Please excuse my message if I’m preaching something we already know or if I’m totally misunderstanding this request. But, I don’t think we can ignore the 0 degree and 90 degree birefringent requirement. I believe it is an integral part of the filter the customer is requesting. Please see attached on antialiasing filters and I highlighted the part which indicates to me that the birefringence is an important factor on this lens. So, I totally agree with the above message from HCO indicating that BK7 probably isn’t the right material to achieve their requirements.