Astrophotography with the Canon 40D
Now that I have had a couple of serious astrophotography experiences with my new Canon 40D, I figured that I should actually write a bit about it. Before the 40D I used a 20D, which was certainly better than film, but I never was really able to achieve the level that I wanted to. The primary problem is invariably achieving a good focus. There are several techniques for getting a good focus, the best of which is probably using a focusing aid like the Kendrick Kwik Focus. The concept is simple - when the telescope is out of focus, the light takes on the shape of the aperture of the telescope, central obstruction and all. Thus, if you put a mask on the front of the telescope with two or three holes, you see two or three spots if the scope is out of focus. But, once the telescope is brought into focus, all of those spots converge, and you see only one image.
Aside: Most focusing aids use circular holes, but if you are making your own focusing aid (and why wouldn’t you?) you probably would do best with triangles at different orientations, rather than circles. This way you get some diffraction spikes as well, and when they are all centered around one point in the field of view, you know you are in focus.
There is one feature on the 40D that hands-down makes it a sheer joy to work with: Live View! Live View lets you get a really good look at what is coming into the camera, before you actually take any photos. This means you can point the telescope at a bright star, switch on Live View, zoom in on the image, and get a really good focus on that star before taking any photos. I have used Live View for both piggyback-mount photography and prime-focus photography, and it is just great for getting a good sharp focus. (I do have to say though, nothing shows the ugly realities about the quality of your camera lenses like focusing on a star. Oh well…)
The Canon 40D has a number of other nice features for use with astrophotography. I always switch the camera into timer-mode and set the mirror to pre-fire so that there is as little motion as possible when the photo is taken. (Even with that, there is still vibration from the shutter, but you can see my previous post for details about that.) I also use the TC80-N3 timer-remote to control the camera without touching it. Of course, the main feature I use is the timed-exposure functionality; the other features of that remote aren’t so relevant to astrophotography. If you switch the camera into M (manual) mode, turn on the camera’s 10-second timer, and set the timer-remote to your desired exposure length plus 10 seconds, everything works great. The only hard part is sitting there waiting to see the results.
That is about all I have to say about using the 40D for astrophotography at this point. I don’t know much about “frequency-response curves,” how suitable the camera is for imaging hydrogen-alpha wavelengths of light, or anything like that. I’m an amateur. I just know that this camera is a whole lot easier to use than the 20D, and it really has me motivated to get out of the city a bit more often to take some pictures of the sky. I’m happy with that!
November 29th, 2007 at 6:42 pm
do you have any of the pics you took with your 40d to share with us? i just got my own 40d and interested in astronomy
January 10th, 2008 at 11:24 am
I found that connecting the 40D to a laptop computer, via the supplied USB Cable, then running the 40D Utility (software supplied with the camera) gives you access to all camera functions and modes.
The Live View can be displayed on the computer screen for a easy to see, and focus, picture. The Live View magnification works with this program which lends itself to accurate focusing of the telescope. Further, all photos are downloaded to the computer providing more image storage than what is available on any Compact Flash Drive.
Since the camera’s shutter release can also be triggered through the computer program the camer remains untouched and steady throughout the photo process.
January 17th, 2008 at 2:44 pm
How do you couple your camera to the telescope??
January 19th, 2008 at 10:41 am
A camera (35mm film or DSLR) can be coupled to a telescope in several ways. First though; what type of telescope are you using: Refractor, Reflector, or Schmidt Cassegrain?
For any telescope you can use a Piggyback mount. This is probably the best way to start learning how to take astrophotos. The camera is simply mounted to the top of the telescope using a bracket. This “camera mount” has a 1/4 x 20 threaded bolt that screws into the bottom of the camera. You can then use any lens on the camera for wide field photography and the telescope is the tracking platform.
With most astrophotography you will need a “T-ring†to couple the camera to the various pieces of equipment. The T-ring has internal threads and is attached to the camera in lieu of a lens. This configuration allows for the attachment of several pieces of astrophotography equipment. These T-rings can be purchased through any telescope dealer and are made for specific cameras. If, for example, you are using a Canon 40D, you will need a T-ring built for a Canon EOS.
For a Schmidt Cassegrain there are several ways to couple the camera to the telescope:
1. Prime Focus Camera Adapter – screws directly into the T-ring. This entire assembly (camera, T-ring, Adapter) then attaches to the back of the telescope in place of the eyepiece. Simply speaking, you are using the telescope as a big lens for the camera.
2. Tele-Extender – is used for high power “eyepiece projection” photography. The Tele-Extender fits over an eyepiece and couples to the camera via a T-ring. This entire assembly attaches to the telescope. The camera takes the place of your eye, so to speak, and looks through the eyepiece. These Tele-Extenders are available for all types of telescopes.
3. Off-Axis Guider – is similar to the Prime Focus Camera Adapter, but has an eyepiece attachment set at 90 degrees to the line of sight. The Off-Axis Guider screws into the T-ring allowing for attachment to the telescope. An Illuminated Reticle Eyepiece (a special eyepiece that has illuminated crosshairs) is set into the Off-Axis Guider. The guider has a small prism that picks off a portion of the field of view and projects it up through the Illuminated Reticle. By finding a star within this small field of view the observer can then use the telescope mounts directional controls to keep the star centered in the Illuminated Reticle during a long exposure. This is a complicated setup which requires good Polar Alignment
and a great deal of patience and practice.
The Prime Focus and Tele-Extender applications can also be used with a Reflector as well as a Schmidt Cassegrain.