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Trees At A Distance

My camera as an image sensor

Building stuff - moon camera part 4

Time to open the throttle and go full scale. This time there was no lens involved at all, just a DSLR image sensor and the 35cm mirror. In this case less means more - more magnification and more difficulty.

I didn't fancy having my D750 open to the dust and flies of the back garden so I used my old Fujifilm S1 Pro, bought second hand many years ago for similar work. Mounted on a tripod with just a 6cm hollow tube as sunshield where the lens should be. The camera had to be accurately positioned in the focal plane of the mirror, just as Warren De La Rue had done with his moon camera.

The depth of focus of a 2000mm f/6 mirror is really small and really not to scale with the size of these optics. But with a little persistence you can get it in position with an image you can recognise - result above. Unimpressed? Take a moment to see stuff in this image.

First of all this is a huge magnification - we are effectively using a 2000mm telephoto here. This tree top is 600m away and just to the right of the chimney in my pre-flight image. Second, there is nothing holding mirror and camera together. Try and take a picture with a camera lens not firmly attached to your DSLR then imagine that at 2000mm focal length. Third, there was 600m of humid, easterly wind with a low sun. I think there was more air than tree in this image. And finally, this mirror still needs a clean and there was a lot of stray light in the camera tube.

But I got the image and the moon camera works. First light achieved for this telescope!