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backfocus


coppernob

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Dear all,

I am relatively new to the concept of backfocus. Please can you advise which spacers would be required for the following combinations of equipment:

1. DSLR to C9.25 and 0.63 Focal Reducer

2. ZWO ASI224MC to C9.25 and 0.63 Focal Reducer

3. ZWO ASI224MC to Altair starwave ED80 with 0.8 Focal reducer

many thanks in Advance. Please be gentle with my ignorance

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It might help if you told people whether you have a focuser on the back of your C9.25 or if you are connecting straight to the visual back or even through a mirror diagonal.

I have the 9.25 with a Revelation Superfocus 2" Rack n Pinion  Focuser. I then plug my 1100D or my QHY5LIIc straight into that. I don't have any problem with obtaining focus with either camera.

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6 hours ago, coppernob said:

Dear all,

I am relatively new to the concept of backfocus. Please can you advise which spacers would be required for the following combinations of equipment:

1. DSLR to C9.25 and 0.63 Focal Reducer

2. ZWO ASI224MC to C9.25 and 0.63 Focal Reducer

3. ZWO ASI224MC to Altair starwave ED80 with 0.8 Focal reducer

many thanks in Advance. Please be gentle with my ignorance

1 DSLR
The focal reducer is supposed to have a back focus of 105mm.
Subtract the DSLR flange focal distance, for Canon 44mm also subtract the t-ring thickness.
If the t-ring is 11mm you will subtract 55mm from the 105 which leaves 50mm for extra spacers.

2 ZWO ASI224MC
In this case the flange focal distance is 12.5mm, this is from sensor to end of mounting threads.
So subtract 12.5 from 105mm which leaves approx 92.5mm for spacers.

3 ZWO ASI224MC
The focal reducer in this case will no doubt have a back focus of 55mm.
Subtract 12.5mm from 55mm which leaves 42.5mm for spacers.

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Thanks guys and yes I forgot to add in a skywatcher dual speed crayford focusser. I dont usually have any problems achieving dslr focus without the focal reducer but adding one in causes me difficulty.

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