What Does "Parfocal" Mean for Filters?
I bought these filters to replace an existing set because they are described as "parfocal". For those who are used to the application of this term to eyepieces or other equipment where "parfocal" means I can swap one item for another and the focus position is unchanged, it is helpful to know that is NOT what "parfocal" means for filters. I think this term is probably used in the same way for all filter manufacturers. In particular, if you measure the focal position of a telescope for the R, G, and B filters relative to the Luminance filter, the filter "offsets" may NOT be 0, 0, and 0. According to the manufacturer, the offsets may be 0, 0,and 0 for a reflector, but they are definitely not for an SCT. I have not seen a discussion of this behavior and it may not be commonly known, especially if buyers do not independently check the focus offsets after purchase and installation. I believe it would be very helpful if the product information would explain what I have just described so that buyers could make a more informed purchase decision.