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.

Rick Veregin avatar
Rick Veregin avatar

Hi Christophe,

The measurement is from the top thread on the reducer, so reducer is not included. What you showed though is not looking right, you need the T-Adapter at 50 mm, the T2 ring at 11 and the 44 flange distance on the DSLR to give 105 mm. The M42 extension ring gives you too much distance, why are you using that?

This should work well, it did for me when I used my 6SE.

As a check if you are too close stars in the corners may look like they have small "tails" pointing toward the center (coma).

If you are too far stars may appear as concentric swirls or arcs.

The f/6.3 reducer is not perfectly flat-field, stars at the edges will not look their best. Focusing on a slightly off-center star helps balance the effect, but you do give up some sharpness in the center of the field if you so that.

I now use the Starizona reducer, it is pricey, but the field is awesome to the corners if you are well collimated.

Here is a good on-line tutorial about the backfocus.

https://www.youtube.com/watch?v=-M7bINU-44Y

Rick

Well written Helpful Engaging
DavesView avatar
DavesView avatar

Like Rick says:

📷 BackFocus.pngBackFocus.png