The next stage of building the really big camera. I had chosen the design used by Warren De La Rue for his moon camera. In my notebook was the mount for the mirror and my D750 DSLR. What I had not mentioned was the DSLR was not there to capture an image; I also use it as a measurement tool when constructing optics.
This may not be an intuitive use but think of it this way. You probably own a smartphone. Is the major use as a phone or do you do lots of other things with it? Maybe reading this page or viewing the IPE?
Same with a camera - the optics can do so much more than simply record an image. I use it to locate the focal point of optics; in this case the mirror. I move it until I can see the "D750" label clearly in focus within the viewfinder and take a measurement with a tape measure to find the distance between camera and mirror. In this case spot on 2000mm.
I didn't need to take a picture to do this. But if I didn't it wouldn't be photography would it! One selfie of my DSLR, shot in an unusual manner.
Tech bit
If you place an object (a DSLR in this case) at the focus of a concave mirror the image of the camera is placed at infinity. So if the DSLR lens is also focussed at infinity it can see itself sharply in focus. I used a 200mm lens to get the scale right, set the focus to infinity and when I could see the front of the camera sharply in focus I had my measurement. The lens aperture was set to f/8, wide enough to place the mirror mount out of focus.
A little bit of veiling glare in the image. Must clean that mirror!