We started to think about buying a telescope last year in October, when one night, we took pictures of the Moon with my camera and a 300mm lens, and that was pretty cool to see some details on the surface. So then we looked for other objects to shoot and pointed the camera towards the Orion Nebula, one of the biggest objects you can observe in Winter. The result was amazing.
NOT. It was not amazing. But we were really excited about seeing this reddish thing surrounded by a lot of stars. Why are the stars looking like white lines? Because the earth is moving, and the higher your lens focal length is (basically the zooming factor), the faster the stars will seem to move in the viewfinder(only two seconds of exposure here).
We didn't know it back then, but this was the beginning of our new passion for astronomy. A few weeks later, after having read all the possible things we could about telescopes, we decided to acquire a telescope, the Skywatcher 150/750mm.
- 150 is for the diameter (in mm) of the principal mirror of this Newtonian telescope, which determines the quantity of light that it will gather. So the larger the mirror, the more light you get. And obviously, the more light you get, the better you will see!
- 750mm is the focal length of the mirror, the length necessary for the light beams hitting the mirror to converge. In the end, it means zooming. The higher the focal length is, the higher the magnification will be.
So, where do we look? As you would do on a microscope, you look through the eyepiece (or ocular lens) , an optical lens magnifying the telescope image. To get the magnification of what you are looking at, you just need to divide the focal length of the telescope mirror by the focal length of the eyepiece. Hence, having different eyepieces enables you to look at objects with different magnifications, the same way you would do with a microscope.
Unfortunately, the magnification has a limit. Indeed, the bigger the magnification is, the smaller the zone of the sky you're looking at is. So this means that the amount of light getting to your eyes gets smaller with the increase of magnification. A general rule of thumb says that the maximum magnification you can reach with a telescope is two times its diameter in mm, so for our telescope, 150*2=300x. If you want to use an eyepiece giving you a magnification higher than this threshold, you can, but don't expect to see anything with such a small amount of light getting to your eyes.
The "tube" and the eyepieces are not the only parts of the telescope. Another very, very important part of the telescope is the mount. Indeed, as we saw earlier, the earth moves and if you want to observe objects in the sky and keep them in your field of vision, you need a way to compensate the rotation of the earth so the celestial objects "stay" in the eyepiece. This is where the equatorial mount comes in.
An equatorial mount is a telescope mount made to easily compensate the rotation of the earth. It has two axis of rotation, one of them being the same axis the earth rotates around! Hence, you can just move the mount around one axis (the right ascension axis) to keep the object in your field of vision. This task can be done by a motor, compensating almost perfectly the rotation of the earth.
The tricky thing with an equatorial mount is to align its rotation axis with the North/Pole axis of our planet. This alignment is fundamental for astronomers who want to take pictures with long exposure times (a few minutes) and don't want the stars to "move" during the shot (like it did on the first Orion nebula picture). For this, the goal is to align the right ascension axis of the mount with a star aligned with the N/S axis of the earth. In the northern hemisphere, Polaris( also known as North Star, Northern Star or Pole Star) is this star. In the south hemisphere, it's a little bit more complicated, because no eye-visible star is close enough to the south pole. People need to use two stars of the Crux constellation to "find the South" on the line joining them.
Our mount is equipped with two motors(one per axis), allowing us to compensate the rotation of the earth but also to move from a location in the sky to another without having to touch the tube. Having two motors is also necessary for what is called the "goto". The goto is a really great feature of telescopes where the user can say for example "I want the telescope to go to the Andromeda Galaxy" and the telescope will automatically position itself to the coordinates of this galaxy. A lot of telescopes are sold with a goto pad, where you can just select where you want the telescope to look at. This feature represents a huge gain of time for astronomers, and especially beginners like us.
We really wanted to have this goto function on the telescope, but since the control pads were so expensive, we decided to build a Picastro, a great hardware "box" based on a PIC micro-controller and designed by Arnaud Gérard( more details here ). The Picastro is able to communicate with a wide range of telescope (there are different protocols for communication) and has a lot of features that most of the goto kits don't have( such as controlling a focus motor). So cheaper and better? Yes. The downside, if it's one, is that you have to build it yourself. A bit like an IKEA goto kit, if you will.
The Picastro can be controlled by a computer, via a serial port, or by a pad, and since it is a DIY object, you can tweak it as you want, and re-use it if you change of telescope one day since it is compatible with many communication protocols. We are currently working on a big project for the Picastro, more news on this coming soon 😉
Finally, the last part of the rig is the camera. Astronomy cameras can be very, very expensive because they use CCD sensors, extremely sensitive to light. They are almost always monochrome, which means that if you want to have colored pictures, you need color filters (RGB filters for example) , and at least 3 pictures (one with just red, one with green and one with blue light) and then recombine them to get a picture. Even if the quality of those pictures can't be matched by a traditional camera, DSLRs (or reflex) cameras can be mounted on the telescope like an eyepiece would be, and give pretty good results for beginners eager to start learning the real science that astrophotography is. The CMOS sensors of a DSLR may be less sensitive than a CCD sensor, but it offers for lower price a much bigger resolution (18 millions of pixels for our Canon 550D) than entry-level CCD cameras (just 1MP). The camera is controlled on the computer via the Canon EOS utility software.
Even if we are still extremely far from being good astrophotographers or astronomers, we are starting to get better and from the first shot of the Orion nebula taken with a reflex camera on a regular tripod, we've come to this:
So far, so good. We will try to post as often as we can new pictures and will soon explain the basics of astrophotography!