Astrophotography Processing Theory

Astrophotography Processing Theory

Jared Holloway – SNC Astrophotography

Note, this guide and any videos associated with it are provided for free. I will attempt to continue to update as time goes one. If you wish to donate to support this type of content and my content in general, please feel free to do so.

Introduction

There are many great books, videos and blogs out there that are going to cover everything I cover in this write up. They range from the extremely technical to the “keep it simple stupid.” It is my belief that knowing as much as possible on the technical side of things is beneficial, but it can also be very daunting for someone beginning in this wonderfully invigorating and frustrating hobby. This write up is not an “end all end all” type of thing, nor is it the “right” way to consider or do things, but rather my personal way of going about imaging photos of the night sky. There will be some who tell you if you don’t do it this way or that way, or that you are doing it wrong, I do not believe that. Yes, what we do is a science, but it is also a hobby, and hobbies need to be enjoyed at your own pace, not necessarily by the standards of others. Also, sometimes you need to figure things out on your own, that’s how I have done the majority of my growth. Other times you simply need to ask a friend, which is how I have done most of the rest. Often, you just need to be pushed (gently) in a certain direction to give you the concept, and then find your own way. My goal with this write up is to give you the basics to formulate your own methods within your own abilities, and hopefully be a helpful guide and introduction into creating images that you are proud of. Saying that, if you have already captured, stacked and processed an image that shows nebulosity or a galaxy, then you have already created something grand and something to be proud of. Hopefully this guide will help you improve upon what you have already done, and maybe help you figure out your own method of doing things. One major note, this guide is designed around using a one shot color camera with a setup suitable for longer tracked and guided exposures. Saying that, the information will remain applicable to other forms of imaging the night sky.

Part 1 – Before Imaging
Part 2 – Connecting your Equipment
Part 3 – Image Capture
Part 4 – Processing

Part 1 – Before Imaging – top

Processing astrophotography images, like processing normal photography images, does not begin in Photoshop or some other photo editing software, but rather, begins before you take your first image of the night. It begins with your location, weather, and equipment. All three of these things are independent of each other and codependent at the same time. Each will affect decisions such as exposure time and filters used, as well as when and how long you focus on one target at a time. It will also determine the settings you use on your camera and other equipment used in capturing your images. A good example of this is when shooting during a new moon phase versus a bright moon phase, or even a full moon. Normally you wouldn’t dream of shooting a nebula during a full moon, but it is possible, with certain filters and by monitoring your exposure times, to get a decent final product. Before we get there, however, lets go over some basics of location, equipment and weather.

1.a – Location

Location is important for several reasons. First, it determines what is visible in the night sky along with its location in the night sky. At certain latitudes, some objects are higher in the sky then others, some will never be visible at all. Some will be too low to capture effectively depending on your light pollution levels. A personal example of this is the Lagoon Nebula, which from my backyard never gets too high on the southern horizon, and is always in heavy light pollution. Not impossible for me to shoot, but difficult nonetheless. To see how the night sky is for your particular location, I recommend some sort of Planetary software. One of them most popular of these is Stellarium, which is free to download on your computer and enables you to enter your location, and even your equipment, which will show you not only how the night sky looks on any particular night, but also shows you your field of view based on your telescope or lens and your camera.

Secondly, as mentioned before, your location also determines your level of light pollution. This is measured in a general sense by what is called Bortle level. Bortle 1 being the least amount of light pollution and Bortle 9 being the most. Most large cities, for example, will be Bortle 8-9 respectively, while the country side, at least in the United States, will typically be Bortle 4 or lower. Do not fear shooting from areas high in light pollution, I typically shoot from Bortle 8 with what I feel are pretty good results. Along with the general light pollution, local lights can and will affect your images. Even blocking a direct light from your neighbor can help improve the quality of your images, even though you still may suffer from some ambient light gradients from them.

1.b – Weather

After location comes weather, which is determined by your location (everything is related!). The obvious issue here is rain and clouds. You cannot image effectively through clouds – though some, such as Trevor Jones of Astrobackyard.com have created some amazing images through some high, semitransparent ones. Not to get into too much detail, there are low, medium and high clouds, as well as even higher ones that can affect your seeing and transparency. Weather apps are a must to really plan your sessions, and these include websites such as ClearOutside, Clear Dark Sky and others.

Seasons and moon phase also have an effect on your ability to image. New moon, or there abouts, gives you a way darker sky then when it is a full moon or close to it. Winter brings longer and colder nights versus the summer, giving you a longer time to shoot and also helping the noise levels from your camera if you are shooting with a camera that cannot maintain a constant temperature on its sensor. Heat and humidity also affect the atmosphere which will have some effect on your images.

I want to note: clouds, thermals and other atmospheric phenomenon that are not visible with the naked eye can make there way into your image, especially with super long exposures. These can and will have an effect on the processing of your final image if you include those frames affected by such things. Many can be fixed easily in the processing of the image, in fact, often you won’t be able to tell. However, sometimes it is best to leave some sub frames out of your final stack or be faced with issues that you cannot easily correct while maintaining the integrity of the final product.

1.c – Equipment

Though mentioned last here, equipment is going to be your biggest factor in determining what you can image, how long you can image and what your final image is going to look like. From your mount / tripod, to your camera, even the way you capture will have an effect on your final image. If you are shooting with only a camera, a lens and a tripod, it is going to be more difficult to shoot some targets versus shooting with a tracking mount, a guide scope & camera, a lens / telescope, and a camera.

Take the following examples:

Setup 1 – Tripod, DSLR and a 50mm lens

Since we are not tracking, we are limited severely in how long we can expose per image to maintain sharp stars. The basic “rule” is the 500 Rule which maintains that to have sharp stars use an exposure no longer then 500 divided by your focal length. In this case we would use an exposure length of 10s at most, however, depending on your camera and the aperture of the lens, you may need to decrease it further. If we switched to a 180mm lens, the max exposure we could use would be 2.7s, so you can see how this setup is limited in what you will be able to capture.

Setup 2 – Tracking mount, 250mm lens / telescope, DSLR using a guide scope and guide camera.

Here we are tracking AND guiding, so are limiting factors are mainly our location (light pollution) and weather. With a “full setup,” we no longer need to worry about the 500 Rule, but we do need to be mindful of our light pollution and weather. If we are in Bortle 8 skies, too long of an exposure can wash out the target we are trying to capture, likewise, if the moon is too bright, it can do the same thing. With weather, if we have intermittent clouds, we may want to lower our exposure time as too not waste collection time if we lose a long exposure or two. Also, if it is really hot and we are using a non-cooled camera, we are going to want to do “shorter” exposures to not overwhelm our sub frames with sensor noise caused by heat. Note, this can also happen in cold weather as well, but much more prevalent in hot weather. Again, everything is connected!

We could talk about equipment all day long and still not be done, so, for the purpose of moving forward, I’m going to make some assumptions: You have a tracking mount and you use a guide scope & camera. If don’t have those, parts of this guide will be relevant still, but in general, for anything other than Milky Way or other very wide filed astrophotography, these two things are a “must.” I will point out that guiding is not necessary unless you are wanting really long exposures, and it is possible to get really good images and detail without guiding, especially from really dark skies, however it does make something difficult which we will discuss coming up. In the next section, I will go over setting up your equipment, and I will also cover a non-guided set up.

Part 2 – Connecting Your Equipment – top

Taking images with a telescope and an equatorial mount can be fun but also challenging. First, you must choose whether or not you want to image using guiding or not, and if you choose guiding, how are you going to connect your telescope and guide setup to your pc or other device. In the following I am going to explain several ways to setup to image based on the equipment that I have and use. Note, not all equipment is the same, but they are typically similar.

2.a – Unguided

When I first setup my Sky Watcher EQR-6 Pro mount with my Meade Series 6000 80mm Triplet to image I did so without a guide camera or guide scope. This is the most basic way to setup your equipment but it comes with the heaviest caveat – your exposure time will be severely limited and without heavy manual input you can (and will for longer exposures) suffer from what is known as “Walking Noise.” Our tracking accuracy setting up this way is going to be solely based on how well you can polar align the mount and you can easily get up to 2+ minutes of accurate tracking with a widefield setup.

Notes:

This setup is suitable for widefield astrophotography with no filter or very “light” filters such as a normal light pollution filter or something like the Optolong L-Pro filter.

The longer your exposure is the more prone to “walking noise” being in your final stack will be. To combat this, limit your exposures to 60s or less and if possible, manually dither

• Dithering is the process of slightly moving the mount between exposures to reduce the amount of noise from your camera’s sensor being in the same location in every image. This only needs to be a few pixels, so very minor adjustments

How the setup looks:

Equipment: Mount, telescope, and camera

Mount will either be controlled by the hand controller or connected directly to the computer. Camera will be connected to the computer. I advise having a finders scope on the telescope / lens.

Setup the mount and scope. Connect the hand controller to the mount, connect the camera to the telescope and to the computer. Polar align using the methods on your mount or by using some sort of polar scope. Align your mount using its alignment features – Example: The EQR6-Pro has a 3-Star alignment process built in. Use your mounts built in tracking to track your target once centered, and then begin to take images.

2.b – Guiding – ST-4 Connection

Your setup is the same as above EXCEPT instead of a finders scope you will connect a guide scope with a camera attached for guiding. The camera will then connect to the computer AND to the mount. The connection to the computer is done by USB and the connection to the mount is done by the ST-4 cable – looks like a phone cable. This will allow your guide camera to talk to mount as well as communication to and from the computer to occur. You will then need to use guiding software, such as PDH2, to run the guiding for you and your mount.

Guiding allows you two things: Much longer exposures and the ability to dither. If you use PDH2, the settings allow you to determine the dither amount. Your capture software will most likely hsve a dither setting as well, BOTH should be set. An example setting, which is the one I use for focal lengths between 250mm and 500mm is a setting of 4 in PDH2 and a setting of 5 in my capture software (I use both Astro Photography Tool – APT and N.I.N.A.).

Notes:

This way of connection only needs to be done if you are still using the hand controller to control the mount and not have it directly connected to your computer.

Equipment: Mount, Telescope, Guide Scope, Camera, and Guide Camera

Setup as before, again, the difference is you will be using the guide scope and guide camera instead of the finderscope. The guide camera needs to be connected to the computer – just like your imaging camera – and the mount with the ST-4 line. Guiding and dithering needs to be setup with your guiding software and your capture software.

2.c – Full Computer Control

This will require you to connect everything to your computer: The mount, guide scope and obviously your computer. There are many benefits to setting up this way: Full control of guiding, full control of your mount and platesolving for framing / centering your targets, all from your computer.

First, things your computer will need:

ASCOM software

Mount drivers

Capture software with optional platesolving

For ASCOM, download the latest from their website. For mount drivers, there are different ways to go here, I personally use EQMOD and the driver suite for my setup is the EQASCOM. For capture software I use both Astro Photography Tool and N.I.N.A. and the both have the ability to add in platesolving. For platesolving look at your capture software’s guide and it will specify what, where and how to get and install the additional software you will need for this feature. Guiding software stays the same and all your connections to it will as well, the main difference is you will no longer be guiding by ST-4 but rather by “pulse,” which is far more accurate and reliable.

Download the software / drivers to your computer. Settings for mount speed and connection (baud rate) speed will need to be set. I recommend finding and watching a video specifically for your mount and what settings to use. They may recommend something other than EQMOD, that is fine and totally acceptable! Note, you may have to power on and connect the mount, etc. to be able to manipulate these settings. I recommend this anyways to test and trouble shoot any issues that may occur from setting up for the first time.

Set up the mount and scope as normal. Your mount, camera and guide camera need to be connected to your computer by USB. I recommend a USB hub or something like the Pegasus Powerbox. I use the Pegasus Powerbox Advanced. Note: You will no longer need the ST-4 cable – with your mount and guide camera attached to the computer, they can both talk to each other.

If you use something like the Pegasus Powerbox, you will be able to connect everything to that and then run one cable to your computer. You will also be able to run power to your mount, camera (if needed) and optional accessories such as dew heaters while only running one power cable to your setup (minus the power for your laptop if you use one). This is very beneficial for cable management as well as being able to monitor your setups power output on your computer.

Assuming you have some sort of hub, this is how your setup will look (I will base this off using the Pegasus Powerbox Advanced)

Setup mount and telescope. Telescope will have the guide scope, camera, guide camera and hub attached. Cables will run from the camera, mount and guide camera into the hub – note: the camera cable will most likely need to be USB3 and plugged into a USB3 port on the hub. Power for your mount will run from the hub to your mount. Power for your camera, if applicable, will run from your hub to your camera. Any dew heaters will connect to your hub. One power cable – a 12v DC supply – will run to your hub. One cable, USB3, will run from your hub to your computer. From here, everything should be connected and beyond troubleshooting settings, you should be all set.

Some common issues:

Port for your connections – particularly the mount

Baud rate for the Port – typically effects the mount most

Mount speed – effects guiding mostly, setup in your mount driver software (the Toolbox for EQMOD)

Not having the imaging camera attached by USB3 – usually just put in the wrong USB slot on hub or computer

Finally, I recommend watching videos on the following: Connecting your particular mount to a computer, setting up guiding using PDH2 and setting up the particular imaging software you plan to use. As mentioned before, some equipment will differ and there are many software applications and computers to use. Though similar in how they function, setting them up properly and efficiently can and will differ. Though I don’t cover it here, using something like a ZWO ASIair for both your hub and your computer can make life easier for you once setup and figured out. Find what works best for you and move forward from there.

Part 3 – Image Capture – top

3a. – A Reference Point

Once you have everything up and running, how you capture your images is the single most important thing you can control leading into how your final image will look. The issue here for a lot of beginners is that there are a lot of different theories that if looked at together can cause a lot of confusion. In addition to this, there are a lot of different factors that will determine what your optimal exposure time should be. This includes your levels of light pollution, the type of filter you may be using, and even the target you are trying to capture. Other objects, such as the moon, can also effect how long you can effectively expose. Lastly, the type of camera and the teelscope you are using will also determine how long you should expose. To give you an idea, below I list some times and settings I use between two of my different cameras.

Canon EOS Ra

For Broadband targets (galaxies, star clusters) I use 400 ISO and expose between 30s and 120s with telescopes that range from f4.5 to f6.0. For Narrowband, I limit myself to 300s exposures at 800 ISO. I do this mainly for the noise created by sensor heat.

ZWO asi2600mc-Pro

For broadband I use a gain of 0 and expose 30s to 120s with a telescope at f6. For Narrowband, I shoot at a gain of 100 with 600s exposures at the most.

As a place to begin testing for yourself, this is not a bad place to start, however, keep in mind that I am shooting in Bortle 8 skies and for broadband targets I am using a filter like the Optolong L-Pro, and for narrowband I am using filters such as the Optolong L-eXtreme and the Antlia ALP-T 5nm Dual Narrowband. Depending on the darkness of your skies, the filters you are using and the f-ratio of your telescope (or aperture of your lens), these exposure times may need to be adjusted. You can also adjust your gain / ISO of your camera, and for that I would research the specifications on your particular camera as well as try to find images shot with that camera in similar conditions as yours.

Telling you how I set mine up though doesn’t really let you know anything of why my exposure times are the length of what they are – other then my max on my Canon Ra. The answer to why they are the length they are is because of the histogram for each sub frame I take. If your images are too bright, they will be difficult to process, no matter how many you take. Ultimately, it is better for your exposure times to be shorter rather then longer, in my opinion, especially if you are under light polluted skies. The rule I follow is that I want my histogram to be 25% or lower, as shown in the following image:

This is a screenshot of the Astro Photography Tool interface with the histogram showing on the lower middle left. This shot is a single 300s exposure using the Canon EOS Ra connected to a Radian Raptor 61mm Triplet Refactor. It was taken using an Optolong L-eXtreme filter. This image was taken rather early in my imaging run, so close to the beginning of astronomical dark. With the Green channel being right at the 25% mark, I know as the night goes on the images will get darker and I will remain below that 25% line of the histogram. Staying at the line or below ensures that I am less likely to blow or over saturate out any stars. It also helps tame any light gradient – which the filter also helps with – that may be introduced by light pollution.

This is something you will need to experiment with considering your equipment and light pollution will be different than mine. The other major factor that will determine your exposure length is how good your guiding is. If you guiding will not support 300s exposures, even though it puts your histogram where you want it, then you are going to have to expose for less time. This is FINE by the way! Less exposure time means more exposures, but in the end, you can still end up with around the same total exposure on target. An example is: 10 x 300s exposures is the same as 5 x 600s exposures. You may be even better off with the 10 shorter exposures versus the 5 longer ones. Some people find that their sweet spot is longer exposures, and some shorter. The key in exposure time is to not feel like you have to push the length, keeping in mind that it is the total time that is most important in general.

A quick note on exposures and dithering. Personally, for any exposure 3 minutes or longer, I dither every frame. 1.5 min to 2.5 min I dither every other frame, and less then 90s I dither every three frames. I’m pretty aggressive in my dithering so you may be able to get away with less, but for me, if it isn’t broke, I’m not going to try and fix it!

To finish off with image capture, in my opinion it is better to have more shorter exposures then try to get fewer at longer exposures if you are concerned about the individual frames being to bright or over saturated. This is even more important when shooting broadband targets!

3b – Filters

I want to touch briefly on filters, since they can be one of the biggest factors in how your images come out as well as how long you can expose. Between color cameras and mono cameras there are a myriad of filter types. Filters are mainly broken down between Broadband and Narrowband. In the broadest sense of the terms, broadband encompasses normal RGB type filters and emissions while narrowband deals with filters and emissions in the Hydrogen Alpha (& Beta), Oxygen III and Suphur II spectrum of light. In simplistic terms, broadband filters let in a lot of light while blocking some while narrowband filters let in some light while blocking a lot. Most light pollution filters, ironically, fall under broadband. This is because they are designed to only block or mitigate the light from light pollution while allowing all other wavelengths in. Note, broadband filters allow in narrowband light as well, so using broadband gives you a fuller color ranger overall. The issue here is that they still let in a lot of light and, unless you are in really dark skies, you will still get some effects from light pollution. Narrowband filters also act as light pollution filters – I like to call them the steroid users of light pollution blockers, and they only allow in as much light within the spectrum that they are rated for. This rating is typically measured in nanometers (nm) with the higher the number the more light let in. An example of this is comparing the Optolong L-eXtreme dual narrowband filter with the Antlia ALP-T 5nm Dual Narrowband filter. the Optolong is 7nm and lets more light in then the Antlia. Though very similar, it is noticeable in processing with the difference in contrast between the two (the 5nm giving better contrast) as well as things like halos around stars may be noticeable at 5nm versus 7nm. The irony here is that sometimes suppression of light is essential to gathering enough of the right light to produce a good image.

A word of caution – it always feels like when using narrowband filters that you must expose for a very long time. In truth, these filters do give you the ability to expose longer than broadband in typical situations, you do not need to feel like you must expose for a really long time. As mentioned before, even though you can expose for 600s at a time doesn’t mean you have to. I have successfully shot as low as 180s with the Optolong L-eXtreme, and I am confident I could go lower if the situation dictated. My recommendation with using different filters is to take 30 min of total exposure time with different exposures, stack, stretch and compare.

3c – Calibration Frames

The most boring part for any astrophotography session is taking calibration frames. Again, many theories out there, many ideas, but for me I stick with what works for me and that is utilizing all 4 main common types of calibration frames. They are: Dark frames, Flat frames, Bias Frames, and Dark Flat frames.

Dark frames are just that, frames taken with the lens or telescope cap on at the same settings and exposure times as your sub or light frames. So, in the previous example shown, I would take dark frames at 300s and ISO 800. If you are using a cooled camera, you will also want to take them at the same temperature that you set your cooling to. If you are using a cooled camera, you have the advantage of building a library of dark frames that you take once and can use for around 6 months or so. If you are using an uncooled camera, I would take dark frames immediately after you imaging session is done. For my dark frame library with my cooled camera, I take up to 50 dark frames with a minimum of 30. On my Canon I try to take at least 20 dark frames.

Flat frames are frames taken with a constant, even light source. They are designed to help with vignetting and any dust you may have in your image train. To take these you can use the morning sky at dawn with a t-shirt over your lens / telescope front to diffuse the light, or use a light panel or light box. Again, depending on your situation, watching a video on how this is done can be really helpful. A note, these should be done every imaging session without changing anything in your image train or your focus point. If you use multiple filters in one night, you need to take flat frames for each filter. On a DSLR, you can set your camera to A or Auto mode, this will help determine the proper exposure time. For dedicated astronomy cameras most capture software has a flats wizard that will help determine the proper exposure time. I like to take between 25 – 30 flats frames.

Bias Frames are taken just like dark frames except they are the fastest except they are taken at the fastest exposure time that the camera can do. For a DSLR this can be something like 1/8000s, and for a dedicated camera you can probably set it at 0.01s. I like to take 100 of these.

Dark Flats are just like dark frames as well, but at the same exposure time as your flats. I take the same number as my flats.

One quick note on flat frames – for narrowband imaging I find that a flat of around 5-6s is best. For broadband imaging I haven’t had issues with a lot quicker. If you are not using a color camera but rather using a mono camera, this may change between filters.

Part 4 – Processing – top

Now for the meat and potatoes, so to speak, of this write up. So now that you have taken your images and you calibration frames, you want to turn them into something beautiful. First, you must stack the images and calibrate them into one final image that you can then take and process. To stack, you can use software like DeepSkyStacker, Siril, Sequator, Pixinsight and others. It is even possible to manually stack them, but I’m going to assume that you don’t want to do that. For most of my projects I use DeepSkyStacker to calibrate and stack my images. For some of my more complicated projects, I use AstroPixelProcessor or Pixinsight. For the most part, you just have to click on the type of frame (light, dark, etc) then select the proper files under each heading and then hit register or stack. There are settings you can tweak, but going with the recommended settings is where I’d advise you to begin. If something is off in the end, you can always change a setting, restack and see how it goes. For the most part though, this part is pretty straight forward. Once you have a stacked image, it is then time to do what I enjoy most, and that is turning that dark image into something beautiful!

4a – Initial Processing and Theory Outline

When you first open you stacked image it will be in what is called linear form. This essentially means the image is not stretched. It will also be in a 32bit format, either Tif or Fits depending on the stacking software you used and what settings you chose. Depending on what software you use to process, you may need to convert the file to 16bit, and if you use Photoshop, the file will need to be in Tif format. For the purpose of this guide moving forward, I am going to go through it as if I am using Photoshop. However, the same principles, more or less, apply to what ever software you may end up using. I may also let you know if something can be done a certain way in another program as a side note.

First, the initial outline I follow is this:

1. Slight crop to remove any stacking artifacts at edges
2. Convert to 16bit Tif using exposure and gamma in PS at default settings
3. Initial Levels
4. Color Balance
5. Initial Stretch
6. Gradient Removal (if necessary)
7. Sharpen
8. Stretch
9. Noise (optional)
10. Color adjustment / saturation
11. Final Background Leveling
12. Final Contrast Adjustment
13. Final Noise (if necessary)

Looks pretty straightforward, and it is, but the rabbit hole only gets deeper the further you allow yourself to go! I mentioned that the above is how I think about it while using Photoshop, and for the most part the same applies when I use other software to process my images as well. What I didn’t mention is that even while using Photoshop, I have different plugins and action sets that I use. These are as follows:

• Astronomy Tools Actions
• StarXterminator
• NoiseXTerminator
• GradientXterminator
• Starnett++
• Topaz AI (rarely)

You don’t have to use plugins or preset actions, but they do help a lot. One little side note, if you are looking for something free to use, you can download a program called Siril. This program is a free, but powerful tool that will get your stacked images to the point of being presentable. However, it is no substitute for using software such as Photoshop or something like Pixinsight.

Next, lets look at processing some data

Image Processing

Stack in Deep Sky Stacker:

• Light Frames
• Calibration Frames:
  • Dark Frames
  • Bias Frames
  • Flat Frames
  • Dark Flat Frames

Basic Outline

• Convert image to 16bit / Crop
• Initial Levels
• Color Balance
• Initial Stretch – light
• Gradient (optional)
• Remove Stars / Create Star Mask (Optional)
  • Adjust color and size of stars
• Levels
• Raw Camera Filter
  • If stars are removed, clarity and texture become options to use
• Noise
  • Should only be done lightly
• Levels / Stretch
  • If you use Astrophotography Tools Actions, Enhance DSO
• Contrast adjustments
• Noise (if needed)
  • Again, very light
• Final Crop (optional)

Process in Photoshop – Note, all new layers created by duplicating the previous layer

• Open Image
• Slight crop and change image to 16bit
  • Use Exposure / Gamma
• Initial Levels (New Layer)
  • Bring shadows to edge of peak, bring highlights in to line
  • Create Threshold Layer –
    • Layer -> Adjustment Layer => Threshold
      • Slider to highlights, use Eyedropper Tool
        • Eyedropper – Select Color Sample Tool with 5 by 5 Average
          • Select two to three spots void of stars (and nebulosity as possible)
          • Select center of bright star
    • Delete Threshold Layer
• Color Balance (New Layer)
  • Image -> Adjustments – > Levels
    • Adjust each level individually to reach balance based on info from Color Sample Tool
      • R G & B should be low (15 – 10) on dark spot and on star try to reach 248 across the board. Lower then 10 is fine, just do not clip the histogram peak
• Stretch (New Layer)
  • Stretch by Levels (RGB at once) or Curves (normal or S-Curve)
    • This is personal preference and I usually try both ways
  • Do not do a full stretch – goal is to get nebulosity visible for star removal
• Gradient (New Layer) – Optional
  • I use GradientXTerminator – Detail = Coarse, Aggressiveness = Low and experiment with checking the Balance background color box
• Starless (New Layer)
  • I use StarXTerminator, you can use Starnet+
    • Starnet+ you will need to flatten image and save as a 16bit Tiff file and run it separate. Undo flatten and open file to add starless image as a new layer on top of existing layers
• Stars Only (New Layer)
  • This layer should be off the gradient layer and sit just below the starless layer
  • Image -> Apply Image
    • Set Layer as “Starless,” set Blending to “Subtract,” set Offset to 2
• Stars Only – Working (New Layer)
  • Deselect “Starless” Layer
  • Duplicate Stars Only Layer, keep below Starless
  • Fix stars
    • Here you have several options – you should just have the stars visible.
    • I use Camera Raw Filter (Filter -> Camera Raw Filter) to lower the saturation in magenta to -100, red to -60 and orange to -40. This is because I use the Optolong L-eNhanced Duo-Band Filter. Other filters or not using a filter may need different options. Experiment!
    • I then use “Make Stars Smaller” from Astronomy Tools Actions, and I typically run it twice
• Move “Stars Only – Working” Layer above “Starless” Layer
  • Set Blending to “Linear Dodge (Add)
  • Reselect “Starless” Layer
    • You should now have a “combined” image again
• Clarity (New Layer) –
  • Should be below “Stars Only – Working” Layer
  • This is where you will begin processing the image to how you like, creating new layers as you go. Remember to make sure your new layers remain below the “Stars Only – Working: Layer
  • Camera RAW Filter
    • Can adjust Texture, Clarity, Dehaze and Contrast
• Levels (New Layer)
  • Adjust Levels to bring highlights forward to the line in the histogram. Do not exceed 200 in the highlight number box
• Raw Camera Filter (New Layer
  • Adjust exposure, contrast, highlights, shadows, whites and blacks. Can adjust vibrance and saturation as well
• Noise (New Layer)
  • Here I use NoiseXterminator at 60%, detail 10
• Noise 2 (New Layer)
  • Here I use Deep Sky Noise Reduction in Astrophotography Tools Actions
• DSO (New Layer)
  • Here I use Enhance DSO Reduce Stars in Astrophotography Tools Actions. Obviously, if stars are removed it will do nothing to the star layer. You can also use levels to boost from the highlights and adjust from the black point
• Boost & Boost Copy (New Layers)
  • Deselect the star layer and create a layer mask using Select -> Color Range -> Highlights. I use 89 Fuzziness and 88 Range. Invert and hit “add mask” in layer panel. On the Mask, move curves to expand the black areas as desired. Use Gaussian Blur at 2.0. Go back to the regular image and move curves down slightly until the desired adjustment is made
  • Merge Boost Copy down into Boost
• Raw Color (New Layer)
  • Adjust color and saturation as desired in Camera Raw Filter)
• Clarity 2 (New Layer)
  • Adjust texture, etc for the final time if desired
• Contrast & Contrast Copy (New Layers)
  • Here I use Local Contrast Enhancement from Astrophotography Tools Action and lower the opacity to 50% or lower in the layers panel. You can use Photoshop’s contrast if you want
  • Merge Contrast Copy down into the Contrast layer
• Noise (New Layer)
  • Final minor noise reduction to reduce noise created by final level boost or stretching
• Final Crop – Optional
• Save a working 16bit TIF copy, save a JPG copy

Notes:

• Keep an eye on the histogram and make sure not to clip into the blacks
  • Clipping into highlights is not as big of a deal but try to avoid it
  • If you do clip into the blacks, see if you can fix it with your current processing, if not, go back to a layer that is not clipped
• Do not over do sharpening or noise reduction – it will create artifacts. Depending on your setup and how the images were taken, not all noise will be able to be removed, nor can all details be sharpened effectively
• Do not over use vibrance and saturation, you will lose detail, especially in nebulosity