How To Guide: Astrophotography With a DSLR

Part 2 of 4 | Beginner Shots

In this part we are going to cover all types of shots that can be taken with your DSLR without a tracking mount – moon shots, starscapes & widefield shots, star trails and milky way wide shots.

Lunar Eclipse & Saturn – Nikon D300, 240mm, f/6.3, 0.3 seconds, ISO 1600 single exposure

Moon Shots

The above shot of the February 20th, 2008 lunar eclipse was taken just with a 70-300mm lens and a tripod – nothing fancy. That brightest point of light on the left side is actually Saturn. A view like this wouldn’t have been possible with a telescope. While close ups of the moon are great you can get interesting shots like this with just a telephoto lens.

Focusing can be tough in Astrophotography but moon shots are the exception. My suggestion is to use your camera to autofocus on the moon and then set it to manual focus and be careful not to adjust the focus. Even though the moon is bright and you can use a fast shutter speed you still need to minimize camera vibrations.

Full moon: f/8, 1/640, ISO 200
Half-moon: f/8, 1/500, ISO 400

The above are only starting points. Exposure time will change with the moon’s position in the sky and other factors. You want it to be bright but be careful not to overexpose the brightest edge. Below are some ideas for other types of shots you can take of the moon (click to enlarge).

The first one is showing off the “earthshine” on the moon’s surface. The overexposed part is what you are normally seeing the the dim part is normally cloaked in shadow but with a long exposure you can see the shadowed section of the moon dimly lit by the earth’s reflected light. The other one is a stylized picture of the moon in some clouds.

Starscapes and Wide-field shots

Starscape over a camp fire – Nikon D700, 24mm, f/4, 60 seconds, ISO 1600 single exposure

Focus: The first thing you need to do is focus on the stars. There are many ways to do this but you may struggle your first few times – don’t get discouraged. If you have an older style lens (the ones that usually have an f-stop ring right on the lens) those lenses typically have a hard infinity focus stop so they are the easiest; focus to infinity and you are done.

This is probably why my 2 favorite astrophotography lenses are my 24mm f2.8 Nikon prime and 50mm f1.8 Nikon prime.  More modern DSLR lenses usually don’t have a hard infinity focus point though so you may need to do some trial and error.

If your lens doesn’t have a hard infinity focus point: See if your camera will autofocus on a bright star (or the moon if its out). If it does, great! Once its autofocused switch the focus to manual and be careful not to adjust it. If your camera fails to autofocus, put it in live-view mode on your tripod and point it at the brightest star (or moon if available).

See if you can focus on the live view mode. If not, manual focus as best as you can in your viewfinder then take a test shot. To make it quick, use a high ISO like 3200 and take a 5 or 10 second picture and check the stars. Keep fine tuning your focus til the stars are solid points of light. They are out of focus if the center is dimmer than the outside.

Exposure: The first thing you want to do is stop your lens down at least 1 stop to improve the quality of stars and reduce vignetting. If the max aperture for your focal length is say f/3.5, try using something like f/5. If you are in moderately dark skies (rural, out of the city) try taking a shot with a 30 second shutter speed and ISO 1600.

Most DSLRs max shutter speed is 30 seconds and to go longer you need to use “BULB” mode. So for longer than 30 seconds you need a remote shutter cable to keep the shutter open (bulb keeps it open as long as its pressed – you can’t do this with your hand or your shot would be a blurry mess). If you have a newer model camera you can probably get away with ISO 3200 to double your light.

Other settings: Disable any camera noise reduction settings. You are better off doing your noise reduction in post. If you don’t want to get too advanced with post processing, you can leave “long exposure noise reduction” on. What this setting does is take a second picture but with the mirror down so no light can hit the sensor.

This is called a dark frame. Then the camera automatically subtracts it from your shot to reduce dark current noise and hot pixels. Personally I prefer taking my own dark frames and subtracting them myself as necessary…plus that way I can take multiple dark frames.

Dark Frames?: It’s a good idea to take these if you had long exposure noise reduction turned off in your camera… you may need them later in processing. To take a dark frame, put your lens cap on and take pictures using the same settings you did with the lens cap off (only ISO and shutter speed is important).

The most crucial thing is to take them in an environment with the same temperature as your regular pictures since temperature affects the noise. I usually take my dark frames while I am packing up all my stuff at the end of the night. So if your shots were 30 seconds, ISO 1600 take a few dark frames at those settings.

Shooting Star Trails

Star trails in black and white – Nikon D700, 24mm, f/4, 6x11min exposures stacked (66min), ISO 640

Star trails are one of the most popular types of Astrophotography. It’s hard to perceive the movement of the sky due to the earth’s rotation unless you are looking through a high power telescope and then you can watch an object pass across the field of view in a matter of seconds. The longer your lens, the longer your star trails.

Using a wide angle lens it takes around 20-30 seconds for trailing to become apparent but with a 300mm lens it happens in less than 5 seconds. There are 2 methods of taking star trails which means you can take them in virtually any environment except metropolitan.

Getting Ready

You will need to achieve proper focus as described in the above “Starscapes” section and make sure your camera’s battery is fully charged (bring a spare if you can). If you are going to use the stacking method things will go easier if you camera has an intervalometer or you buy a remote shutter cable that can be programmed to take shots at specific intervals such as the Canon TC80N3 or Nikon MC-36.

There are also cheaper third party models on eBay that also work for more camera models (Nikon remotes and Canon remotes). Look for the ones with the little LCD screens.

Regardless of what method you use, you don’t want to leave your lens wide open. It’s best to stop down at least 2 full stops so you get sharper stars across the whole image. If you are rocking $2000 glass I suppose you can ignore my suggestion. If your max aperture for the focal length you chose is say f/3.5, try shooting somewhere between f/5.6 – f/8.

Single Exposure Method

With this method you just take one extremely long exposure. Anywhere from a few minutes to hours! There are some strict requirements for being able to do this method though. The first one is there can’t be any moon in the sky. It will end up overexposing not only your sky but also any landscape your happen to have in your shot. The only exception being you can sometimes manage shots under 30 minutes if you use ISO 100.

The second requirement is extremely dark skies. If you are within ~100 miles of any city or town, forget about doing that hour long exposure you were thinking about – the light pollution will wash out the sky and your stars. Cameras can drain battery fast when they leave the shutter open. It will vary from camera to camera but you may need to use a power adapter for your camera instead of a battery. 

Finally, some cameras will just have a finite amount of times that they can keep their shutter open (and thus their CCD or CMOS amplified) before “amp glow” becomes an issue. Amp glow is caused by heat in the camera and usually starts at the edges of the image and gets worse. If you are taking pictures in cold temperatures you might avoid it all together.

Any trails you see there will be the exact same trails you see on longer exposures, only they will stretch longer. One setting you can try first is this: f/5.6, ISO 100, 30 min (1800 sec) shutter speed. By using that as a baseline you can see if your sky is too bright or too dark, if your landscape is exposed properly, if there is any amp glow, etc. The single exposure method requires a lot of trial and error but if done properly results in some very otherworldly looking images.

Stacking Method

With this method you have a lot more flexibility. You can do this in moderately light polluted environments and when the moon is out as well. The basic idea is instead of taking one single 60 minute exposure you would instead take 60 seperate 1 minute exposures and stack them to make it look like a single continuous star trail. You don’t want to have more than a second in between shots or else the gaps between images will be too large.

You can use exposures with higher ISOs to gather more stars compared to the single exposure method. It’s quite simple; find the exposure setting you want for your picture and by taking multiples of that same exposure and stacking them, the star trails will grow and nothing else in the image will change.

I will discuss this again in the processing section of this guide, but the basic method to stack in Photoshop is to load up all your frames into separate layers and set all the layers to the “Lighten” blending mode. Better yet, there is a fantastic little program called Startrails that does this for you and also lets you load in dark frames. You can download it here at Startrails.de.

Milky Way Widefield Shots

The Milky Way – Nikon D700, 24mm f/3.5, ISO 2000, single exposure

The Milky Way appears as a long cloud arcing across the sky when you view it with the naked eye, but that “cloud” is actually the combined light of billions of stars so tightly packed together and so far away that our eyes can’t distinguish them. With a telephoto lens you can distinguish a lot of them though and what you are presented with is a wall of stars.

The darker regions in the Milky Way are vast clouds of interstellar dust blocking the light of stars behind them. The constellation Sagittarius is roughly the center point of the galaxy and when you look there you are peering to the middle of our galaxy though most of it is obscured since we are looking at it from the side.

Getting a good Milky Way shot is highly dependent on the light pollution levels. If you can find your location on a light pollution map (such as the one I linked to at the bottom of page 1) see what shade of color you are in. If it’s anything worse than green, forget it. Blue is better and Black is perfect.

The shot you see above was taken on the edge of a green zone, the problem was there was a small town directly south about 10-15km which is where this shot is pointing. Had the town been North instead I would have been able to capture more detail in the Milky Way.

Suggested exposure for your first try: f/4, 30 sec, ISO 1600. Try one at ISO 3200 as well if your camera’s noise doesn’t get out of control at that setting. The Milky Way should be immediately apparent on your rear LCD when you preview the picture.

Finally, don’t forget to take a series of dark frames for all the exposure settings you used.

If you want to take close ups of sections of the Milky Way or photography other deep sky objects like nebula and galaxies you will need to move onto the Advanced section of this guide. You will now need something to move your camera with the motion of the sky.

The Milky Way – Nikon D700, 24mm f/5, ISO 3200, single 1 min exposure

Continue Reading Part 3 Of The Guide…..