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Old 19-01-2019, 01:01 PM
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PRejto (Peter)
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Join Date: Jan 2011
Location: Rylstone, NSW, Australia
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Thank you all for the thoughtful replies. I really appreciate them. Playing devil's advocate for the sake of discussion....



Consider the following. The Blue DS filter appears to be "out" by 1097 + 866 = 1963 steps = 216 microns. Worst case by design for my TEC180FL should be 60 microns, If this is correct (and it appears to be in spec using the R/B Type 2 filters) then I must account for an additional 155 micron shift between Red and Blue.


Consider thickness: Glass will shift the focus by ca 1/3 thickness. Astronomics are 1mm (1,000 microns) thick. To shift by 155 mm compared to the other 1 mm filters, the offending filter would need to vary in thickness by 465 microns! I cannot imagine that any reasonable manufacturing process designed to make a 1 mm thick filter could result in a filter thicker or thinner by nearly 50%.


As far as focal shift due to wavelength and index of refraction, I estimate that BK glass 1 mm thick will shift the R/B by 14 microns. I don't know what type of glass is used in these filters but it seems highly unlikely that refraction effects of the media would account for 155 microns.



So, perhaps the offending filters are leaking UV or IR. How might I confirm or refute this possibility?


Please let me know if I've stuffed up any of these calculations.


Thanks,


Peter
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