backfocus

[Andy Li]

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

[Gary Palmer]

The C9.25 will be around 105mm from the sensor to the back of the reducer. It would be checking with Celestron on this as its for an older camera Canon 450d. The asi 224 wont be so critical due to size of the sensor. The Starwave 80mm will be according to the type of 0.8 reducer as they all differ in spacing. So a Skywatcher reducer wont work on all telescopes though they generally say they do.

[Andy Li]

Thanks Gary

the 0.8 reducer is from altair astro. There is also the complication of a skywatcher 2 speed crayford on my c9.25. When people quote spacer distances – they do mean approximate distances don’t they?

[Gary Palmer]

Hi Andy, it does depend on the size of sensor. The larger like a DSLR the more critical it has to be but 1 or 2 mm is not a problem in most cases. The 0.8 Reducer needs 55mm back focus so you will need the sensor to class measurement from ZWO for the camera then add spacers tp make it 55mm to back of 0.8 reducer not the lens inside.

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