Astrophotography How-To Guide, Part I

[astrostu]

Prefaces​

If you have any suggestions or problems with this guide, or if you find any gross errors, please let me know by e-mailing me.

Purpose

The purpose of this guide is to explain how to photograph common "stuff" in the night sky. The main part of the guide is organized by increasing level of equipment, starting with a simple hand-held point-and-shoot and going up to an SLR camera mounted on a clock-driven equatorial mount. This guide is for digital cameras. Though the techniques are likely the same for film, I do not have a film camera and so it is not written for that equipment.

The opinions and techniques expressed herein are solely mine. I give no guarantee of success for you when using them, but I would be surprised if you are unsuccessful if you follow my instructions. There may be other, better, techniques out there; the ones expressed here are simply what I've learned over the past few years of doing this.

This guide is meant to be read straight-through. Though you can use it as a common internet FAQ, I don't recommend it because it is not laid out that way. As it progresses, I introduce new concepts, ideas, and notes that I do not necessarily repeat later on, even though they may be relevant.

I have tried to make it easy to follow by including common features at the beginning of each new section that describes what new things I will be addressing.

Assumptions

This guide assumes that you can find objects in the sky. It simply gives names and advice on how to photograph, and it should not be used as a how-to-find-stuff manual.

This guide assumes you know how to work your camera and how to manipulate aperture and exposure length. It also assumes you know how to manually focus.

This guide assumes you know what objects are in the sky or can look them up elsewhere. I explain some terms, like "Earthshine," if it's a specific thing to be photographed. But, common astronomical objects (like the Pleiades cluster, the Summer Triangle asterism, or the Andromeda Galaxy) I will not provide explanations for because that is not the purpose of this guide. Google is your friend.

Terminology

Field of View - This is how much you can see through your optics. A large field of view means you can see a lot, whereas a narrow field of view means that you can only see a little. Field of views are usually measured in angles.

Angular Measurements - There are 360° (degrees) in a circle. Thus, if you could see everything around you, you would have a 360° field of view. If you can only see objects from directly left to directly right, you have a 180° field of view. One degree (1°) is divided into 60' (minutes, or arcmin), while 1 arcmin is composed of 60" (seconds, or arcsec), much like an hour is divided into minutes and seconds.

Magnitude - This is a scale that astronomers use to refer to brightness. It is a logarithmic scale: This means that if Object A is 10 times brighter than Object B, the magnitude of Object A is not 10 times different from Object B. Rather, because of the way the scale is defined, Object A is 2.5 magnitudes brighter than Object B. Every change in brightness of a factor of 10 results in a change in magnitude of 2.5. Additionally, fainter objects have a larger magnitude, while brighter objects have a lower magnitude (this is for historical reasons). Pluto has a magnitude of around +13, while the moon has a magnitude of around -12. This is a difference of 25 magnitudes, which means that the moon is 10^((+13--12)/2.5) = 10^(25/2.5) = 10^10 times brighter than Pluto.

Exposure Time/Length - The amount of time the shutter on your camera is open and the detector (or film) is exposed to light. Longer exposure times result in a brighter image because the detector has had more time to collect light.

ISO - This tells the camera how much to boost the signal that is recorded by the detector. A larger ISO number will "increase" the sensitivity of the detector and hence the brightness of the image. "Increase" is in quotes because the sensitivity isn't actually increased, it's just that the gain on the detector's output is increased. This means that you're not actually getting more light, just whatever the detector says was collected is multiplied by a larger number when it gets recorded onto your memory card. Because of this, while the end result is that the brightness of the image is increased, the brightness of the noise is also increased. I recommend using the lowest ISO setting possible.

Noise - Any fluctuations in brightness that aren't part of the object you're trying to image. In electronic devices, noise is generally the result of heat: The molecules that make up the detector move around, and they move more the hotter they are. The more they move, the larger the chance is that they will accidentally release an electron which then gets recorded by the detector as a piece of light, creating noise. The cooler the detector or the better made the detector, the less noise there will be. In modern electronic devices, the amount of noise is generally linear with time: An exposure 2x longer will record 2x as much noise, and so on.

Mean, Median, and Mode - Ah, those geometry terms you thought you would never ever need again. Mean is the same as average, which is the sum of all values divided by the number of numbers that went into the sum. Median is the middle number of a ranked list. That means that if a list is {1, 5, 2, 3, 6}, then the median is 3 since that is the middle number once the list is sorted. If there are an even number of values in the list, then the median is the average of the two middle numbers. Mode is the most frequently occurring number, and as such there can be more than one mode in a set of numbers. For example, if a list is {1, 5, 3, 2, 3, 6, 4, 5}, then the two modes are 3 and 5.

Seeing - A term astronomers use to describe the atmosphere's blurring effect. Because Earth's atmosphere has turbulence - wind moving in various directions at different altitudes creating eddies - it will act to blur the light passing through it, making astronomical objects appear fuzzier than they should, even if your focus is spot-on.

Sizes of Common Objects

Sun and/or Moon - 0.5°. So if you have a true 300 mm lens (so on my Digital Rebel because of the 1.6x crop factor this would be a 188 mm lens to get a 300 mm equivalent), then you will have a field of view of about 4.5°x7°, meaning the full moon will take up ~3% of the total area. With my 1000 mm lens (1600 mm equivalent), the full moon covers just about the entire vertical size of my detector. Note that the moon's size does change, oscillating between ~29.5' and 33'.

Stars - Stars are point-objects (except for the Sun) as seen from Earth. Unless you're going for an interesting effect, then the stars should be no larger than 1 pixel, otherwise you're using a poor lens, your lens is out of focus, or the seeing isn't all that good.

Constellations - This varies by constellation. There are very small constellations like Dolphinus or Canes Minor to very large constellations like Virgo, Draco, or Ursa Major. I'm not going to list the sizes of all 88 constellations, especially because the size varies depending upon how much you want to see. A few of the more photogenic Northern Hemisphere ones, with their largest dimension to get most of the good stars are:

Orion - 27°
Hercules - 34°
Leo - 30°
Cygnus - 27°
Gemini - 22°
Böotes - 30°
Canes Major - 19°​

Asterisms - These are not "official" constellations, but they are other groupings of stars that have common names, such as the Big and Little Dippers, the Hyades, or the Summer Triangle. Some of the more common ones:

Little Dipper - 20°
Big Dipper - 26°
Hyades - 20° or 4° (depending upon how much)
Summer Triangle - 38°​

Planets - Very small. The largest as seen from Earth is Venus, which when new gets up to 1 arcminute, but when full shrinks down to about 10 arcseconds. Jupiter is second-largest, which gets up to nearly 50 arcseconds. Saturn can appear nearly as large as Jupiter because of its massive rings. Mars gets up to around 25 arcsec, but it usually hovers closer to 5. Uranus, Neptune, and Mercury are closer to a few arcminutes, and Pluto is a point-source. What is impressive and can be captured in a very big zoom lens (generally need more than 500 mm equivalent) are the four main moons of Jupiter (AKA Galilean Satellites), which can span several arcminutes across. You won't be able to resolve the disk of the moons, but you'll easily be able to see their positions relative to the larger, brighter planet.

Other Astronomical Objects - Without a telescope, there are really only two other things I can think of that you can capture with a normal camera lens: The Pleiades (AKA M45) and The Orion Nebula (AKA M42). The former is 1.8° in extent while the latter is about 50 arcmin (the part visible with reasonable exposures, anyway … it's over 80 arcmin).

About Photographing the Sky at Night​

Photographing the sky or objects in the sky at night is very different from photographing during the day. This is mainly because of two things. First, objects in the night sky are much fainter than objects are during the day. The brightest nighttime object - the moon - is a factor of one million times fainter than the sun. Because objects are much fainter, they require longer exposure time. This brings up the second issue, which is that the night sky rotates over Earth (this is a true statement -- relative to Earth, the night sky moves). So, in order to not see this motion, you need to take short exposures, or have some way to compensate for this motion.