• Mike Ridgway

Learning to View the Heavens Part 2

My intention when starting this blog was to post regularly as I gained experience in the realm of Astrophotography. As happens sometimes, life placed other demands on me. However, I am now at a point in my life where I am ready to dedicate myself to this endeavor. So, here we go!

In part 2 of this series, we will explore the 3 key settings in astrophotography in depth. I have gained a wealth of experience since releasing part 1, which I will be passing on along with examples of my work. To quickly recap: it is necessary to find a location with dark skies in order to shoot the night sky. Dark Site Finder is a great resource for this. You can access this free online resource at

However, I have since found light pollution map, which is another free online resource. I now prefer this site, since it provides much more detailed map information. It is actually a google map with light pollution information displayed on it. This resource can be found at

It is important to find a site with a lower Bortle class number. The Bortle scale assigns a number from 1-9 for locations based on the amount of light pollution. Bortle class 1 is considered an excellent dark sky, while Bortle class 9 is considered an inner-city sky. For the purposes of shooting Astrophotography, you are going to need to shoot in a location that is Bortle 4 or lower. Anything above 4 is going to require special light pollution filters to achieve decent results. If you live on the west coast, there are numerous excellent locations with very dark skies (although you made need to drive a few hours to get to them). If you live on the east coast, there are fewer options. Being located in Maryland, my favorite locations are Spruce Knob in West Virginia, Cherry Springs in Pennsylvania, Assateague Island in Maryland/Virginia, and Shenandoah National Park in Virginia. You really need to look around and figure out which location will work best for you.

Once you find a location, you need clear skies. This is the most frustrating and challenging part of Astrophotography. If you are shooting Milky way Panorama or Milky way Landscape Astrophotography, a few clouds here or there is okay. If you are shooting Deep Sky Astrophotography, you need solid clear skies for at least a substantial portion of the night (4 – 5 hours). This comes down to one of the most important aspects of being a successful Astrophotographer: planning and preparation. Let’s call it the 2 P’s of Astro. The 2 P’s of Astro will comprise 40-50% of a shoot. Another 40% will involve post-editing (especially for deep sky shooting). The rest comprises the actual shoot itself.

Once you have found a good location with clear skies, you need to understand the 3 important camera settings for Astrophotography (and for Photography in general). Shutter speed, Aperture, and ISO.

Shutter speed is essentially the length of time a camera shutter is open, and thus the length of time that light is getting exposed to the camera sensor. This is also known as exposure time. In photography, different shutter speeds are used for different types of shots. Let’s say a sports photographer wants to capture a shot of a soccer player just as he is kicking the ball. In this case, the photographer would use a fast (or high) shutter speed to avoid any motion blur. For example, I would use a shutter speed of 1600 – 2500, or 1/1600-1/2500 (1/1600 being 1/1600th of a second) for a shot like this. The goal is to capture that split second where the soccer player looks frozen in place kicking the ball. If you used a slower (or lower) shutter speed, light will continue to expose to the sensor for longer (microseconds make a huge difference!) and the soccer player will look blurred from the captured motion.

In photography, motion blur is often used for amazing shots of night time traffic trails captured over time. This type of photography is all about catching more light in a single shot. Astrophotography involves the same concept. When photographing objects that can be hundreds or thousands of light years away, the camera sensor needs to absorb as much light as possible to produce an image that is visible. If you tried shooting a picture of the Orion Nebula with an exposure time of 2.9 seconds, or a shutter speed of 2.9 (a fairly long exposure time by normal photography standards), you would end up with a few stars and very minor details. The below image was a 2.9 second exposure of the Orion Nebula.

Image 1: M 42 Orion Nebula – 2.9 second exposure

As you can see in Image 1, the camera did capture some stars and a little bit of the Orion Nebula in the middle of the photo. The sensor simply cannot capture enough light from Orion in such a short amount of time to produce any sort of usable image. This means that for Astrophotography we need to use much, much slower shutter speeds than would be used in any other form of photography. In the following image, you will see the difference when decreasing the shutter speed drastically to a 240 second exposure.

Image 2: M 42 Orion Nebula – 240 second exposure

That is not a mistake…… Image 2 was a 4-minute exposure! This exposure time allows enough light to be recorded by the sensor to produce an image with lots of details. This type of exposure time requires a star tracker mount for your camera, which will be covered in an upcoming blog. Without a star tracker the image would be ruined by star trailing (since the night sky is always moving, or more accurately the earth is spinning). With the use of a star tracker you can take multiple long exposure images and then stack them in post processing. The following image was the end result of shooting the Orion Nebula last December. Image 3 consists of 22 4-minute exposure shots stacked with 21 4-minute dark exposures in a software application called deepskystacker.

Image 3: The final image after post-processing.

Image 3 consists of a total of 88 minutes of light exposure time and 84 minutes of dark exposure time. Image stacking is important for Astrophotography in general, but essential for Deep Sky Astrophotography. This will be covered in a future blog as well.

Deep Sky Astrophotography requires a star tracker mount to obtain this kind of exposure time. But what if you don’t have a star tracker? Shooting the Milky way does not require a star tracker at all! When shooting the Milky way without a tracker, you can shoot exposures up to 20 seconds long without any star trailing effect. I have pushed it as high as 25 seconds, but you definitely don’t want longer exposures than this.

In summary, shutter speed is the means by which you control the amount of time that light is exposed to your camera sensor. The more light you allow to reach your sensor, the more visible detail you will get out of your deep sky image. For Deep Sky Astrophotography, you want to use longer exposure times. Generally, you want at least 2 minutes per exposure. For Milky way shots with a star tracker, you generally want 1 to 2-minute exposures (depending on the Camera). Without a star tracker, you want to stick to 15-20 second exposures.

In my next blog I will continue with the next 2 important settings: aperture and ISO.

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