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Learning to View the Heavens Part 3

In part 2 of this blog series, we started exploring the basic fundamentals of shooting Astrophotography. In part 3 we will explore the importance of aperture and ISO. Mastering these two settings along with shutter speed will provide the beginner Astrophotographer with a solid foundation to begin photographing the night sky.


APERTURE

The Aperture of a camera can be defined as the opening in a camera’s lens that allows light to pass through into the camera. The human eye is very similar in functionality to a lens aperture. As a person moves from a dark room to a light room, the iris in their eyes contracts to control the size of the pupil. This prevents too much light from entering in a bright environment, or allows more light to enter in a dark environment.


In photography, a camera lens can either have a variable aperture or a fixed aperture. A variable aperture can be adjusted to allow the camera sensor to capture subjects in different levels of light. The ability to control the aperture of a camera lens provides many benefits in photography, including the ability to control depth of field, isolating your subject, and controlling when aspects of your composition are visibly sharp.


In Astrophotography, everything depends on capturing as much light as possible. Adjusting your shutter speed and using longer exposure’s enable the camera sensor to capture more light. However, this is only effective if you use a lens or telescope that let’s in as much light as possible. The more light a lens lets in, the shorter your exposure times will need to be. If the lens lets less light in, your exposure times will need to be longer. The longer your exposure time, the more likely something (a stray cloud, a breeze shaking your camera, someone walking in front of the lens, etc.) will ruin your shot. Believe me, it will happen.


Lens Aperture is expressed as a number known as “f-stop” or “f-number”. For example, a lens Aperture of 2.8 would be expressed as f/2.8. For those that are new to photography, this can be a bit confusing initially. The larger the Aperture (opening in your lens), the lower the f-stop. The smaller the Aperture, the higher the f-stop. The reason for this is that Aperture is expressed as a fraction. An Aperture of f/8 can be thought of as 1/8, while f/4 can be thought of as 1/4. The fraction 1/4 is larger than 1/8, just as f/4 is a larger opening than f/8. Here is an Aperture scale provided by photographer.org, which visualizes how various f-stops can affect the amount of light passing through to your camera.


(Photographer.org., 2020)



The specific f-stop you use depends on which form of Astrophotography you are shooting. For shooting the Milkyway, you usually want to use a larger aperture of f/1.8 – f/3.5. I personally have found f/2.8 to be a sweet spot, being an effective Aperture in most situations. However, this can vary depending on the lens, camera, and conditions you are shooting in.


The Aperture you use when shooting Deep Sky Objects is typically going to be smaller. Deep Sky Objects represent much smaller targets than shooting something as large as the Milkyway. You will also be using much longer exposure times on these subjects, so an Aperture of f/2.8 is often going to provide more light than is desired for optimal results. An Aperture of f/4 to f/6.3 is usually better. Remember, the larger the aperture you use, the shorter your exposure time will need to be. This can depend greatly on which object you are shooting, your equipment, Bortle class, etc.


ISO

Since this blog series is dedicated to beginner Astrophotographers, we won’t go deep into the subject of ISO. Instead, we will provide a brief overview of what ISO is and recommend ISO settings for various forms of Astrophotography. An in-depth guide blog on ISO will be released at a later date.


In the world of digital photography, ISO is a capability that refers to the sensitivity of the camera’s sensor, or basically the sensor’s signal gain. To simplify it, ISO either makes the image darker or lighter, digitally. Having read this, you may ask “why bother so much with shutter speed and f-stop? Why not just kick the ISO up?” The answer is that there can sometimes be consequences to using higher ISO settings. One consequence is the potential to end up with more digital grain, or noise in the image. While this is a real possibility, it is not always the case in Astrophotography. Generally speaking, the rule of thumb in Astrophotography is to use longer exposures, with larger apertures, with moderate ISO ranges.


When shooting the Milkyway, you typically want to use higher ISO ranges from 1600 – 3200. Shooting with a Canon 6D Mark II, I have found that ISO 2000 is usually the sweet spot for shooting the Milkyway. However, I have obtained some detailed results as high as 3200. Be careful though, since the higher the ISO, the brighter the image and the more likely it will end up grainy.


Milkyway Panorama: 119 Second Exposure, F/2.8, ISO 2500


When shooting Deep Sky Objects, the ISO setting really varies. Keep in mind that the lower your ISO, the longer your exposure will need to be. If shooting a lower ISO of 400 to 1000, you will need to gather longer exposure shots. Generally, aim for 3-4 minutes per shot, with 4-5 hours of shoot time per object. If shooting at higher ISOs, you can shorten the exposure time to 2-3 minutes. However, always try to gather as much overall light frame time as possible. This can even result in multi-night shoots for the same object. The more overall exposure time you have to stack, the more detail you will have in your final image. Just make sure if you’re shooting across multiple nights that you’re using the same settings (shutter speed, aperture, ISO), and that you have a temperature-controlled camera. Using different ISO settings in the same stack can result in poor images.


Andromeda: 238.9 Second Exposure x22, f/4, ISO 400



North American Nebula: 179.9 Second Exposure x88, f/4, ISO 1600

You will notice that with Andromeda I captured much longer individual exposures with a lower ISO. The North American Nebula was captured using much shorter individual exposures, but a much higher ISO. Each of the images above were stacked using light and dark exposures, but no other calibration frames. I would highly recommend focusing on one deep sky image per night, and gathering as much light frame exposure time as possible. Once the light frames are complete, immediately begin capturing dark frame exposures. We will cover calibration frames in depth in a future blog.


SUMMARY

Shooting Astrophotography images requires mastery of shutter speed, aperture, and ISO. Shutter speed controls the length of the exposure. Aperture controls the amount of light that passes through the lens to the camera sensor. ISO represents boosting the digital signal, thereby making the image lighter or darker. These settings can vary depending on the equipment you are using. Overall, here are my recommended settings to use as a guide for getting started.


Milkyway Astrophotography (without a star tracker)

Shutter Speed: 15 – 25 seconds

Aperture: 2.8 (or as large as your lens will go)

ISO: 1600 – 3200

Make certain to capture multiple exposures of the same composition, so you can stack them later for greater detail.


Milkyway Astrophotography (with a star tracker)

Shutter Speed: 60 – 120 seconds

Aperture: 2.8 (or as large as your lens will go)

ISO: 1600 – 2500

Capturing multiple exposures and stacking can be done here as well. I have found that typically a single 1 to 2-minute exposure will gather plenty of details.


Deep Sky Astrophotography (with a star tracker, which is required for Deep Sky Objects)

Shutter Speed: 120 – 180 seconds at higher ISOs, 180 – 240 seconds at lower ISOs

Aperture: 2.8 (or as large as your lens will go)

ISO: 400-1000 (low range), or 1000 – 1600 (high range)

You must gather multiple exposures and stack later using stacking software. Aim to gather at least 20 single light exposures.


Part 4 of this blog series will explore star trackers. This will include a look at how to use star trackers, setting them up, shooting with them, the different types of star trackers available, and how much they cost. The iOptron Star Guider Pro, the Skywatcher Star Adventurer, and the Skywatcher EQ6-R Pro will be covered in depth.


References

Photographer.org. (2020). Aperture & F-Stop. Accessed from

https://www.photographer.org/aperture-f-stop/

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