Astro Photography

Probably I am not the only person looking up into the stars during a clear night and consider, what equipment might be necessary to capture at least some reasonable astro photos. Searching that topic in the internet will find some great images, some taken with quite regular equipment like a decent camera and standard lens. In particular the milky way, our home galaxy, will be a wonderful target in a clear night, either to create nice light traces from a longer exposure time or more defined stars using a higher ISO setting.

Please check out my separate page on taking astro images using regular photo equipment: Astrophotography, give it a try.

For more serious astro photography, some more preparation and propably more equipment is required.Without the use of a motorized equatorial mount, a four minute exposure from the milky way using a 16mm lens would look like this:

Let me place a short warning first. Once you successfully captured a first image of some sky detail, it may be complicated to turn away from that track and not start spending extra money in new gear and even more time to get better results.

Astro photography has little in common with taking pictures during daylight. Taking a single astro photo requires some planning, patience while collecting photons (taking the actual series of pictures) and continues with a serious amount of data processing before you may finally see the first result in the photo editor of your choice for further refinement. Please check out the internet since there are quite good tutorials for astro photography so I do not get into the details here. But I've assembled a small Astro Glossary with many links to more detailed information which may serve as a quick reference.

This kind of photography gets even more complex, if you increase the focal length to capture smaller objects or their finer details. This picture of the Pleiades was taken with the D750 through a corrected telescope with a focal length of 580mm on an equatorial mount:

Before we dive further into this topic, we need to classify some aspects of astro photography.

  • Larger scale objects like the milky way
    Such targets can be captured with regular camera equipment in acceptable quality. With a smaller focal length you may not even need an equatorial mount. For sharp, non-elongated stars a short exposure time and higher ISO setting may required. In order to reduce sensor noise, many pictures are taken and combined with a so called stacker application. In this step not only noise will be reduced significantly, stars will be properly aligned as well..
  • Objects within our solar system
    These include the planets and, of course, the moon. While these are easy to spot in the night sky (if visible at all), taking proper images is quite more demanding. First of all there are turbulences in airflow which affect light rays in a somewhat random way, degrading the quality of the image. This effect is named seeing. In addition the planets are quite tiny and require a decent focal length (1500mm and more) to create a reasonable image on your sensor. Of course such focal length require a more robust equatorial mount. To compensate for seeing issues, several thousand images with short exposure times may be taken (for example a video stream) and the stacking tool (like AutoStakkert) may use only the best ten percent to create the final image.
  • Targets within the milky way
    While more distant compared to the objects in our solar system, the requirements are way less demanding. You may even start with a regular lens and even try without an equatorial mount (using many short exposures). Typical range of focal length for many of these objects is about 200-600mm. Most interesing objects in this class are the various Nebulae which exist in arbitrary shapes. Some are self-illuminating, some reflect light of a bright star.  Other consist of dust and cover stars behind. In general these are much more faint compared to the surrounding stars and therefore require a quite long total exposure time (we are talking about hours).
  • Targets beyond the milky way, our home galaxy
    Of course there are great targets outside the milky way, outside our home galaxy. In general these combine the previous two points, they are both quite small and quite faint and, again, require decent equipment to be captured right. Of course there are exceptions, like our neighbor galaxy Andromeda. With a distance of about 2.5 million lightyears, Andromeda is that large and bright that it may be seen with the naked eye under good conditions and it always is a great target for binoculars or a telephoto lens. We'll see soon how this may look like.
  • The sun
    Spotting the sun is something completely different and demands an express warning to not to try this using a regular lens or telescope. You do risk permanent damage to your eyes. Equipped with a proper sun filter observing sunspots on the surface over few days is a really interesting topic, though.

If you still like to follow this route, wait for a clear night, pick your camera and tripod, point it to one of the larger objects in the night sky and take several (more than 100) short exposures (few seconds) at a higher ISO setting (for example ISO3200) and combine these in a stacking software (see below for the tools I use). If you crank up the ISO even further, you may already see the faint object on your camera screen.

Which targets work at your particular location depends on several factors. You may check out websites like Telescopius.com for good targets and use applications like Stellarium to find them in the night sky. And also check out the app store on your smartphone for applications which use the integrated accelerometer to simplify finding these targets even more.

And please do not give up if the first frame taken from the Orion Nebula looks like this in the camera (single frame D750, ISO1600, 120s, 580mm):

Both the image of the Orion Nebular and the one from the Pleiades were captured from my home location under moderate conditions (Bortle 5, Light Pollution Map Karben) including light pollution and a typical winter dust. These pictures were never intended for publishing and were taken as a first light with a new telescope to check focus and guiding quality of the equatorial mount.

But this exact image is a nice example to demonstrate, what astro photography is all about. Regardless which type of camera was used to capture this picture, to obtain a colorful and detailed result some massive tuning on the gradation curve will be required. Only then those fine structures and gradients will reveal. After application of a quick "astro"-curve we may get a result like this:

Only that extreme curve shown at the left was applied to the original image, any we already recognize good colors and contrast. The curve actually starts with the light pollution (the left peak), which already throws away about half of the brightness values. Then the steep slope stretches about 10% of all brightness values across the full range (from black to white). For this reasont this step is called "stretching".

Everyone familiar using these curves is aware of the fact, that such massive adjustments reveal image artefacts like sensor noise. On the right you'll see an enlarged crop from the center for better visualization.

Sad point is, that this noise can not be controlled using exposure time only. In this example a longer exposure time would increase the base brightness from light pollution in first place. Then the already too bright core would get even more overexposed. In the end this will reduce the usable brightness range hence decreasing image quality. On the other hand a reduced exposure time would make sensor noise more dominant.

The relevant keyword in astro photography is Signal-to-Noise-Ratio (SNR), effectively the distance between the signal of interest and the omnipresent noise-floor. We simply need to maximize that signal to noise ratio. For a single exposure the SNR is basically defined by the bit resolution of the sensor (quantization noise), the number of brightness levels which can be differentiated. For JPG files this would be 8 Bits (per color), so we may differentiate only 256 brightness levels. Raw image, in contrast, provide at least 4096 (12 Bit), most of the times 16384 (14 Bit) or even 65536 (16 Bit) brightness levels (per pixel and color channel), which result in a less noisy image after applying such a curve. But the source of this image originates from a 14 Bit RAW file of the D750 (specialized astro cameras not necessarily have a higher bit depth) and still does not look nice.

Now data processing takes over and we need many more images from the same object. The individual pixels of all images will be integrated and the resulting color value evaluated. Since sensore noise occurs more randomly it is reduced in this step the more images are processed. This not only compensates the limited bit depth of the camera sensor, but removes other noise like satellite trails as well.

After stacking 28 of these images along with flats and darks and some later corrections, the result may look like this (D750, 580mm, f/5.8, ISO1600, 28x 120s):

From a mathematical standpoint a duplication of the available data quantity halves the quantization noise. The tiny series from the region horsehead nebula illustrates this pretty well.

As an experiment I took four exposures of 300 seconds each (ZWO ASI2600MC Pro) during full moon right into the light pollution of Frankfurt. The series of 1, 2, 3 and four integrated images was then stretched in Photoshop. With this little quanity of images the quantization noise decreases rather quick. For a similar significant further reduction, we'd need 8 images, then 16, 32, 64 and so on. 60 images of 300 seconds equals a total exposure time of 5 hours. This is where these long exposure times originate from in astro photography.

But let us get back in time a little and check out one of my first deep sky image taken in 2006, the Andromeda galaxy.

The source images were captured with a Nikon D70 mounted on a simple equatorial mount with clear skies at an altitude of 1500m in Switzerland. I already knew that my exposure time should not exceed 30 seconds with an effective focal length of 300mm (70-200VR, f/2.8) so I took a total of 25 exposures at ISO1250 which were stacked using Deep Sky Stacker.

I remember being quite excited to manage capturing an image at all of something that far away from us. But the result was way off from what I expected to get so focus was lost on this topic.

Nowadays, some 15 years later, that image probably still arouse pity. When searching the internet for astro images you will found plenty of great shots which sometimes were taken under much worse conditions. Specialized software tools for astro photography evolved a lot since. They seem to do a much better job than in those past times and, of course, I've learned a lot as well.

I intentionally presented that original image here to clarify, that taking some exposures is not the end of the story. In fact it is the first step within the workflow. A nice image needs some attention in post processing and editing. So I decided to take the original images (no correction frames) and push them through a modern workflow. So this is the result (identical framing):

Of course this still is not an exceptional image, but clearly demonstrates the capabilities of modern imaging tools, even if there are only compressed JPEG files without correction frames available.

After realizing this, some curiosity came up. So I picked one of my telescopes, a somewhat reasonable 6" reflector telescope with a focal length of 750mm and f/5 aperture. The only goal was to determine, if there at least is a bit of a chance to image some deep sky objects from my home. If this could prove at least some success, there was a chance to revive that abandoned hobby.

The obtained result was never intended to be presented anywhere, since I was totally aware about the shortcomings in this setup. But to provide you a better idea of what to expect, or better, not to expect from such a standard telescope, which still is great for visual observations, I'll show it anyway (D750, 28x 120sec, ISO3200):

On first sight this does not look that bad, except for some dust on the mirrors. But after that initial impression some severe focus issues are pretty obvious. One issue is the mechanical stability of the eyepiece extension. As soon as something heavy is attached, like a DSLR, it bends a little so the focal plane is tilted slightly. As a result we get a focal gradient across the image.

While this probably could be fixed mechanically, a loss of focus towards the corners originates from the simple construction of this reflector telescope. An uncorrected "Newtonian" telescope creates a spherical focus at the eyepiece, which is ok for visual astronomy, but highly useless for astro photography, which requires a focus plane. Both issues can not be corrected afterwards.

Depending on the type of telescope different countermeasures exist to generate a proper focal plane for astro photography. In general this is realized with some more or less complex lens systems. And, like any kind of specialized optical system, these tend not to be cheap. If you intend to purchase a new telescope for astro photography and try to save some bucks here, you'll be "rewarded" with images like the one above.

It took me a while to decide for a new telescope with regards to optical quality, targets of interest and budget. In the end I now own a, so called, apochromatic refractor with a focal length of 580mm and an aperture of f/5.8 (hence a diameter of 100mm). Images taken with this telescope are on quite a different level and razor sharp up to the corners. Here is Andromeda again, taken from the same spot as the previous image (D750, 32x 120sek, ISO640):

Similar to regular photography, a decent lens is essential to obtain appealing images in astro photography as well. In addition you do need a stable mount and ideally a guiding system to compensate for mechanical errors within the mount (which always exist for consumer grade mounts).

If you still intend to enter the territory of astro photography and purchase some gear, you'll first need to decide which objects you are interested in. For larger objects like nebulae I personally prefer refractor telescopes (those with lenses) and a focal length of about 350-500mm. If you prefer planets or distant galaxies, you will need both more focal length and a sturdier mount. In that case you probably go for some reflector telescope (those with mirrors). If you prefer portability, size and weight may be most relevant and consequently limits maximum focal length.

But we did not yet reach the end of our journey, since a setup with a properly guided telescope still has some potential for optimizations.

In my case the target was to obtain an autonomous setup which runs without a laptop and only a minimum of cabling. Nowadays this can be achieved with tiny computers and some patience for configuration and collecting the required software pieces. Due to their compact size they can be mounted right at the telescope using short wires only. Controlled from a tablet using a wireless connection, I can focus on other stuff while the telescope performs its tasks outside in the cold.

Then the DSLR should be replaced with a dedicated astro camera, which, in my opinion, has some advantages. First of all it does not have any moving parts, which is quite good during the colder seasons. Since such a camera is connected to the control computer as well, we easily get auto-focus capability which is nice, since temperature changes may require re-focusing from time to time. In addition to that a dedicated astro camera in general has a cooled sensor, which may reduce read noise significantly. On top of this such cameras may collect light more efficiently, so quality per time increases.

My very first image using that astro camera had been the Pelican Nebula (IC5070). With a total exposure time of 90 minutes (31x 180 seconds) I got this:

My current setup looks like this:

  • Teleskop: TS-Optics TSQ-100ED 580mm focal length, f/5.8, apochromatic refractor
  • Mount: Skywatcher HEQ-5 Pro
  • Polefinder: ALCCD QHY PoleMaster
  • Guiding-Camera: ZWO ASI120mm Mini
  • Main-Kamera: ZWO ASI2600MC Pro (26MPixel, APS-C)
  • Fokuser: ZWO EAF Motorfocus
  • Control computer: ZWO ASIair Plus
  • Power supply: Jackery Explorer 500
  • Stacking-Software: AstroPixelProcessor
  • Editing: Photoshop CC, StarXTerminator, Topaz AI

The dominant amount of ZWO components was set by my decision to go for a ZWO ASIair, which (at the moment) only supports components from the own eco-system (except for mounts). But there are high quality alternatives without such restrictions like the open source software N.I.N.A..

With this setup I am able to capture many medium sized deep sky objects with a very decend resolution. But since the camera has an APS-C sensor, the setup effectively has a focal length of 870mm, so larger objects like the Andromeda galaxy do no longer fit into a single frame. For such objects I still may use the DSLR. On the other hand that focal length is still way to short for distant galaxies or planets.

To determine your favorite focal length you may use tools like Telescopius.com or Stellarium. These provide the function to enter both focal length of a telescope and a camera (sensor size) and to place the resulting frame rectangle into the virtual sky.

This final image is a detail of the constellation cygnus within the milky way. With the bright star Sadr bottom right, there is the Crescent nebula (NGC6888) on the left and the star cluster M29 right on top. All surrounded by a massive amount of stars from the milky way. Taken with the D750 at ISO640, integrated from a total of 70x 118 seconds subframes, a total of about two hours exposure time: