Beginning astrophotography part 1: basic types of AP
This is an image of Messier Object 51 (aka M51) the Whirlpool Galaxy taken from my back yard outside of Houston, Texas.
While I captured the data for this image, I have to admit that I didn't do the processing myself, I had help from a friend on Reddit r/telescopes.
Ok, so I regularly have discussions with people concerning what kind of equipment to get to get started in astrophotography (AP).
This is a really, really tough topic, because AP isn't just a single, monolithic subject. There are several types of AP, and each has its own particular needs as far as equipment and technique. I'm going to try to tackle some of this in several posts.
Let me first break this down by starting to distinguish between different types of AP. This can basically be broken into four categories based on field of view and exposure.
By field of view, I mean the size of the patch of sky we are trying to image. This isn't quite the same as, but is somewhat analogous to magnification. If I am trying to capture an image of a large astronomical object, such as a full constellation, I need a wide field of view so that the entire constellation can fit into it. If I'm trying to capture an image of something like a planet, which appears very small in the night sky - about the same as a star - I'm going to need a narrow field of view. The final image of both these targets might be the same size, say large enough to serve as the background wallpaper on a computer screen... but the fields of view will be significantly different.
Exposure refers to the length of time that the camera's image sensor is exposed to the light of the target. This is also often referred to as shutter speed, which is similar in nature, but not quite the same thing (but for most purposes these terms can be treated somewhat interchangeably).
In conventional photography, we often see exposure times measured in fractions of a second. For example, if I'm trying to get a picture of something outside on a bright, sunny day, I might need a very fast exposure time, something like 1/1000th of a second. Anything longer than that and the image might look overly bright and washed out. Not enough exposure and the image will be too dark. If I'm trying to take a picture indoors with good interior lighting, my exposure time might be between 1/250th and 1/125th of a second. Most flash photography is done around 1/90th or 1/60th of a second.
Buy for AP, we are typically trying to capture images of things that are very faint. To do this, we need long exposure times, usually measured in whole seconds, minutes, or even hours. When I'm imaging galaxies and nebulae, my common exposure times are 120 seconds, 300 seconds, or even 600 seconds. But herein lies a problem.
When you look up at the stars at night, it might seem like they're stationary, motionless, fixed in place. And, to a great extent, they are... but we aren't. We're standing on the surface of a sphere that is rotating at the rate of 1 revolution every 24 hours. Becuase of this, the stars above appear to be moving as well. If you watch a star for a while, you'll start to notice this. Look at a star near the Eastern horizon and watch for several minutes and it will appear that the star is getting higher above the horizon... because it is. Conversely, look at one on the Western horizon and you'll watch as it slides down toward and below the horizon. Stars rise and set exactly the same way as does the sun, and for exactly the same reason: the Earth's rotation.
While this rate seems fairly slow, its speed is relative to the size of your field of view. We use angular measure for this purpose. If you want a good explanation of this, I'd refer you to the Forth Worth Astronomical Society's website where they discuss Measuring the Sky. But let me try to break it down a little.
Angular measure is the size of an angle you get if you draw a line from one side of an object, to your eye, and back out to the opposite side of it. For example, If you look at the full moon and draw a line from its North pole, to your eye, and back out to its South pole, the angle formed by those two lines is approximately 1/2 of one degree (or 30 arcseconds, an arcsecond being 1/60th of a degree). There are, of course, 360 degrees in a circle. A star in the night sky will make a full 360 degree circle around the Earth in 24 hours (actually it's about 4 minutes less than 24 hours, due to the orbit of the Earth around the Sun... but let's not nit pick here). If you divide that 360 degrees by 24 hours, you find that the star moves about 15 degrees per hour. As I mentioned before that a degree can be divided into 60 arcminutes. If you do the math, then, you find that that star will move about 15 arcminutes - about 1/2 the width of the full moon - per minute. Arcminutes can further be divided into arcseconds, which are 1/60th of an arcminute. And here you'll find that star moves 15 arcseconds in a second. Now that may not seem like a lot of motion... but then we have to take into account the field of view of our telescope and camera setup.
The Celestron NexStar Evolution 6 is an 8 inch Scmidt Cassegrain Telescope (or SCT). The ZWO ASI120MC-S is a 1.2 Megapixel USB3.0 color camera that makes a good starter planetary imaging camera. If you were to use these two together, you'd get a field of view about 8.4 arcminutes, or 504 arcseconds, wide. It would look something like this: PUT IMAGE HERE. Jupiter is really bright, so we don't actually need a long exposure, but let's just say we do. And let's say there's a background star in the picture. If we were to take a 10 second exposure without compensating for the motion of the stars, that star wouldn't look like a star - like a white dot in the image - but a streak that's about 50-60 pixels long. Why? because the size of the field of view means that, in this case, each pixel is about 0.38 arcseconds across. In 10 seconds the Earth rotates about 150 arcseconds, and 150/0.38 is 57. So the light from that star would smear across 57 pixels.
"Star trail" photos like this one are taken by using long-exposures with no compensation for the rotation of the earth.
(Not my picture, credit goes to @DanSandy on imgur.com).
Now, when you are trying to capture images of faint objects like galaxies and nebulae and need to use long exposures, if you don't have a way to compensate for the motion of stars across the night sky, you end up with a smudge for your object and star trails. And the smaller your field of view, the worse the problem is.
There's an old rule of thumb in photography known as the 500 rule: the longest un-guided exposure you can capture of the night sky can be determined by dividing the number 500 by the focal length of the lens or telescope you're using. Most DSLR cameras (for example the Canon EOS Rebel T8i, come with an 18-55mm lens. At it's 18mm (widest-angle setting) you get 500/18 or about 27.8 seconds before the motion of the stars is noticable. If you "zoom" in the camera to the 55mm setting, you get 500/55 or about 9.1 seconds. My main telescope for imaging has a focal length of 800mm. So my maximum un-guided exposure time is 500/800 or about 0.625 seconds. If I want to capture images of anything that require long exposures - and I do - I need to compensate for that motion with a telescope mount that can "track" the object across the sky.
So back to my classifications. I can break down astrophotography into the following basic groups:
Short exposure - wide field of view
There's not a lot here, actually. The moon is about the only thing you can do with short exposure times and a wide field.
Short exposure - narrow field of view
This is lunar and planetary astrophotography. The moon can fall into both as you can take wide-field images of the whole moon or narrow-field images of portions of the moon's surface. Venus is the largest planetary target, getting up to 66 arcseconds (1 arcminute and 6 arcseconds) in diameter at its absolute maximum. But Venus isn't a very good target for reasons I'll discuss at another time. Jupiter is the next-largest and one of the best, and at its maximum it gets up to about 50 arcseconds in diameter. So planets are small targets that need small fields of view to produce images that show detail.
Long exposure - wide field of view
This is the realm of nebulae. Most of the really spectacular and colorful nebulae fall into this category. For example, M8, the Lagoon Nebula, is a prime example. While this kind of astrophotography does require a mount that can track objects, depending on the focal length of the telescope and size of the camera's sensor, the demands on the mount can be fairly easily met. This is where most beginners will start out for deep sky astrophotography (deep sky referring ot anything outside of our solar system).
The Lagoon Nebula, also known as M8. This is an example of long-exposure, wide field of view imaging.
Long exposure - narrow field of view
This is the really tough category. This requires longer focal length telescopes, which provide narrower fields of view, and usually fairly large apertures to capture more light. They also typically need highly sensitive imaging cameras. And they definitely need telescope mounts that can handle the weight of the equipment and still provide extremely precise motion to track objects across the sky. This is typically highly expensive and the realm of experts.
Now there's three more categories I'd like to mention. These aren't broken down by exposure and field of view. They're really in a class of their own.
Star Trail photography
I mentioned this before. This is actually the easiest type of AP to get started with. It requires a camera and something to hold the camera still (like a tripod). You point the camera at the stars (most people point them at the North or South celestial poles to get the effect of the stars "wheeling around" overhead) and start the exposure. A good exposure time to start with is 5 minutes (this typically requires an extra piece of equipment to time the exposure and/or a way to trip the shutter without touching the camera (i.e. a remote of some sort). Technically, this is a sub-set of long exposure - wide field of view imaging.
Nightscape photography
This is also a sub-set of long exposure - wide field of view photography. These are images where the night sky is shown behind a foreground of terrestrial scenery (such as trees or mountains). This requires a different set of techniques, though the equipment is typically a camera and tripod like with star trail images.
Solar imaging
This is significantly different. It's a short-exposure image, as the sun is very bright, and relatively narrow-field, but it requires specialty equipment. The two most common types use a white light solar filter to show sunspots or a hydrogen-alpha solar telescope to capture details of the sun and its corona. I may discuss this at a later time, but for now, it suffices to say this is a whole different ballgame.
In coming posts, I intend to examine the first four categories in some detail.
In the meantime, let me offer you a few reccommendations for further reading: