Astrophotography with a Canon 40D

Revised 25th June 2011

 Tips and foibles

The Canon 40D is a very versatile camera but if you use it for Astrophotography there are a few pitfalls and tips that I have discovered that may help you with your Astrophotography. These tips are for the 40D but may well apply to later models as well.

I have used my 40D to take Astro photographs both with Canon lenses for wide angle views and mounted at the prime focus of both Schmidt-Cassegrain telescopes and a Skywatcher Pro 80ED refractor.

If you use Canon Lenses with auto-stabilisation on a tripod at high elevations to take astronomical pictures it appears that the circuit that detects that the camera is mounted on a tripod fails to turn the auto-stabilisation off. This results in some very strange star trails. The solution - turn the stabilisation off using the manual switch on the lens. The images below show the effects you get - it was very confusing at first as it only occurs when the camera is pointed above a certain angle.

The Live View mode is very useful for focusing on astronomical objects as it provides live feedback on how close to focus you are so you can make that last little tweak to get it correct. This is enhanced by the ability to zoom into the image by up to 10 times. However live view warms the CCD up and increases the noise level so you have to leave the camera for several minutes to cool down before you take an image after using live view.
There is a way round this which is to use the supplied video cable to view the live image on an external TV or monitor. When I read the manual I thought the external video cable could only be used to view images during playback but it appears that anything that is shown on the display can be viewed on the remote monitor. This includes Live View as well as all the menus.
This also helps solve the heating problem - I have not found out if it helps the battery life significantly.
The use of an external monitor gets over the other problem of being able to see the screen at high elevations without laying on your back !
It looks as though you can use this mode to record a signal that can be processed directly by Registax or similar software but I have not tried this yet. Remember to turn the recorder off before you look at the menus.
You may find the image is rather dim - if so you need to put the camera into Manual mode and then increase the exposure time. As the shutter is open all the time in live view I am not sure why this works but it does !

Most of us are plagued by Light Pollution in particular the orange skies resulting from light pollution from Sodium Street Lamps. There is a quick way of reducing the effect of this - alter the colour temperature setting of your camera or use the electric light bulb setting if your camera does not allow you to set the colour temperature. If you reduce the colour temperature this reduces the cameras red sensitivity and increases the Blue Sensitivity. As a 'orange' glow is towards the red end of the spectrum this results in the sky turning from orange to black. This only effects JPEG images not RAW images. The only downside is that it reduces the cameras sensitivity to red objects such as H-Alpha regions but you will not be able to image these from a light polluted location anyway.

A couple of useful programmes have been recommended to me recently:

The first is BackyardEOS at  http://www.backyardeos.com/ this programme allows you to control a Canon EOS series camera remotely and has a number of features designed to assist you when using an EOS camera to take astronomical images. I have not tried it yet but it comes with a recommendation from a fellow EOS imager.

One thing you may notice when you look at the instructions is that BackyardEOS measures the camera temperature when a image is taken. After a bit of hunting around the Internet I found that the EOS cameras do measure the temperature of the imaging chip, presumably for the noise reduction algorithm & to operate the sensor over temperature warning. This information is recorded in the EXIF data in both .JPG & RAW files but the Canon software does not display it. After some more hunting around the internet I discovered ExifTool at http://www.sno.phy.queensu.ca/~phil/exiftool/ which allows you to download all the Exif data including the temperature. You can write scripts using this so that the temperature the image was taken as can be written as part of the filename. This then allows you to see which darks & flats match the temperature the image was taken at. Monitoring the temperature also allows you to see how effective your cooling strategy is.

John Murrell

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