Edge HD 8 Backfocus

Here’s another common issue with AP imaging using the Celestron Edge HD 8, backfocus.

Because there is so much gear variance in AP, sometimes the manuals just don’t have what you need in the format you need it. Backfocus in inches not mm, information in weird places, etc.

When I first got my Edge and I knew I needed to use an off-axis guider (OAG) it took me way too long to get the right backfocus dialed in based on disparate information from multiple sources. At the end of the day, combining the information from the scope and reducer manual was the trick to getting what I needed done.

Natively, the backfocus requirement for the Edge HD 8 is 133.5mm. With the .7x reducer, it’s 105mm. Both the Celestron T adapter and Starizona one come in a convertable cofiguration, with a 27.5mm spacer that can be removed to hit 105. What those products do is extend the SCT adapter to marry up with the standard 55 mm backspacing as specified in most imaging manuals for the cameras, for me, the ZWO guides are the resources to figure that out based on your individual config.

So, to put it in short, both T adapters will make up the difference for both methods, all you do is hit the 55mm related to your equipment.

A quick note on off axis guiding, the total distance in backfocus has to be the same for both the guide camera and imaging camera, making them parfocal. The 105 or 133.5 mm spacing is direct from the rear element of either the SCT or the reducer to the imaging sensor, and along the L shaped trajectory (with the pick off prism at the axis) to the guide camera sensor. It takes a bit of fiddling, so do this in daylight. It’s much easier to get the main camera in focus then work on the guide camera.

For the visually inclined, video below.