Cleaning and handling of optical parts

1. Introduction

 The cleaning of optics can be very easy if you know what you are doing. This document explains how to clean and how to handle the optics.

Dust and fingerprints are the main reasons of cleaning optics. If you don’t clean this in time the component will decrease in quality of reflectivity or transmission. A good handling of the optics is essential for the long live of high quality.

Never touch an optical surface, always hold a component by the ground surfaces.

2. Dirt type and cleaning

Dust:

Dust consists out of a great amount of silica particles, which are used in grinding glass during the manufacture. A good and simple method of cleaning dust off of a component is to simply blow it away with dry nitrogen or CO2 snow. If the dust is sticky you have to use a liquid like propanol or water + detergent.

Grease:

Grease, like fingerprints, is corrosive to coatings especial the unprotected coatings. Therefore grease has to be removed as soon as it is spotted on a optical component

By applying liquid gently on the surface with a tissue you can wash off dust and grease.

For cleaning grease you always have to use a liquid like propanol pr water + detergent. When there is no dust left on the component you can increase the pressure on the cloth when cleaning with the liquid. Clean the component until the grease is all off.

Drying marks:

These marks are most of the time from water. Water contains calcium that can scratch the optical surface. When removing these marks use plenty liquid and don’t apply a lot of pressure.

If the drying marks are left from the propanol use condense formed by breathing on to the surface to clean it.

3. Optical surfaces

Gratings:

NEVER CLEAN GRATINGS!

Grating should never be cleaned with a tissue, cloth or liquid. They can only be cleaned by gently blowing the dust off.

Filters:

Most filters can be cleaned by blowing the dust off and if necessary using a liquid to remove fingerprints. Always be sure if the filter is an overcoated or unprotected one. The overcoated filters are more robust against cleaning with cloth or tissue. Unprotected filters have to be handled with special care.

Lenses:

Lenses can be AR coated with an over coating or can be unprotected. Handle them the same as the filters. Blow the dust off and clean if necessary with a liquid.

Mirrors:

Big mirrors are usually unprotected. These mirrors can only be cleaned by blowing away the dust or using CO2 snow.

Small mirrors can be overcoated or unprotected. Make sure you know which one you are handling. If in doubt of whether you are handling an overcoated or unprotected component ask and most of the time treat it as an unprotected component.

Overcoated mirrors can be handled as the over coated filters.

For unprotected aluminium or silver mirrors you cannot use a cloth because these will make scratches in the coating or the glass it self. You have to use dry nitrogen or CO2 snow to remove any dust. Grease cannot be cleaned for unprotected mirrors and have to be left alone. The only way to clean the grease is to recoat or wash the mirror.

4. Handling the optics when cleaning

Holding optics:

As said before don’t ever touch an optical surface. Always hold the component by its ground surfaces. If there are no ground surfaces make sure you wear gloves when holding the optical surface.

Working clean:

Makes sure you have clean hand when cleaning / handling optics. If you are not sure wear gloves to reduce the risk of scratching the optical surface.

When using a tissue or lint-free cloth make sure that they are clean. Never use a used tissue or cloth. Also when one has fallen on the ground use a new one.

When you use propanol make sure there is no residue in the bottle. Some bottles have “used propanol” in them and cannot be used for optical cleaning.

Before you start cleaning make sure you have a clean area to work. Clean a desk and put some E-tork on it. This will create a good clean area.

5. Cleaning material:

Dust can:

When using a dust can to blow of the dust always hold the can right up. When the can is tilted you risk spraying the liquid nitrogen on the surface.

Tissue or cloth:

The main material for cleaning optics is the tissue. There are two types of tissue; the E-tork and the Whattman lens tissue. The E-tork is mostly used to clean the surface before the final touch with the Whattman tissue. The disadvantage of these two tissues is that they leave lint, which you have to blow off afterwards.

Cloth, mostly a white cotton cloth, is mainly used when lint / dust is not wanted. You have to be sure that the surface to clean is really dust free because cloth is a bit harder on the surface than the tissue is.

Liquids:

Mostly used for cleaning optics is the propanol. After propanol dries it leaves a residue on the surface. This residue is an oxidation of the propanol and some element in the air. The best way to get rid of this residue is to breath on the surface and clean the surface again with a tissue or cloth.

Also distilled water with a detergent, like soap, may be used. The detergent is used to reduce the surface tension of the water. With the water you wash the optical surface clean of dust. It might clean a bit of grease as well.

Balzer’s No.2 is a mixture of alcohol, distilled water and a neutral detergent. Because of its mixture it does not leave any residue. Only use Balzer’s No. 2 when the surface is already very clean.

6. Cleaning method

The best way of cleaning optics is first to blow the dust off the surface.

By applying plenty propanol with the E-tork you can get rid of the grease and the sticky dust. Don’t use a lot of pressure on the surface because you might scratch it. After you have cleaned the surface breath on it to get a layer of condense. With a Whattman tissue clean the condense from the surface.

Make sure that the last wipe with the propanol and the condense covers the whole surface and leaves no dirt.

If there is any lint or lose dust on the surface you can blow this of with the dust can.

If you clean the component with distilled water you can use some neutral soap. Water normally leaves some drying marks. Clean them with propanol and the breathing method.

If you don’t know how to clean the surface please ask before touching it.

/Maarten Blanken/