I’ve been interested in astonomy my whole life, and a few years back, I got a second-hand Meade LX-10 telescope. This is a 8″ diameter scope… definitely not a toy. It is great for planetary viewing and can even track the planets as the Earth turns. I have used it from time to time to try to take some pictures of celestial objects, but not very good ones. Astrophotography is a growing interest of mine now and I found the learning curve quite steep. I’m throwing together everything I’ve learned over the past couple of years including links to software and such into a few posts. There are multiple steps involved in this project and it took me a long time, working here and there and amongst about 1000 other projects, to finally get everything together for this.
To take amazing amateur astrophotography images you need to decide whether you want to look at deep space objects (DSO) like galaxies or nebulae, or if you want to focus on planetary viewing (within our own solar system). You don’t really want to zoom very much if you want DSO images because zooming narrows your field of view and most DSOs you want to get are very dim so you don’t want to zoom in to make their light spread out across your eye or camera sensor too much, you want bright pictures. You can take some amazingly cool pictures with just a DSLR and a “barn door” tracker. (This is just a device that rotates counter to the Earth’s rotation so your camera will continue pointing at the same object for minutes or hours at a time, meaning it moves a full 360 degrees in 24 hours).
In my case, I want to do a little of both planetary and DSO. So I have my LX-10 telescope, which has an 8″ diameter mirror, meaning it can collect a lot of light, and it has a tracker so I can point a camera at one point in the sky as long as I want. However, unless you perfectly align the scope with true celestial north, the scope will still drift a bit over the night. This is because it is an open control system. You just point it at a star, turn it on and hope you aligned it well when you started. The best images are taken with closed-loop control systems. That is, they continuously look to see if it is drifting off target, then takes actions to move back on target when needed. This guy has my telescope and has taken some amazing pictures of planets, sunspots, and DSOs. Some are mindlbowingly good! To be fair, he’s using some reducer lenses like this one to change his F-stop to make it much quicker (for example F/3.75 and F/4.6 in some images), oh and he’s using a 4x Powermate lens a lot which costs as much as I spent on my entire telescope… But what is great about his site is that he tells all the settings he used for the images. This is similar to Reddit’s astrophotography subreddit.
My scope is what they call “slow,” “long,” or “dark.” This refers to the F-stop number. This is also called the focal ratio or relative aperture. It is a ratio of the len’s focal length and the diameter of the scope. THe focal length of my telescope is 2000mm. And the aperture is 8″ or 203mm. The F-stop of this would be about 2000mm / 200mm = 10. So this means my scope is a F/10 lens. The higher the f-stop number is, the darker the image will be (all other things being equal) when compared to a lens with a smaller f-stop number. For example an F/4 is considered a pretty “fast” telescope. My F/10 number basically means that to take a nice bright picture of Jupiter for instance, I need to play with the “sensitivity” or ISO number of the camera sensor (used to be film) as well as the duration I leave the shutter open on the camera.
You might say, “Well I can just crank up the sensitivity then. Higher sensitivity will make the image come out brighter, right?” The answer is yes and no. You can raise the sensitivity, but then a lot of other things (such as heat) can trigger a pixel to register a value in a digital camera. This will increase the noise in the image, making it staticy.
You might then think “Ok, the other option is to just open the shutter for a long time, exposing the camera sensor to more light over a longer time period.” yeah… not really. This can work as well, however you will increase the overall light pollution in your images and worse, you risk blurring the image. Since the Earth keeps on spinning, objects in a fixed telescope’s field of view move. Without tracking perfectly, long exposures will be blurred.
Even if you can fix all that, there’s blurring you simply cannot fix. This is due to heat inversion in the air column between you and the object you are viewing. The Hubble space telescope was designed to fix this problem… by simply being above all the air on earth orbiting in a pretty high orbital plane (at the farthest reach that the space shuttles could fly). Air at different temperatures has different densities. This acts like a prism to bend the shape of a beam of light. Look at a straw or pencil in a glass of water. Notice how it looks like it’s broken in half at the interface of the water and the air? That’s an example of the different densities acting like a prism. Since air is, well, air… it is gaseous and mixes and moves around a lot. It is in constant motion (unlike the water in your glass compared to the air sitting on top of it.) Hot air rises and cool air falls, making all sorts of weird prism effects in our viewfinder. In the column of air between you and the top of the atmosphere where Hubble is, there’s also a lot of dust. The dust as well as the temperature inversions is what makes starts look like they are twinkling.
Side note: Planets don’t twinkle in the sky when viewed by eye. This is because a star is so far away, you are only seeing it as a point light. This makes it easy for dust or temperature inversion to affect your view of it. But planets are much closer, and their light is spread slightly wider across the retina in your eye. This means there much less of a chance that a mote of dust will block it, or temperature inversion will guide the light too far from your retina.
So how do you fix the problem of having a dark image from the telescope without getting errors from too high of a sensitivity setting or blurring from too long an exposure and temperature inversions? You can thank Woz for the home computer! Using a laptop, I’m going to connect a closed-loop control system to my computer to track the objects I want to image very closely, reducing blur of long exposure images. I’m also going to take multiple images of the same object, then do a process called “stacking” where a computer algorithm will take the sharpest views of different parts of the planet, for instance, and stitch them together into a composite image that is overall much sharper. Then I’ll be able to do some image processing on the composite image to get some great results. Stay tuned for more posts on this theme!