johnpd I have heard that filters should be positioned immediately before the camera with no intervening extensions. Is this correct?
A qualified yes.
It is true for most cases when you are doing prime focus imaging, where the rays converge from a large objective/main mirror onto the sensor.
However, if you have a flattener or reducer whose exit pupil is smaller than the camera sensor diagonal, then the opposite may be true.
You can roughly measure the diameter of the last glass element of the OTA that is closest to the camera. If the glass diameter is larger than the diagonal of the sensor, then the filter should be closer to the sensor (unless that glass is oversized -- that is where the ray diagrams come in very helpful). However, if that glass is smaller than the sensor, then it is better to place the filter closer to the reducer or Petzval element.
Chen
