My Canon 200mm SLR imaging setup

I have been in the hunt for a wide field imaging setup for my SXV-H9 camera. I had tried a Takahashi FS-60C but was unhappy with the color correction and it wasn't quite as wide a field as I was seeking. It is a wonderful little visual gem but not what I needed for imaging. I read about using Canon SLR lenses for astroimaging and decided to give it a try. The Canon 200mm f/2.8 EF lens is a well regarded lens and cost less than the focal reducer plus adapters I needed for the FS-60C! So I sold the FS-60C and bought the Canon 200mm lens as well as another daytime lens with the money from the sale.

There are some difficulties with using a SLR lens with a traditional filter wheel/CCD setup. First I needed a custom adapter so I could attach the SLR lens to my filter wheel. I found a fellow imager who had an adapter custom made who just happened to want to sell it, so I solved that part of the problem. The other potential difficulty is the SLR lens wants a short distance (approx 44 mm for the Canon EF) between the lens back and the sensor chip. Fortunately the setup I have (FLI CFW-1 filter wheel and a SXV-H9 CCD) gives me exactly the right distance.

Finally there is the problem of how to mount the SLR lens. Typically these are meant to hang off the front of the camera and do not always have a tripod collar available. My solution was to use some guide scope rings to secure the SLR lens as well as the CCD camera. The SXV cameras have a nice round metal casing which you can secure with a ring the same as you would a telescope. I put the rings on Losamandy DA parts which let me slide the rings to where I need them and then tighten them down. All this is mounted on a Losmandy DMM (male to male) plate which slides its the Robin Casady saddle on my Atlas EQ-G mount. I just adjust the 3 retaining screws on the 2 rings until I get the lens/CCD combination pointing approximately streight w/o any tilt.

I wanted to be able to autofocus the SLR lens so I took a Robofocus motor from my C8 which has a nice sprocket on its shaft. I mounted this using some aluminum plates which come with the Robofocus motor. It is attached to another Losmandy DA plate that I can slide back and forth so the motor is lined up with the focus ring on the SLR lens. The Canon lens I have uses internal focusing, so the length of the lens does not change as you focus. This is important so that you can attach a ring to the SLR lens.

I found a vacuum cleaner belt at the local hardware store which fit snuggly around the SLR lens focus ring and the sprocket on the Robofocus motor. Everything is either on a sliding platform or has lots of screw holes so I can move everything around until its snug and then I tighten all the parts down. Then I added the all important rubber band you can see in the picture to keep the belt well tensioned. The belt is still loose enough that if the lens hits its stop at either end of it focus travel that the belt will slip and not bind up the stepper motor.

I added a guide scope to the front so I can image and guide with a very small package that easily fits on my lightweight Sirius EQ-G mount.

Return to articles index