Back focus distance for Celestron 0.63x reducer on NexStar 6SE

Celestron NexStar 6SE
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Christophe tonnelier avatar
Christophe tonnelier avatar

Hi everyone,

I'm trying to nail down the correct back focus distance for the Celestron 0.63x reducer #94175 on my NexStar 6SE, and I'm getting confused by conflicting information.

My current imaging train:

- NexStar 6SE

- Celestron 0.63x reducer #94175 (measured thickness: 29mm)

- Celestron T-Adapter SC #93633-A (measured length: 50mm)

- M42 extension ring (12mm)

- T2 Canon adapter ring (11mm)

- Canon 6D (modified, back focus: 44mm)

I've read that the 94175 reducer requires 105mm of back focus, but I cannot find a clear answer to a specific question:

Is the 105mm back focus measured from the rear face of the reducer, or from the front face?

With my current setup I'm getting full frame coverage on the Canon 6D (no vignetting), but the stars are not perfectly sharp across the whole field and I suspect a back focus issue.

Has anyone used this exact reducer with a Canon DSLR and found the correct spacing? Any measured train that works well would be very helpful.

Thanks in advance!

Christophe

Well written Respectful Concise
Spacey avatar
Spacey avatar

Hi there, don’t add the reducer thickness to the measurements, measure from the rear of the reducer, same if the reducer is not attached.

You should not be getting a sharp stars across the entire field a standard Celestron, especially on a full frame sensor.