In the few days I had the camera I was determined to acquire some astrophotography test shots with the EOS-M camera. Even if it meant getting up at 3am to have some dark sky after moonset. It would have been easier a few days before, but a Pacific storm system had provided several days of overcast with occasional rain. This particular morning was just about perfect, clear skies, decent seeing and no wind to bounce the telescope around.
For testing I used the same setup I often use with my Canon 20Da or 60D. An Astro-Tech 6″ (150mm) Ritchey–Chrétien telescope riding atop a Losmandy G11 mount. A 0.8x focal reducer has T-thread at the rear allowing a Canon EOS lens adapter. To attach the EOS-M I used the Canon M Mount to EOS Mount adapter. An SBIG STi autoguider completes the setup.
The result is an f/7 optical system with 1080mm focal length. This gives a field of view of about 72×48 arc-minutes (1.2 x 0.8 degrees) on the sky when using a camera with an APS-C sensor.
I have previously written about the unpleasant discovery that Canon has removed remote or tethered shooting features from the camera. This one feature deficit ruins any real possibility that the EOS-M is a real consideration for astrophotography, or indeed any sort of time-lapse or similar application. With the camera in hand, and the RMA number pending, I still wanted to give it a real try. There is hope that a later software revision, or another model using the M-mount will be a possibility.
Focus was no problem, a few pokes at the camera’s touch screen provided a magnified view of Sirius. Put a Bahtinov mask on the front of a telescope and adjust the focus to achieve the correct diffraction pattern on the screen.
The first issue was finding that the camera would not release the shutter when attached to the telescope. This led to several minutes of reading the manual without finding the solution. Followed by several more minutes of poking through the camera menus. I was guessing that the failure to release the shutter was some sort of focus issue, I was looking through the settings for manual focus. Eventually I located the custom function that allowed the shutter to release without a lens attached. Once set, the camera would now take an exposure on the telescope. Another small victory.
Without remote control or an intervalometer, it was frustrating to get the sequences of exposures needed to assemble an astrophoto. I found that when setting the camera for touch screen shutter a mere brush on the screen would trigger another exposure. With a slight enough touch that the autoguider would not complain. Add a two second shutter delay to allow any vibration to settle and it worked. Through forty 30 second exposures I sat beside the telescope playing cribbage on my iPad, lightly touching the back of the camera every time I heard the shutter close. Then do it again for the necessary dark frames.
Longer bulb exposures can be realized using the IR shutter release or through using the touch screen shuttering. When set for bulb, touching the screen will start an exposure, another touch will end the exposure, with a timer presented on the screen throughout the exposure.
The lack of a shutter release connection may be a serious issue for star trail photography. Any delay between exposures will create small gaps in the trail. With an electrical connection this gap can be set quite small, a pulse a few hundred milliseconds long works well with other EOS cameras. It seems unlikely that this will be possible using the IR trigger in bulb mode.
The short battery life will also be an issue for long sequences, with only one to two hours of continuous shooting possible. The EOS 60D will do at least six hours on one battery, even on a cold night. There is a small opening through the battery compartment door that can be used for a dummy battery style external power supply. The manual lists the availability of an AC Adapter Kit ACK-E12. Though why this in the least desirable without remote control of the camera is a mystery to me.
For anyone who would like to look at unprocessed data from the camera I have included three files. These are 16bit TIF files converted from the camera raw using Images Plus 4.25, they have been debayered to “At Cature White Balance” and nothing else. There are two images of M42, one at 240 seconds at ISO 12,800, another at 30s with ISO 3200. I have also included a 30 second dark taken at ISO 12800. If you feel the need to see some other file, or the raws themselves, drop me a comment with your email in the appropriate field, I’ll send it to you.
The camera dark frames are particularly pleasant to survey. A notable lack of hot pixels, no particular amplifier glow or other signals visible, no objectionable pattern noise, only a minor top to bottom gradient. Dark frames like these are just what you want for low-light shooting!
I would like to say that the EOS-M is a great astro camera. It could be, if it were not for the elimination of key features by Canon. The lack of either an external shutter release connector and the ability to remotely control the camera over the USB port has completely crippled an otherwise good camera. The photos I did take show the potential of the camera for astro applications. I can only hope that Canon wises up and either includes remote control in a later software revision, or releases another camera with these features that uses the mirrorless EOS-M lens mount.