Obviously our goal is to have a sharp image with zero star trailing, so what we would hope to accomplish with autoguiding is to keep the movements of our stars within a single pixel. Now we will see all of the periodic error of the sky tracker. Let's say we switch out the 14mm lens for a 400mm lens with a sampling rate of 2.78 arcseconds per pixel. Would it be useful to autoguide with this setup? Not at all. These movements from the periodic error fall below our imaging sampling rate, so our camera physically cannot capture them. Will our stars ever trail from this periodic error? No. Let’s also say for example that the periodic error on the tracker creates movements that move the stars up and down 30 arcseconds. With this imaging system, your sampling rate is equal to 80 arcseconds per pixel, this means every pixel in this system fits a square of the sky that is 80 arcseconds wide. ![]() For example, let's say you’re using a Canon 6D with a 14mm lens and a sky tracker. But as you could’ve guessed, the answer to that question depends on another question, how much can the stars move before we care? Our autoguiding setup is not the only setup that has a sampling rate, our imaging scopes also have a sampling rate. So if the sampling rate of one huge pixel isn’t good enough, we need to find out what size of pixel, and what focal length telescope we will need to account for the small motion of the stars. With just one pixel we will never be able to detect the small motions that stars make while imaging. With one pixel, our whole telescope field of view is measured by just that one pixel, so we would say that the sampling rate is very low. ![]() Let’s say that we are trying to measure the movement of a star using a camera and a telescope, and that our camera only has one huge pixel. The smaller the pixels, and the more zoomed in our telescope, the smaller that piece of sky gets on each pixel. Think of it like this: each pixel in our camera fits a little piece of the sky inside of it. Typically this unit is noted as arcseconds per pixel, like how in our gold miner analogy our sample rate was defined as feet between samples. If we think about the light from our scope landing on our sensor as a plot of land, we can also sample it at a particular rate across the dimension of space. The distance between our gold samples is the sampling rate. The point is that we can change how good the measurement is by changing how frequently we sample for gold. We could also check for gold every 10ft, for every inch, or for every nanometer. Let's say you decide to take more samples to get a better idea, and you sample a piece of dirt every 100ft in a grid across your plot, now you’re doing better. Obviously this won’t do very good at measuring the gold in the plot. Let's say you take a dirt sample at each corner of your plot. You want to survey the plot to see how much gold you have. For example, let's say you’re a gold miner who has a plot of land with an area of one square mile. A sampling rate is simply how many times you measure a signal over some dimension. ![]() In astrophotography (and many other fields of signal processing) there is an important term called the sampling rate. off-axis guiders, it's important to discuss some general knowledge about how autoguiding works. Autoguiding can allow for a system that takes trailed 2-minute long exposure to take exposures exceeding 30-minutes, so long as it is properly tuned and you make the correct decision about how you are going to autoguide.īefore we get into guide scopes vs. By doing so, you can correct for any kind of problems that may pop up while trying to perfectly track the sky, like polar alignment, periodic error, or wind gusts. Autoguiding is a method of active telescope control that makes your telescope ‘lock on’ to a particular star in the sky. For most users, this will not be possible without some kind of autoguiding. In astrophotography, we want the sharpest long exposure images possible.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |