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Bikeman55

Coma corrector backfocus vs camera backfocus

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Bikeman55    7

I have an Orion 8" astrograph f/3.9. I'm looking to purchase a Baader 2" Multi Purpose Coma Corrector - MK III, I'm currently imaging with a DSLR which the backfocus should be fine. My question pertains to looking into the future. If I purchase this coma corrector now, which says it has a backfocus of 55mm, and later upgrade to a different camera, say a ZWO ASI178MM, will I still be able to use this coma corrector with that camera when the ZWO camera says it needs a backfocus of 17.5mm.

How does the backfocus of the coma corrector relate to what the camera wants for backfocus.

Hopefully I've worded my question well enough. Any help would be appreciated.

Thanks in advance.

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symmetal    79

The term backfocus is very misleading and means different things to different people.

The 55mm 'backfocus' of your CC is the distance required between the CC back plate and the image sensor. A Canon DSLR has a 'backfocus' distance of 44.1mm between the sensor and the front of the lens flange. The T adapter has a depth of nominally 11mm. 

The 44.1 + 11 is about 55, the distance of your CC 'backfocus'. Hurrah!

Your 17.5mm 'backfocus' of the ZWO is the distance between the sensor and the front plate of the camera. This means you would need a T extension of 55 - 17.5 = 37.5mm to achieve the CC 'backfocus' distance.

The small chip size of the ZWO compared to your DSLR means that the CC probably isn't needed as only an area  equivalent to a centre cut-out of your DSLR is being used so the coma error will be much reduced.

Alan  

Edited by symmetal
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Bikeman55    7

Fantastic. Thanks so much for the response. That perfectly answered my question.

Question seems stupid now that I know the answer, I should have known that but thank you for the quick response. Much appreciated.

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symmetal    79

Glad to help.:smile: The question isn't stupid, but having the term backfocus to mean how far the sensor is mounted from the front of the camera is. :wink2:

Alan

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