elaboration of "keeping dust off optics" (2009Mar01)

Realizing that I really should go to the trouble of at least diagraming what I am about to describe, as I have in the past I will wince and proceed, preferring to get the idea out rather than wait on a diagram that I may never get around to creating. As a stop-gap, I will end with links to several relevant Wikipedia articles that do include diagrams and/or images.

In the original post on this topic, I only described the air flow, without going into detail about filtration and drying of the air used. I intend to amend those omissions here.

Since the needed volumetric supply rate of air is fairly small, significant compression (enough to ensure no condensation on the optical elements) should not represent an onerous power requirement. However, even simple impeller turbines can be worn by dust particles, and filters last longer if coarse material is removed before reaching them, so the first stage should be a cyclonic separator.

As the input to the compressor, a slight vacuum will be pulled on the air as it passes through this first stage. If that air is at or very near dew point, condensation may occur inside the separator. (If condensation does occur, most of the heat removed from the condensing water vapor will remain with the air, raising its dew point.) Packing the center of the separator with a downward pointing cone of stainless steel wool will provide that condensation with a surface to adhere to and wick down, collecting in drops at the point of the cone. The moist stainless steel wool will also help remove finer dust particles that would otherwise pass through the cyclonic separator. (The finer the stainless steel wool, the more effective it is likely to be, but also the more likely that it will need an occasional back-flush with steam.)

Next comes the compressor. This should be a very small impeller turning very fast, to achieve significant compression with a low rate of flow. The chamber enclosing the impeller may also act as a second-stage cyclonic separator. Any condensation remaining in the air flowing into the compressor should be flung against the outer wall of that enclosing chamber or reevaporated instantly.

Following the compressor is a HEPA filter, to remove any remaining dust. The clean air from the HEPA filter is then directed into a box containing the optics (lenses and image sensors). That box will need to be strong enough to withstand mild pressurization, and should be sealed except as described below, although the pressurization will help ensure that any imperfections in the seal don't result in dust infiltration.

Rather than exposing the optics directly to the outside air, they should be protected behind a lens cover in the form of a wafer of flat, very clear glass or crystal. That wafer should be etched on its outer edge with widely-spaced grooves, perpendicular to the flat sides, to serve as air channels, and on it its outward-facing side with groves which begin at the groves in the edge, tapering to nothing as they spiral toward the center at a shallow angle. By mounting such a wafer in such a way that air passing out through the groves around the outer edge is forced to turn and flow in an inward spiral across the face of the glass, an air-flow, reminiscent of that in a hurricane, will be set up, with filtered air spiraling inward next to the lens cover, pushing out a bit, and then spiraling back outward a short distance in front of the lens cover, keeping the lens cover itself free of dust and small droplets in the ambient air.

Now for the promised Wikipedia links…

• cyclonic separation
• dew point
• capillary action
• centrifugal compressor
• HEPA
• laminar flow