A Question of Back Focus

borgboy

Hi ZWO,

The back focus on the ZWO ASI 1600 is quoted as 6.5mm. And its shown as a physical dimension on the drawings.

When working out the correct spacers for my focal reducer I need to add 0.7nm for the optical path difference caused by the glass thickness of my filters.

Do I need to add anything for the optical path difference caused by the thickness of the protective window on the camera? Is the 6.5mm figure a true optical path length, or a physical path length?

If the 6.5mm is a physical distance, then can you tell me the thickness of the protective window please.

Apologies if this has been asked before.

Regards

BorgBoy

Sam@ZWO

6.5 mm already calculated the thickness of protect window

dmcarlsn

Hi Sam,

Please provide further clarification on this subject.

Which of the following statements is true:

1) The 6.5 mm number is the physical/mechanical distance between the sensor and the protect window and does NOT compensate for the refractive effect of the protect window.

2) The 6.5 mm number is the effective optical path length between the sensor and the protect window and DOES compensate for the refractive effect of the protect window.

Thanks,

Dan Carlson

merlin66

Dan,

I think you'll find Sam was saying #2 is correct - 6.5mm being the "effective" back focus with the protective window in place.

Support@ZWO

Effective optical path length of our camera is 6.5mm.

dmcarlsn

Thanks Sean!

Regards,

Dan Carlson