When an optical system requires the sensor of the camera to be located at a specific distance from the last optical element, mechanical spacer adapters are used to position the camera at the right location. If the camera system also includes a filter wheel and an OAG, for example, between the camera and the telescope, the space required for these items must be taken into account. As a simplified explanation, for example, if a telescope’s field flattener requires the camera focal plane to be 55mm behind the last optical element, and the filter wheel is 20mm thick, and the OAG is 10mm thick and the sensor is set 10mm inside the camera then these items all take up 20+10+10=40mm of the back focal distance. Still, the camera needs to be 55mm away so we need to add spacers that take up this remaining 15mm. This also means that if you have an optical system that requires 55mm of backfocus distance and your filter wheel is 25mm thick and the OAG is 20mm thick and the sensor in the camera is 17mm from the front plate, then these items take up 25+20+17=62mm of backfocus. Too much! In this case something has to go to reduce the total back focus distance of the camera to 55mm or a different optical configuration must be found.
For the convenience of calculation, all the data displayed are the increase in the back focal length taken up by using the adapter, which is not necessarily equal to the thickness of the component itself. All the adapter data provided by QHYCCD are based on the focal length increment.