Light Pollution Filters

Our neighborhood is a somewhat odd case. A large development surrounded by miles of empty land. Get just outside the neighborhood and the skies are quite dark. Inside the neighborhood one has to deal with the usual house lights and a plethora of streetlights. Still, I can see the Milky Way from the driveway, pick out M31 with the unaided eye, and make out a number of star clusters. There is one streetlight directly across the street from my front yard. A notable problem, only partly dealt with by way of a strategically planted Royal Poinciana. A few years old now, the tree has begun to shade the driveway from the worst local light source.

Despite the fact that the neighborhood is disgustingly overlit, there is a mitigating factor. All of the streetlights are low pressure sodium type lights. These lights emit all of their power at 589nm, a sharp emission which can be filtered at the camera. Filters for this and other common light pollution wavelengths are readily available from several manufacturers in a number of sizes and formats.

Light Polluted Orion
One minute Orion without LPR filter

While taking some wide field shots using a 50mm lens I had recent opportunity to see the difference with and without the filter. I did not have a filter that fit the 52mm thread on the lens, but rather simply set my 48mm filter on the front of the lens. I had not performed this simple experiment before as the usual mounting location of the filter is buried inside the setup. On this occasion it was a simple matter to take identical frames with and without the filter.

The resulting frames can be seen at the left. These are taken from the camera raw images, imported with daylight color balance, cropped and sized for display. Both images have been handled identically. Neither is a “pretty” picture, these are unprocessed images, none of the stacking, stretching and sharpening that would vastly improve the visual appearance.

Continue reading “Light Pollution Filters”

Oops, a little too much power there…

How powerful is the K1 AO laser?

For someone who has lately been used to working around relatively harmless power levels, beams of a few milliwatts, this is a reminder that lasers are potentially very dangerous. With a milliwatt power level beam there is no danger in getting a hand in the beam, be mindful of your eyes, but otherwise not a lot of concern. This is vastly different, beam power here is measured in tens of watts… The bright yellow beam looks so innocent, appears so harmless. Give that beam a chance, a momentary slip, and it will burn you… badly.

Anything in the beam is at risk, even components we thought were robust enough to withstand the power levels. In this case a reflective ND filter that was to reflect most of the beam into a beam dump, allowing a small amount to continue up the beam train for use in alignments. So much for the ratings on the manufacturer’s data sheet, the beam punched through the coating and even started to melt the glass…

Burned Filter
A reflective ND filter burned through by the K1 AO Laser