This is a new page in development. Please comment below if there is more terminology you would like added, and I will consider it. I will try to add on when I can.
Terminology:
If you have recently joined some astrophotography groups, you have probably heard a bunch of terms going around, and been wondering what all that is about. There is a good chance someone sent you here to help you out.
Let's get right to it.
Trailing (star trailing)
- The sky, while it appears to be stationary, is moving all the time.
- Well.... technically that is wrong and right at the same time. While the objects in the sky are certainly moving, they are mostly at such great distances (excluding the moon, and to a lesser extent, the planets) that their motion is so slight from our perspective that their movement doesn't really matter for astrophotography.
- EARTH IS SPINNING - This is the motion that primarily concerns us as astrophotographers. As earth rotates, the entire sky above us, from our perspective, appears to move around us.
- Trailing is caused by the motion of the sky (from our perspective). This motion restricts our ability to have long exposures, because the longer we keep the shutter open, the further the sky moves across our frame. A very tiny amount of movement will not really effect the image noticably, but more movement will over course result in a bunch of lines across our image, including the object we are trying to capture, blurring across the screen.
- The perceived motion of the sky at night is constant, and rotates around the north pole.
- As you zoom in to a small section of the sky, you are MAGNIFYING that motion, so objects in that smaller portion of sky you have zoomed into, will move faster across your sensor frame.
- This means the more focal length, or zoom you use, the faster your shutter speed has to be in order to keep your object from blurring across the frame of your image... also called "trailing" across your frame.
- The reverse is also true. The wider angle lens, or less focal length, the wider your field of sky is in your frame, the longer shutter exposure times you can get away with. Hence, you can potentially get upto a 30 second exposure with no tracking (we will talk about that later) if you have a very wide angle lens, taking perhaps an image of the milkyway.
- Here is a great video on explaining what is called the rule of 500, and also how to adjust that for better accuracy for modern cameras:
Light Frames (Subs), Dark Frames, Flat Frames, Bias Frames
- Light Frames (also called "Subs") - these are just your images of the object you are taking pictures of. "Light frames" just means these are the frames you are trying to expose well for your actually deep sky object. When you are also heard the term "Subs" or "Sub frames", this is the same thing. People sometimes refer to their individual images as subs, because most astrophotographers are always combining a lot of images to produce a final image.
- Dark, Flat, and Bias frames - these are all about trying to clean up sensor noise in your light frames (your images). When shooting long exposures in the dark, you are going to get noise in your images for several different reasons. Unless you can manage long enough exposures to properly expose your object at ISO 100, you will be boosting your ISO in order to compensate for not being able to keep your shutter open long enough, and this creates some noise. Also, you will likely get a little noise just from having your sensor active for long exposures. All of these "frames" are actually images, but they are trying to directly image the noise, not your object. Then these frames are loaded into software while stacking all of your images, and the software does it's best to correct for the noise. Without these special frames, the software would not know whether the noise in the original image was part of the object or not, and therefore could not correct for it.
- For more information on Dark, Flat, and Bias frames, follow this link:
Stacking programs
- There are a lot of them. Fortunately, a lot of them, but not all, are free.
- Examples of stacker programs:
- DSS - Deep Sky Stacker
- http://deepskystacker.free.fr/english/index.html
- Sequator
- https://sites.google.com/site/sequatorglobal/
- Stacker programs take your images (light frames or subs), dark frames, flat frames, and bias frames, and stack them for you to create a stacked image.
- After stacking these images, you will save a .tiff file.
Light Room & Photoshop - If you are going to do this, you need these programs. You just really do. You can do a subscription for about $10 per month that will get your both programs. It's worth it.
Photoshop - Stretching an image
- For your best work flow, you generally import your .tiff file into Photoshop. In photoshop you will use levels and curve adjustments to adjust the brightness of the image. The way this is done, it doesn't stretch the images, like distort it's proportions... instead, it stretches the data, and pulls the exposure data out.
- Here is a great instructional video on this:
- If you are super confortable with photoshop, you might just finish your post processing here but adjusting saturation, contrast, etc until you are happy. Or, you can save after stretching, and take it into Light Room. I like taking it into light room.
- Watch a lot of youtube videos on photoshop and lightroom. Include the word "astrophotography" in your search, and you will find a lot of resources on youtube.
Bahtinov Focusing Mask
- OMG. This is a gift to all astrophotographers. You can NOT use autofocus on your camera for astrophotography... it just doesn't work. It needs to be able to see stuff to focus, and it just can't see well enough... not even the most expensive DSLR / Mirrorless. Nope. ....and you have to have your focus SPOT ON for astrophotography, and you have to do it manually.
- So, what do you do?
- One method, is to turn your ISO up ALL THE WAY, and change to Live View mode, to display on the back screen. Try to find a really bright star, or a bright very distant light, and do your best to focus on that. Good luck.
- Earlier in the day, focus on the most distance object you can, and note the exact position of your focus ring. Exact. Mark it if you can.
- Best solution: GET A BAHTINOV FOCUSING MASK for your lens.
- You might have to have someone 3-D print one for your specific lens. I did, and it's worth it.
- Here is a link to a youtube video that will explain how you use this mask better than I ever could:
- I will add, that when you are using the mask to find focus, turn up your ISO very high, so you can see the spikes clearly. (you will know what I am talking about after you watch the video) Then make sure you adjust your ISO back down to how you actually want to expose for your image.
Tracking Mount:
- We talked earlier in this page about how the sky is in motion (from our perspective), and about how that motion affects how long of an exposure you can have without having star trailing / motion blur in your image. How do people get these great images of very dim deep sky objects, from earth, when they have to use long focal lengths? In order to get these very nice detailed images, they can't just boost ISO to rediculous numbers, because that just creates more terrible amounts of noise, and while you can use stacking to help with that, stacking will only help so much.
- What a lot of people are doing, is countering the motion of the sky, using a tracking mount. A tracking mount, is an attachment you put on a sturdy tripod, that will move your camera WITH the sky. Because your camera is moving along with the sky, at the same speed, in the same direction, you can increase your exposure length significantly, letting a LOT more light into your image to the extent that you can even lower your ISO, which means less noise in your image.
- To use a tracking mount, it's not just a matter of mounting one on your tripod, mounting your camera, pointing it at your target, and away you go. You have to do that is called a "Polar Alignment" of your tracking mount first. The sky (from our perspective) rotates around the north pole. Hence, if you have your tracking mount NOT pointed at the north pole, but instead, say, it's pointed east, you have actually made your problem much worse! Tracking mounts will have some sort of polar alignment scope, which you generally use to align to the North pole, or if you live in the southern hemisphere, you would align it to the South pole. This can be tricky, but there are a lot of youtube videos on how to polar align a tracking mount, and maybe I will do a separate tab that discusses what I know on this topic at some point.
- There is no free lunch. Tracking mounts do cost money. Different amounts of money will get you tracking mounts with different levels of capabilities. You should think long and hard on what kind of equipment you will be using, not just now, but also for the foreseeable future, that you will need the tracking mount to be able to manage. Different tracking mounts have different weight capacities before they start to fail. Some mounts, when polar aligned and setup correctly, will be able to take you right to a target on their own, while others you will need to find your targets on your own. Some can handle very heavy telescopes, while others can not. Others have great polar alignment scopes, others have basically a straw to look through. Getting a tracking mount will definitely level up your astrophotography... but there is expense involved, a learning curve, and you will want to do your homework on the available models out there before choosing one to purchase. Also remember, if you underbuy... that is, buy a tracker that isn't going to fullfil your needs because it costs less.. it will wind up costing you more later on, when you have to also buy a whole different tracker to actually accomplish your goals.
Slew / Goto Mounts:
- Just a really quick note on these terms you will here. When someone talks about "Slewing to a target", it means they are talking about what is called a Goto Mount. A Goto Mount is a tracking mount that can automatically move on both X & Y axis, and has a control until of some kind, where you can choose a target (after polar alignment) and your mount will move directly to the target you choose. This simplifies things quite a bit, but these mounts are of course quite a bit more expensive. It's worth it if you can afford it, but that is up to you :)
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