Dan Brown:
While lying in bed trying fall asleep last night I had a thought. Is it possible that filters improperly mounted in a filter wheel could cause image defects? What spurred this thought was my Pegasus filter wheel. The manufacturer's intent is to use three screws with washers to hold the filters in place. If the screws are tightened I would expect the filter to be deformed. What I don't know is how much affect a flat filter that is slightly distorted will impact our images. Any thoughts?
Dan
First off, I totally agree about the poor way that filters are mounted in the Pegasus FW. It will distort the glass and can even chip a filter if the screws are over tightened.
When a plane parallel plate is used in transmission, the transmitted wavefront is very insensitive to mechanical deformation (assuming that it's not made from a birefringent material). That's because the beam deviation at the front surface is proportional to n-1, where n is the refractive index and the deviation at the front surface is undone by the rear surface. The refractive index of glass is about 1.5 so a refractive surface only introduces about half the optical deviation of the surface. In reflection, any surface deviation is doubled so a reflected beam is deviated about 4 times more than a refracted beam so the accuracy requirements for a mirror are considerably higher than for a refracting element.
The bottom line: You can't bend your filters enough to make any difference in image quality.
John
