Wed, 10/10/2018 - 12:20
Hello! Going through the CCD manual for review, I found a statement at the beginning that suggested that photometric filters degrade over time, so inspection and transform updates annually are needed.
In the absence of dust, dirt, crystals, etc., forming on a filter surface, does the band pass change over time so that a filter needs to be replaced even though it is clean, without dirt, dust, etc.? Does the band pass need to be checked periodically to ensure that it matches the standard system?
I use St-402 with BVIC filters. Mechanically, they seem fine, though I am sure my eye is not trained as much as others to discern when dust, dirt, etc., become a problem.
Thank you and best regards.
Mike
Hi Mike,
Colored glass filters do degrade, primarily through the introduction of moisture into both the glass and the sandwich cement. This usually results in hazing/fogging, as well as actual crystalline structure. The most common filter with problems is V, where the BG39 glass is hygroscopic.
For dielectric/interference filters, the most common issue is scratching or etching of the interference layer. This permits out-of-band light to pass through the filter. However, the coated layer is quite hard and actual pinholes or scratches are pretty rare.
The best approach is to (a) inspect your filters, and watch for visual changes; and (b) determine your transformation coefficients on a seasonal basis, and look for changes. The transformation coefficients include total system throughput, and so other changes, like accumulating dust on the mirror or optical surfaces, probably overwhelm any bandpass changes, but they at least help in determining when something along the way is degrading.
The ST402, with its internal, dielectric filters, probably would remain stable for years. I wouldn't worry too much about replacing those filters. Like any precision instrument, just be careful when handling/cleaning the filter wheel.
Arne
Mike
Thank you!
Mike