What is this about?

This post documents how I do deep-sky astrophotography with only “standard” photography equipment. Hardware, software, and everything in between. It kind of works, but depends on what you want to photograph (and your expectations). I do recommend getting at least a star tracker. I do not have one, but I hear they are a must. In this post I will skip some details and information, as I expect you to do some research yourself. Or watch some videos. This is more of a reference, or an “is this even worth it?” thing with a bit of basic stuff thrown it.

Disclaimer: I do not get any money for mentioning products. They are just what I use.

What is deep-sky astrophotography?

Deep sky objects (DSOs) are things deep in the sky. This includes: Nebulae, star clusters, galaxies, so basically anything outside our solar system. Sounds great, right?

Well… except that most of the DSOs are so faint that you cannot see them with the naked eye. Additionally, our sky (or atmosphere really) is in the way, absorbing or scattering light from DSOs. And people do not tend to like darkness, so they put lamps everywhere that outshine distant galaxies.

Despite these odds, deep-sky astrophotography tries to capture every precious photon emitted thousands to millions of years ago in a galaxy far, far away. Or in our galaxy, it is still far away.

How can you even do deep-sky astrophotography?

Like with every good hobby, by putting every single cent into the hobby. You have other hobbies? Not anymore. Real pros put complete observatories for tens of thousands of € (or $ (or whatever)) in their garden. They buy their house specifically so they can put an observatory in their garden. Or they built large metal structures and shoot them into space and call them space telescopes, so the pesky atmosphere is not in the way.

Ok that might be a bit extreme, but a good rig is not really cheaper than buying good lenses. Multiple.

You can of course settle for one of these smart-self-calibrating-goto-all-in-one-telescopes, that are probably cheaper and result in a lot less hassle, but where is the fun in that?

Or, if you are poor like everyone, you can just use whatever expensive camera and lens is lying in the drawer next to you from your last FOMO buy. This is what I choose to do for now.

Equipment

Camera

I already said, whatever is in your drawer and lets you attach a lens. It should be mountable on a tripod, or else I can’t help you.

Bring extra batteries. Multiple.

I use a decade old Nikon D500.

Lens

I recommend something below 150 mm effective, otherwise you will get issues without a star tracker, because the earth just rotates too fast. Other than that it depends on your desired target.

I use a Sigma 105mm f/2.8 EX DG. Because nothing is cooler than using a macro lens to photograph things that are 1438031031832166400 km in diameter.

And it is the only fast telephoto lens I have in this range. Wider lenses allow for epic Milky Way shots, but this post is not about that.

Tripod

Your shaky hands won’t cut it.

I use a Rollei Compact Traveler No. 1 Carbon with the ball head of the Rollei C5i Alimunum.

The sturdier, the better. I go for portability so I can transport it in a backpack and hang the backpack or a bean bag on the tripod to make it more stable.

Shutter remote

If your camera does not allow taking photos for time-lapses (interval shooting), you need a remote shutter to do this. I have a Rollei Cable remote release that I do not use.

Torchlight

A headlamp with additional red light is best. I use a Ledlenser MH5. If your surrounding is bright enough that you do not need a lamp, you are doing it wrong.

Backpack

If you have to travel to the dark skies. I use an Atlas Athlete together with the Atlas Adventure Hip Belt. Slaps top of bag: This bad boy can fit so much gear in it!

Computer

As beefy as possible. Fast current CPU with lots of free SSD space (2 TB is great, external is fine, I would not recommend using an HDD) and RAM (32 to 64 GB). If you do not have this, an observatory is probably cheaper in the current pc market.

In the end you trade money for speed. Except for the SSD space, as this method requires processing a lot of images. Nevertheless, it will not be fun with a slow computer. Well, nothing is fun with a slow computer to be honest.

Smartphone

For some useful apps, and to have something to do while you wait for the photons to hit your sensor. Bring a powerbank, if you are outside for longer.

Method TL;DR

So, how can you capture an object that moves quickly across the sky? You choose the longest exposure time that is short enough that the object seems to stand still. Combine it with a wide open aperture and the highest ISO you can stomach. Then you put your camera on a tripod, point it at the thing in the sky and let it shoot thousand of images, while sometimes making sure the thing in the sky does not escape from view.

Planning

The first step is to plan. And to learn.

Apps

I use the following apps and websites to plan:

• stellarium
  • Find your next target and plan when and where it is visible
  • It has the option to input your camera and lens data, so you can roughly see what your sensor will see
  • Useful if you are outside and want to know what that is
  • Available for PC as well
• PhotoPills
  • Plan the location, including when and where the sun, moon and Milky Way are
  • It also has useful calculators for setting the correct exposure time to not get elongated stars
• Windy
  • All things weather, clear skies are mandatory
• Clear Outside
  • The weather app for astrophotography, good for the last minute “is it worth to close Netflix and go outside” decision
• Light Pollution Map
  • If it is too bright at night, go somewhere where it isn’t
• CoMaps
  • If you have to go somewhere use this, so you do not get lost at night in a dark forest

Learn Astronomy

You have to learn about DSOs, their size, magnitude, and where they are. Without a star tracker, go for bright objects that fit into the view of your camera/lens combination. Fainter objects will require too many exposures and thus more disk space and longer processing time.

Pro tip: The higher the object is in the sky, the less atmosphere is between your sensor and the elusive photons. Also light pollution from nearby cities has less of an effect. So try to avoid shooting near the horizon.

Pro tip: Wait for astronomical night, so no light scattered from the sun annoys your sensor.

Pro tip: Do not do astrophotography under the full moon. Although I did in the case shown below. At least choose a target on the other side of the sky. Or do whatever you like. Who cares.

The Shoot

The second step is the actual shoot.

Preparation

• Go to the dark place
• Set up your tripod and weigh it down, if possible
• Put your chosen lens on your camera and the camera on the tripod
• Set the camera and lens up, in my case with the 105 mm on the D500
  • Image quality: RAW
  • Mode: Manual
  • Exposure time: 2 s
  • Aperture: f2.8
  • ISO: 3200
  • Disable autofocus
  • Disable image stabilization
  • Close viewfinder shutter
  • Use quiet shutter (QC, reduces vibrations (maybe))
  • Set up interval shooting
• Focus the lens to infinity
  • This is crucial to get right, unless you like blobs as stars
  • Enable live view, so you see on the LCD screen what the camera sees
  • Enable the live view exposure simulation (Pv-Button on Nikon), if available
    • The live view will be very noisy, but much brighter, so you can see all the stars
  • Point the camera at the sky and choose a few stars
  • Gently move the manual focus until the stars become points
  • Rock the focus back and forth to get a feeling where the sharpest point is
    • Use both hands, if possible, to move the focus ring more precisely
  • Lock the focus, if the lens allows it
• Take a test image to check if the stars are pointy
• Check stellarium (or your favorite planetarium app) to see in which direction the DSO is and which constellation is nearest
• Move from brighter to fainter stars, until you find the exact stars which are near the DSO
  • First by eye until you have the rough location, then via viewfinder
• Make sure to line the view up the way you want the DSO to be imaged
• Then move the camera view center to where the DSO will be in a few minutes, so that the DSO is still in view, but not in the center of the view. By doing this, it stays in view for a longer time, while it moves across the camera’s view despite the earths’ rotation, without having to adjust the camera.
  • Check in the planetarium app (best with your camera/lens data) so you know in which direction the object will move
  • Make sure the object has some space to move in the view, otherwise you will have to adjust the camera a lot
  • If you are fancy, you can check how long it will take to move through the view and set a timer to adjust the camera
    • Pro tip: Standard lenses are sharpest at the center, so try to have your main point of interest always near the center
      • This will require you to adjust the camera more often, but the end-result will be sharper
  • Except if you have a star tracker, then, lucky you, you can ignore all this
• Make another test image and check the
  • Sharpness
  • Correct star positions
  • DSO (For brighter ones you might already see it in the image)

Shooting for the stars

• Start your intervalometer
• Check the latest image after a few minutes to see if the DSO is still in view (depends heavily on the camera/lens and DSO)
  • Adjust the camera view if necessary
    • Pro tip: The objects in the sky do not move linearly, they also rotate. Keep that in mind when adjusting the view
  • Check the focus from time to time
    • Depending on the lens, it is possible to get focus drift, especially when pointing straight up
      • This happened when I forgot to turn off the lens’s image stabilizer
• Repeat until the sun comes up, clouds come in, your feet hurt, you’re cold because you did not need that jacket, your batteries died or your SD card is full
• Keep an eye on the dew point (e.g. with Clear Outside), so your lens does not get all dewy on you
• Pack your stuff and go home
  • Check with the torchlight, it is easy to forget and loose something in the dark

Post-processing

This is an example for one of my last astrophotography sessions. I photographed the Sadr Region around the star Sadr (γ Cygni) in the Cygnus constellation. For processing I used Siril 1.4.2 and GIMP 3.2.0 and used the general processing steps from this great tutorial from Astrofotografie aus Norddeutschland (in German). Others like Deep Space Astro also have great tutorials in English. Just check that the Software Version is current, as it is currently very actively developed.

I use Linux, so not everything might be the same on Windows or MacOS.

Warning: Processing ~3500 2 s exposures taken with my 20.9 MP camera required about 1.2 TB of disk space and took about 5 hours with my 32 CPU cores and 64 GB of RAM. Registration and stacking takes the longest time. If you have a potato or toaster as a PC (no judging!) this might not work at all.

For my workflow to work, you need to have Siril prepared with GraXpert with a Background Extraction and Denoising model, as well as StarNet. A local object catalogue is also helpful.

Pro tip: Siril stores all its files in the selected current working directory. If you save the intermediate progress as FITS files, you can jump back to a previous snapshot of your image even after closing Siril (recommended after stacking is complete).

The following steps are the ones I took. They might not be the best steps, but at least they are my steps. Except most of the steps which I borrowed from the Astrofotografie aus Norddeutschland video.

Siril - Stacking

• Get your images from your camera (I use Rapid Photo Downloader)
• Open your favorite photo management software (I use digiKam)
• Go through the images you took and weed out the obviously bad images (you can do this later in Siril as well, but I prefer to check the images at least roughly myself)
  • Most are likely from checking/adjusting the view while the intervalometer is running
• Open Siril
  • Choose the correct current working directory (house button in the top left) that has enough space to house the large files (depending on the number of images, something around 1 to 2 TB might be required)
  • Change to the Conversion tab and drop all your images in there
    • Choose FITS sequence in the drop-down and check Debayer for color images
    • Click on Convert
  • At the bottom center select AutoStretch with High Definition enabled from the drop-down

• Change to Sequence and Open Frame List
  • Check the images and select one where the framing of the DSO is as you want the final image to be
  • Click on the reference checkbox to mark it as the reference for registration
• Change to Registration and press “Go register”
  • This makes sure that each image is transformed so that the stars align with the reference image
  • The default using Global Star Alignment has always worked well for me
  • This can take a long time (hours, depending on the number of exposures and capability of your computer)
• Change to Plot
  • Here you can see the Full Width at Half Maximum (FWHM)) of the stars in each single image
  • The higher the image position in the diagram (higher FWHM), the more blobby and less pointy the stars are
  • By selecting images which are less sharp and excluding them, the final image will be sharper
    • You can select them directly in the plot and then right click -> Exclude selected points
  • Here you can also see focus drift, how well you refocused, or if the atmosphere, clouds or dew decreased sharpness
• Change to the Stacking tab and select Average stacking with rejection
  • You can check here how many images will be stacked
  • Start stacking by pressing Start stacking
  • This again will take a long time as well

Siril - Preparing image for stretching

• Select an area of the image for cropping (left click and drag), to remove lesser exposed edge areas
  • Right click -> Crop to crop
• Extract the background using GraXpert
  • Select Scripts -> Python Scripts -> Processing -> GraXpert-AI.py
  • Select Background Extraction with Smoothing 1.00 and subtraction
  • Click Apply
    • This can take a few minutes, depending on your system

• The next step is plate solving found under Tools -> Astrometry -> Image Plate Solver
  • Input your DSO or nearby star into the search field and click Ok
  • When everything worked you can now click on the galaxy symbol at the bottom and get an annotation of all the things in the image (adjust it with right-clicking it)
  • For some reason I did not bother looking up, this sometimes does not work, especially for processed images, so do it now (also the next step is not possible without plate solving)
• Color calibration time! Go to Image Processing -> Color Calibration -> Spectrophotometric Color Calabiration
  • This is even more complex, so RTFM
  • Short version: Select Average Spiral Galaxy as the white point in the drop-down and click Ok
    • If you have filters or want to add atmospheric corrections you can include them
• Remove green noise via Image Processing -> Remove Green Noise -> Apply
  • This does not lose you any data despite some people claiming so. It just moves the G histogram of the RGB image around a bit. Useful, as camera sensors are actually something like RGGB and not RGB (they have double the green pixels).
• Next comes the deconvolution which I do not do, as it does not seem to work well with my setup
• Next! Noise reduction using GraXpert
  • Select Scripts -> Python Scripts -> Processing -> GraXpert-AI.py
  • Choose Denoising as the operation with Strength around 0.9
  • Click Apply
  • This can take a bit, depending on if you can utilize a GPU (My AMD GPU is not recognized 🙁)

• Image Processin' -> Star processin' -> Desaturate Stars
• Now the magic trick: Separate stars from the rest!
  • Image Processing -> Star processing -> StarNet Star Removal
  • Check Pre-stretch linear image and Generate star mask
  • Execute!

Siril - Stretching

• We were in AutoStretch until now, change it to Linear and make sure the value range sliders are all the way right (for the top slider) and left (for the bottom slider)

• Now the most important part: Stretching the image!
  • Bring out some of the colors
    • Image Processing -> Stretches -> Asinh Transformation
      • Stretch factor around 100 and press Apply
  • Star Wars: Return of the DSO
    • Image Processing -> Stretches -> Generalised Hyperbolic Stretc
    • Check the Logarithmic scale box at the top and monitor the histogram at all times to make sure no pixel values are clipped (become white or black)
    • Loop until the image looks good:
      • Click on histogram maximum to set the Symmetry point
      • Set Stretch factor to < 1.0
      • Set Highlight protection between 0.6 to 0.7
      • Click Apply
      • If the histogram has moved further to the right (the background gets bright) and no pixel values are in the darker ranges
        • Switch Type of stretch to Linear stretch (BP shift) and shift the Black Point until the histogram starts just short of complete black
        • Apply
• Increase color saturation Image Processing -> Color Saturation
  • Increase Global Amount with low Background factor and Apply
  • Whatever looks good
• Save the starless image as a TIF

• Stretching the stars is next
  • Make sure the view is in Linear with the full range again
  • Bring out the colors
    • Image Processing -> Stretches -> Asinh Transformation
      • Stretch factor around 200 to 300 (depends on image) and press Apply
      • Make sure in the histogram (with log-scale) that no clipping has occurred, redo and adjust Stretch factor if necessary
  • Change to Image Processing -> Stretches -> Histogram Transformation
    • Loop until image looks good:
      • Pull the middle arrow below the histogram slightly to the left
      • Click Apply
• Increase color saturation Image Processing -> Color Saturation
  • Increase Global Amount with low Background factor and Apply
  • Whatever looks good
  • No colorful confetti please, but many stars are blue or orange
• Save the starmask image as a TIF
• We are done with Siril for now

GIMP - Finalizing

• Open GIMP and load both starless and starmask images into layers (starmask on top)
• Set starmask layer mode to Screen

• Use Levels tool to move black point of each R, G, B
  • Make sure all three color component left-most peaks (background) are on top of each other
  • Do this for both the starless and starmask images
• Use the Curve tool to adjust contrast to your liking
• Duplicate starless layer
  • Add mask based on grayscale
  • Increase contrast of mask via Curve tool to include the bright areas of the DSO
  • Add Gaussian blur on mask (not the image)
  • Copy the mask (not the image and do not paste yet)
  • Increase Saturation
  • Increase Sharpness
  • Merge down with original starless image
• Duplicate starless image
  • Create any mask
  • Paste mask from above and anchor it
  • Invert mask
  • Reduce saturation of the image
  • Add Gaussian blur (this time to the image)
  • Merge down with original starless
• Check how the image looks with and without the stars and adjust until you like it

• Select starmask image
  • Use Select by Color tool to select stars
  • Grow selection so stars are completely selected
  • Use Distort -> Value Propagate with the More black option to make stars look better
• Final Curve and Saturation and whatever else adjustments with both starless and starmask images

• Export final image as jpg
• Preparation for publishing back in digiKam
  • Resize and watermark the image via Batch Queue Manager and saved Workflow
  • Some final sharpening of the resized image
  • Custom ExifTool script for adding original image metadata from the RAW-file and adding Copyright information
    • This is meant for non-astro images; I currently do not have a good way to merge original metadata and FITS metadata for the final jpg

Congratulations! If you are still reading this: I hope any of this was helpful. Here is a bonus image that shows how an astro-photo looks like when stacking with the Pixel maximum stacking method in Siril: The future of earth-based astrophotography!

Example images done with this technique

Just a few images that I took and processed similarly. To see details, check out the links below each image.

---

I hope you had fun, I know I did!