Back Focus

Backfocus Theory

The back focus distance is actually a property set by the optics (i.e. the telescope, the focal reducer, field flattener, etc). The backfocus distance is the distance required by a perticular optical element in order to project a focused image on top of a camera sensor. Everything that you put between the optics and the camera sensor, must add up to the total back focus distance required by the optics. 

When you use a focal reducer or a field flattener, the backfocus distance that you need to achieve is often the number given by the manufacturer of the focal reducer or the field flattener that you are using.

In the case of the cameras OGMA AP26CC and AP26MC, the distance from the front face of the camera to the sensor is 17.5mm. We refer to this parameter as the backfocus distance of the camera.

For example, in order to achieve proper focus with a telescope that requires 55mm of backfocus, you'll need to substract 17.5mm from 55mm. The resulting 37.5mm is the distance that you have to put between the camera and the telescope. You can achieve this distance by combining filter holders, extension rings, adapters, filter wheels, OAGs, etc.

Since it is impossible for us to know what devices you own, we can't tell you exactly how you will achieve these 37.5mm (from the example above). 

If you are using a refractor telescope, often the focuser will provide enough movement to reach the backfocus distance without the need to add other devices or spacers between the telescope and the camera.

If you can't reach the backfocus distance by moving the focuser, (this is often the case when using a focal reducer) you will have to look at the specifications of the elements that you plan to connect between the camera and the telescope and fill the gap with extension rings.

For example, M48x0.75 extension rings from different brands can be found at Amazon. Depending on the diameter and thread on the back of your telescope, you may need different adapters or extension rings.

Backfocus Formula

A formula to calculate how much extension rings you need to fill the gap to achieve the backfocus set by the optics is the following:

ER = BFopt - BFcam - BFother

Where:

ER: Width of extension rings

BFopt: Backfocus distance required by the last optical element (i.e. the telescope, focal reducer, or field flattener)

BFcam: Backfocus distance of the camera

BFother:  Backfocus distance of all other elements between the camera and the last optical element (i.e. filter holders, extension rings, adapters, filter wheels, OAGs, etc)