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I am setting up an Explore Scientific HR Coma Corrector and the manual states 55mm back focus but doesn't say where from !

Would you say its from face A in the first photo or face B in the second photo ?? There is a 3mm difference when the adapter is screwed on.

Thanks in advance.




Edited by Skipper Billy
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Hard to tell, but easy to find out:

It looks like the M48-M42 converter has a 3mm optical length when totally screwed on. 
By the looks of it and the place of the setting screw, it is meant to be able to fine adjust the distance.

So I would go some where in the middle: 55- 3(+2mm range for fineadjust)= 50mm 

Your cam and fw need 35 mm, so a 15 mm spacer is needed.
So I think that with a 15 mm spacer you will have enough adjusting room to get a good flat field.

You will have to find the sweet spot (distance wise) anyway. It is never exactly what they tell you...


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I emailed ES in Germany and they confirmed that the spacing distance should be measured from face A as shown in the top photo - just posting in case anyone comes across this thread looking for the answer to this question.

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  • 2 years later...


I've just bought this coma corrector  - and i intend to connect it directly to my QSI camera,  and based on imaging at 1m F/L the working distance is 56mm.  The outer flange on camera is 50mm from the sensor so i need to add in an additional 6mm. So am i right in reading this, that the included adaptor adds 3mm, so i need to add in an additional 3mm ?

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