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Back focus spacing for WO61 refractor and flattener


Photosbykev
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I've just acquired a ZWO533MC-Pro to mount on the back of a William Optics Zenithstar 61 refractor with the dedicated PFLAT61A flattener.

I have been using the refractor and flattener with a Canon DSLR and have set the adjustable flattener to 12.9mm when using the T2 mount and the stars are nice an sharp corner to corner.

Anyone using the WO61 with dedicated Flat61A adjustable flattener with a cmos camera? The back focus according to WO is 67.7mm from the rear shoulder of the flattener to the sensor BUT is this with the flattener adjustment set to zero mm adjustment rather than 12.9mm? and can I use that screw adjustment to set the back focus to exactly 67.7mm after fitting extension tubes and filter drawer in the optical path?

regards

Kev

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Edited by Photosbykev
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The 67.7mm distance is with the flattener adjustment at zero. Using a DSLR and T2 adapter you have the standard 55mm between the front of the T2 adapter and the sensor. Hence the setting of 12.9 on the flattener adjustment to make the total 67.7mm distance, (within 0.2mm).

You can use this 0-15mm adjustment on the flattener to achieve the total of 67.7 when using your filter drawer and extension tubes etc.

Alan

 

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Thank you both for confirming my thoughts on this. I suspect I won't run into any issues with stars in the corners using the small sensor on the ASI533 even if I removed the flattener. As it doesn't change the focal length of the refractor I may as well use the screw adjustment built-in to fine tune the back focus adjustment.

17.5mm - ASI533MC-Pro camera front face to sensor

17.5mm - Starizona filter drawer

so I need around 32.7mm extension + 0.6 or 0.8mm for the filter thickness so approx 32.5mm extension tubes

Kev

Edited by Photosbykev
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Yes, you may as well use the flattener as it can only make the stars better, considering adjusting the necessary spacing is fairly easy with this model. A standard 30mm T2 extension tube with the flattener set to around 3mm should be a good starting point. :smile:

Alan

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