4 inch Cassegrain Field Flattener Back Focus

[nakbrooks]

The specification for the ASA 4CAKORRFL 4” field flattener (http://www.astrosysteme.com/download/4-cassegrain-field-flattener/?wpdmdl=2977) says that the field flattener should be placed so that its centre is 124.5mm from the CCD.

 

The adapter supplied to me by ASA puts the centre of my field flattener 127.4mm from the CCD, 2.9mm more than the specification suggests.

 

The filters and camera glass are in the light path so maybe their refractive index affects the calculation, but surely not by as much as 2.9mm.

 

Is this difference likely to be critical to the performance of the field flattener? Should I insist ASA provide a revised adapter?

 

[nakbrooks]

Just checked the Astrodon and FLI specs.  The filters are 3mm thick and the camera window 3.18mm so this implies a focus offset of around 2.2mm I think.  So maybe the ASA-supplied adaptor is putting the flattener around 0.7mm too far away from the CCD.  I doubt if that is enough to make a practical difference to the performance of the flattener so maybe this isn't the source of my ongoing focus problems.

[lukepower]

Nigel, have you checked the distance primary-corrector and secondary-corrector? While it is no thighly critical, the optician from where I got my mirrors (Zen from Venice) told me that it should be within +/- 5mm of the correct position, otherwise it wouldn't be able to correct all errors correctly.

In my case, I missed the correct distance by about 2cm and it made a difference of about 1.8" FWHM stellar diameter (plus quite bad correction off-axis)

[nakbrooks]

I'll look at that Luke.