Celestron 8" SCT, with f/6.3 reducer and ZWO ASI Cooled cam

orangemaze

What is the correct back focus? I am being told 100 different things by everyone I ask. I know the cooled camera have a normal back focus of 55mm. With a focal reducer, where do I start the measurement from? The end of the glass or the end of the t-adapter?

Support@ZWO

Our cooled camera have 2 nature back focus.

6.5mm BACKFOCUS, M42 male threads, without 11mm ring.

17.5mm BACKFOCUS, M42 female threads, with 11mm ring.

I don't know the back focus of f6.3 reducer, maybe you can ask celestron for a drawing, so I can tell you how to measure the back focus.

orangemaze

So the length can be variable, and is not a hard number (I am not counting the male / female threads part). Depending on different optics, it can be longer, would the distance from the glass to the sensor of 115mm sound about right?

nadiraybek

it depends on which focal reducer f6.3 .

Because Antares , Celestron and Meade or other suppliers have f6.3 but with different focal length reducers . All of them have different back focus .

orangemaze

On this page (https://astronomy-imaging-camera.com/manuals/QuickGuide-cooled.pdf) under 'Connecting the Camera to the telescope' it shows a 55mm back focus. If we deduct distance of the sensor to the camera case front of 6.5mm shown two images below that, we get 48.5mm. Or does the back focus change, depending on the reducer? If so, the manual needs to be more clearer, and perhaps give the formula needed to determine the correct back focus if possible.

orangemaze

Also on the diagram showing the various options of connecting. I am doing 8 (focal reducer) to 5 (5. M42-M48 16.5mm extender), 5 does not fit on 8.

Support@ZWO

It's depand on the back focus of reducer, we provide some adapters to help you reach back focus of reducer, but if it can't be, maybe need some customize adapters for your reducer.