High quality, fast refractors have a lot going for them when it comes to astrophotography but they come with a downside - field curvature. This is where the image comes to sharp focus on a curved surface. The problem is that the sensors in cameras are flat. The larger the sensor - like those used in cameras with APS-C sized or larger sensors or DSLR cameras - the more effect is noticed.
A field flattener brings the image to focus on a flat surface, like your camera's sensor. At the same time, the basic optical performance of your telescope remains the same.
You will need 55mm of backfocus.
The Bintel T-thread Spacer Ring Kit (sold separately) is recommended to achieve proper distance for use with CMOS imagers.
Use with a DSLR only requires a T-ring for the camera (T-ring sold separately).