Cleaning optical filters

hi Thomas,

Maarten uses alcohol or propanol to clean filters and sometimes reverts to water and soap in special circumstances.

Normally he treats all filters as if they have soft coatings:
- lay a folded-out optical tissue on the filter
- put some alcohol on the tissue
- slide off the tissue by pulling it from one side
- dont apply pressure on the filter at all if not necesary

If the stain is sticky then he applies a bit of pressure either with an optical tissue or with a cotton-ball stick. Dip in alcohol and carefully rub the filter.

cheers, John

Hi Thomas and John,

At the NOT it has (as long as I've been here) been the duty of the astronomers to clean the filters before installing them. That is, first by a ligth blow to take away dust, and if there are stains or something sticky, then with the optical cleaning liquid in the cupboard (blue liquid in the plastic bottle). It should be done very carefully with optical paper dipped in this liquid. Never use an optical paper twice! And never rub. Just slide the wet paper over the surface and then slide dry (new!) paper over it afterwards. Also, before putting filters back in their envelopes (in fresh optical paper free of any dust), I usually blow away any dust that has accumulated during the time it has been mounted. As far as I know, we've never used alcohol.

A lot of optical paper is spent this way, but it is better not to try to save paper. Also, filters should not be cleaned more than strictly necessary since it involves a risk.

Cheers, Amanda