Andreo of RBA Premium Astrophotography.  It is a chromatic noise reduction tool that attempts to remove green noise and the green casts this noise may cause in your astrophotography image.  It is based on PixInsight’s SCNR Average Neutral algorithm.  If you don’t already have this useful filter for Photoshop, I highly recommend it, it’s free!  You can download the plugin here:

Hasta La Vista, Green!

HLVG Filter for astrophotography

Results and Thoughts

I must admit, this post became a bit of a nightmare.  I began to document my processing steps one by one, taking screenshots of progress along the way.  I wanted to provide a detailed tutorial of how I turned this forgotten data into a masterpiece, despite having no associated dark frames, and only an hour’s worth of exposure time.  As I experimented using different methods of noise reduction, and various orders of operations, I became very discouraged with my final image results.  I spent hours taking different roads with all of my trusted astrophotography tools at my disposal, and the results continued to be unimpressive.  By adjusting the curves enough to show any substantial detail on the nebula, I introduced a frightening amount of noise into the background space.  No amount of noise reduction could remove it, without turning the entire image into a blurry mess.

I just couldn’t bring myself to post a tutorial with the end result turning out like it did.  So I scrapped the idea, and settled for a forgettable image of the Flaming Star Nebula.  Surely this gorgeous nebula that spans 5 light years across deserves more than that.

Astrophotography Processing Tutorial

My unused processing tutorial screenshots

At the end of the day:

No amount of processing can make up for lack of exposure time!

I guess you could say I was doomed from the start.  I am not going to spend any more time on this image until I am able to capture at least another 2 hours of data on it.  I hope you can learn from my experiences in astrophotography, in both victories and failures.  But I guess that’s why you’re here 🙂  Please follow AstroBackyard on Facebook for the latest updates.

 

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Selective Processing for More Detail

Staying Inside – Image Processing

The unseasonably cold weather and precipitation we have experienced here in Southern Ontario have given me the perfect opportunity to go through my old astrophotography images and reprocess the data.  I have been advancing my image-processing skills by studying current astronomy images taken by the pros.  Being a creative professional myself, I have always understood and appreciated the power of inspiration. I am always interested in new image-processing techniques, photoshop tutorials and new software that can enhance my work.  Through selective processing, I have been able to squeeze out the most amount of detail from my astro images.

Western Veil Nebula

The Western Veil Nebula – I reduced the stars to show more contrast in the nebula

My latest take on The Swan Nebula is my favourite version yet. Through selective processing, I was able to tame the background stars, while intensifying the gorgeous pinks and reds in the nebula itself.  I also recently reprocessed my wide-field image of the Western Veil Nebula, with a focus on reducing star size, and overall image contrast and color. The “witch’s broom nebula” is a tough process, especially if you have to deal with a severe gradient behind all of those stars. After assessing the gradient in photoshop, (mostly due to heavy light-pollution) I can easily even out the sky background using the Gradient Xterminator plugin.

I am quite pleased at my latest results of the Eagle Nebula as well.  I went through my astrophotography folders from the past 4 years (like I said, it’s been cloudy!)  and found a set of almost 2 hours of frames on M16 that I had not previously used!  I combined all of the data together from May 2012, and May 2013 in Deep Sky Stacker to create an image with over 3 hours of exposure time.  I decided to keep the extremely wide-field view captured by my 80mm telescope, rather than cropping the photo around the nebula. This image really benefitted from the selective processing technique. By reducing the stars on a separate layer, I was able to keep all of the detail found in the nebula.

Eagle Nebula - 80mm Telescope

Wide field image of the Eagle Nebula with my 80mm telescope

Image Processing Techniques

One of the processing techniques I have been implementing into my photos is to process different elements of the image separately. By this, I mean to process the background, the stars and the nebulosity on their own.  I am able to do this by selecting each element of the image and stretching the data without affecting the other areas. For example, I can boost the vibrance and saturation of the nebula or galaxy without adding additional noise to the background of space and stars.




As I have stated many times, I prefer to tame the stars in the image to be as small as possible.  Normally, I would run the “make stars smaller” action to the entire image in Photoshop. This actually starts to diminish the precious detail in your deep-sky object that you worked so hard to capture! Many other actions that are intended to correct issues with the background space and stars can take away from your subject as well.

You can also manually Remove the Stars Completely from your image using photoshop.

Swan Nebula - 8 Inch telescope

My latest version of the Swan Nebula

Selective Processing

There are several ways to accomplish the selective processing technique to your astronomy photos.  You can create multiple adjustment layers of your image in Photoshop, and apply the various actions to each element of the image on a separate layer.  Once you have applied your desired settings applied to each layer, you can use layer masks to combine all aspects of the photograph into one.  This means you will likely have layers for:

  • The Background Space – With a balanced black-point set

  • The Background Stars – Small, sharp and with lots of accurate colour

  • The Brighter Stars – Soft, or with Diffraction Spikes and Color

  • The Deep-Sky Object – Full of luminance, color and detail

  • The Core or Brightest Area of the DSO – reduced to show detail, not blown out

 

Selective Processing - Astrophotography

Processing the nebulosity separately from the background stars in Photoshop

You can also process the selected elements of your images as separate documents.  Sometime I prefer to do this to really focus on achieving the best possible result for my focus area, without the temptation to poke around at another feature.  Once you have processed each version of the image with your focus area maximized, you can then combine the images using layer masks.  The blending and layer masking is definitely the most delicate stage of the process.  You can really make a mess of an image by failing to inspect all areas of your image before flattening.

I find it helpful to use a reference image of your deep-sky target. This is the best way to make sure you have not overstretched your image data, and that your colors and details are an accurate portrayal of that particular deep sky wonder. I often look for inspiration on APOD!  To stay connected with me and my latest astrophotography images, please follow my Facebook Page.  I hope you are all excited about the wonderful deep-sky targets that will be gracing our night sky the coming months, I sure am!

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