Astrophotography, give it a try

If you consider to try astro photography yourself but do not want to spend money first, then continue reading.

As long as you already own a decent camera with manual exposure control, a wide angle lens (20mm effective or less) and a stable ground to place it (a tripod will work as well, of course), the only additional thing you need is a night with a clear sky. Set your camera to a somewhat high sensitivity like ISO 3200, whitebalance to daylight, exposure time to 30 seconds and focus to infinite. With the lens pointing straight up, use the auto shutter release (not touching the camera) to take your short.

For more flexibility during edit the image should be recorded in a raw format instead of the regular compressed JPEG. The raw image retains much more gradient details within the darker areas which is our primary interest. But as you can see from my example, even shooting JPEG some details may already be recovered from such a compressed image.

If you are lucky, you may already see on your display an image like this (D750, ISO1600, 30s JPG, Nikkor 16mm, f/4, tonecurve adjusted):

The "stripe" across the center is known as the Milkyway, in fact it is the view across our home galaxy. The number of visible stars in this picture is much larger than that, what you saw in the sky using the bare eyes. And while you may recognize some constellation in the sky, these are much harder to see within this picture. But if you look to the details, you may recognize some faint brighter spot in on the right, just above the center. This is not a star but your neighbor galaxy Andromeda.

For orientation I recreated that frame using the astronomy tool Stellarium. The position of the Andromeda Galaxy has the blue marker (sorry for the german constellation names):

The Andromeda Galaxy is that big and bright, that you may observe it using a binocular, as long as you know where to search for it. Under good conditions you may recognize it even with your bare eyes.

It is totally unknown if Andromeda is still in the shape we see it from earth. Since its light traveld about 2.5 million years to reach us, many thingy may have happend, even in a stellar scale. The light of stars, which we use to build constellations in the sky, is way younger and just some hundred or few thousand years old. That being said, observing the night sky is always a kind of travel back in time.

In terms of brightness the Andromeda galaxy is one of the few exceptions in the night sky, most object we want to photograph in the night sky are much fainter and nearly impossible to spot with bare eyes. To capture these in a picture, much longer exposure times are required. Sad point is, that earth is not that kind and does not stop rotating until we are done with our long exposure. As a result, tiny stars will elongate into lines in our picture. This may be interesting but not what we are looking for.

To compensate for earth rotation, the camera has to be placed on a motorized mount, so the lens stays pointing to the same spot in the sky. But let's try with out such extra gear.

The good thing is, that only the total exposure time is relevant to capture light. As long as we use a raw format, it is less relevant if we capture 10 minutes of light in a single exposure or from 600 exposures of one second each. When adding all images afterwards in a process called stacking, the sum of captured photons is (nearly) the same. But movement of stars is much smaller in such short exposures so we do not get (that long) star trails in the final picture.

To compensate for the shorter exposure time just crank up the ISO setting. Of course the noise will be more visible in the picture (ISO setting does not primarily increase noise level, it just makes it more visible), but since we integrate hundreds of images while stacking, that noise will be significantly reduced. The more pictures you take, the better the result will be.

For my little experiment I just pointed the camera using a 50mm lens towards Orion and took sub exposures of 7 seconds each at ISO3200. The individual frames do not look impressive at all:

If you take a closer look, you will already spot the Orion Nebula right in the middle of the frame, and that's what we are looking for. But you will also notice some vignetting in the corners (getting darker) and a light gradient across the whole image. Since Orion is south in the sky from my location, I photograph directly into the light dome of Frankfurt. We'll see in a minute how worse this will get.

Let's first have a look at the vignetting. Since we do need a massive push of the gradient curve to work out the fainter details, such a falloff in the corners is nothing we want to have. To compensate for that, we do need reference shots to document this falloff with the same lens setup (focal length, focus). For this we point the camera to a neutral object, like the daylight sky or, if we do not want to wait for the next day, use a tablet with a gray light source. Adjust only the exposure time to obtain a medium gray. As a nice side effect, this frame also documents any dirt on the lens or sensor which will be corrected as well. These pictures are named "Flat Frames".

But we are not done yet. Imaging sensors tend to produce some color shadow during longer exposures, sometimes also referred as amp-glow. You may easily observe this by taking a longer expose with the lens cap attached. The expected picture should be black, but in most case it isn't. It is essential to take these frames with the camera at the same temperature as the main sequence. The stacking software uses such "Dark Frames" to compensate for that as well.

Sadly we need a third type of correction frame, which documents the "base level" of the sensor, which is, in general, not zero. This value will later be subtracted from all pixels to define the floor value. To eliminate long-exposure artefacts these frames are taken with a quite short exposure time and are calles "Bias Frames". I personally take these frames with the same exposure setting used for the flat frames, so they are called "Dark Flat Frames".

There is no need to overdue that for this small experiment, 10-15 exposures of each should be sufficient.

From the 300 images (7 seconds each) I took for this experiment, only 170 could be used (due to slow moving clouds), so the total exposure time had been around 20 minutes. The final result after stacking, removing the light gradient and tiny adjustment of the tone curve looks like this:

More total exposure time would have improved this image and the various deep sky objects (the nebulae in the center) would stand out even further. Feel free to push the limits.

That image was stacked using AstroPixelProcessor, which worked with a minimum efford. For your first experiments the trial period should be sufficient, afterwards there are several other solutions, even some free versions like Siril or DeepSkyStacker. Just use the search engine of your choice to find a solution which best fits your needs and also check out YouTube for tutorials how to use them.

Of course there are limits in this workflow, focal lengths above 100mm a probably not usable. But to get a first impression how astro photography works and if this is something for you before investing plenty of money, it may be sufficient. But you still may obtain some nice results, like the following from the Plejades next to Mars:

This stripe was taken with the D750 on a regular tripod and a focal length of 100mm (70-200 f/2.8). To keep the stars round, exposure time was reduced to 2 seconds at ISO3200. 860 Subs were stacked to obtain a total exposure time of about 30 minutes. Since I was too lazy to correct the view often enough, only this small stipe in the center was usable.

A Nikon Z50 along with it's 50-250mm kit lens was used to capture this image of the Andromeda Galaxy. In order to not have star trails in 100 exposures of 20 seconds each (33min) at ISO2000, the camera was mounted on a plain old motorized mont, an Astro 5. I intentionally only coarse aligned the mount towards north. While there is some slight visible chromatic aberration in the corners, this could be the seconds step into astro photography with a rather small invest. And there are several nice targets to capture, as long as there is a clear sky.

This image of our Milky Way (Cygnus/Lyra region) was taken without any kind of tracking. I simply mounted my D750 along with a Micro Nikkor 105mm f/2.8 AI-s (built around 1980) on a tripod and took about 250 images at ISO2000, 8 seconds each. The framing was adjusted only once in the middle.

Of course this lens is not perfectly sharp at f/2.8 and needs to be stopped down twice to reduce chromatic errors and enhance sharpness significantly (see Aperture). Mounted on an unguided skytracker (using my old Astro 5 mount) exposure time can be extended to compensate for this. The result may look like this:

This image from the region of the Garnet Star near Cepheus was created from 17 subs of 180 seconds each (51 minutes total). While an astro camera (ASI2600MC Pro at ISO100) was used for this image, the result should be similar when using a regular camera. Except for the red areas which will be less prominent due to the infrared filter.