[APML]: Filter Cleaning Test Results

[Chuck Vaughn <aa6g@aa6g.org>]

About a month or so ago there was a thread about cleaning interference
filters. Michael Richmond and I disagreed about how sensitive these filters
are to cleaning. The results are in.

I ran the test as follows:

I used a Lumicon Deep Sky filter that I purchased in 1986 (maybe 1987).
It was the size that screwed onto the back of a C8. It was my first
scope for astrophotography. I later adapted it to use with my 350mm lens
but otherwise it has seen no use over the last 12 years.

Before cleaning I had a transmission curve run on the filter for a baseline.
I cleaned the filter each day for 25 days after leaving it outside with the
dielectric side facing the sky. It rained on it 14 times including one hail
storm. It was rather dirty each day. I carefully cleaned it each day by
spraying it with MS-260 and gently wiping it with a cotton towel. No
special or extraordinary measures were taken.

An image showing the before and after transmission curves plus the four
points measured by Lumicon before I purchased the filter can be downloaded
here:

ftp://shell3.ba.best.com/web2/aa6g/clean_test.gif

You'll see that the curves are substantially the same with only some slight
differences that could be attributed to testing runs as easily as cleaning.
The Lumicon points fall right on the curves at three out of four places,
with one being 1% worse.

The usual disclaimers: These results do not mean I think you should throw
caution to the wind and disregard good cleaning practices. This test is
certainly severe but does not indicate that an interference filter with a
hard coating will be destroyed with reasonable handling.

Questions have arisen over the stability of the coatings of these filters.
This filter is 12 years old and shows no degradation in that time. I expect
it to be good for years to come.

Chuck  <aa6g@aa6g.org>