[Aavso-photometry] crud on filters

[Brian D. Warner]

Hi Michael,

John Menke ran into this very problem and solved it using some proper optics
cleaners. Here's an excerpt from the second edition of my book:

It would seem obvious that clean filters would make getting flats
easier because there aren't any dust particles on the filter to
create "dust donuts". However, the problem of clean filters extends
beyond simply keeping dust off the filters. Fellow photometrist,
John Menke, wrote some emails detailing his troubles with getting
good flats. It turns out that his filters had developed a thin layer
of "snowlike stuff". Why the layer formed may be related to his
location, the type of coating on the filter, or any number of other
reasons but the effect on his photometry was significant and - most
important - inconsistent.

John outlined several suggestions as he was trying to fix the problem.
First, do not use general glass cleaners, especially with cotton swabs.
You may just smear the material and not actually remove it. If you use
any cleaning fluid, make sure it's specifically for optical surfaces.
Remember that some filters have special anti-reflecting (A/R) coatings
that may be damaged by household cleaners.

Do use very soft cloth. A T-shirt washed many times is one possibility
but much better are the special lens cleaning cloths that you can buy
from opticians and in the pharmacy section of your local grocery store
(at least in the U.S.). Most important is do not spray any cleaner
directly on the filter. Apply it to the cloth first. Many filters are
sandwiches of two or more pieces of glass. If you get fluid between the
layers, you can count on buying a new filter.


You can reach John at  john at menkescientific.com

Good luck!


Clear Skies,
Brian D. Warner
Palmer Divide Observatory (716)
http://www.MinorPlanetObserver.com

Collaborative Asteroid Lightcurve Link
http://www.MinorPlanetObserver.com/astlc/default.htm