filter – Page 2 – A Darker View

Safe Transit Viewing

You have heard it before, but it really is true! Looking at the Sun for longer than a few brief moments with an unprotected eye can lead to permanent damage. Looking at the Sun with any sort of optical deceive that has not been properly filtered can lead to instantaneous eye damage.

Discussed below are the only safe methods I am aware of to view the Sun. There are some dodgy methods out there. Take chance with your irreplaceable eyesight? I think not. Be careful and do it right if you want a look.

For the unaided eye there are a couple options to view the Sun safely…

Solar Viewing Glasses Proper solar viewing filters are made from a thin plastic film, usually mylar, coated with metal to properly filter the Sun’s light for viewing with the unaided eye. These are widely available and quite cheap. Local astronomy organizations have been giving them away in preparation for the transit. Do not attempt to use these viewers in conjunction with binoculars or small telescopes, they do not provide sufficient filtering for optics!
Welding Glass Filters used for viewing welding offer much the same protection as solar filters. A shade 13 or darker welding filter can provide good protection for the unaided eye. Again, do not use a welding filter with any sort of optical device such as binoculars, they are not designed for such use and may not block enough light.

A much better view of the event can be seen if modest magnification is used. Do keep in mind that any soft of binoculars or telescope also concentrates much more light that the eye alone. Remember those childhood “experiments” involving a magnifying glass and ants? The best options here are the use of solar filters designed to be used on a telescope, or indirect means such as image projection.

Solar Filter — A solar filter mounted on a refracting telescope

Solar Filters for Telescopes Filters specifically designed for telescopes consist of either a plastic film, or a sheet of glass, coated with a thin layer of metal to block the Sun’s light. These filters pass about 1/1000 of one percent of the light. More importantly, they block the harmful infrared and ultraviolet light that could so easily cause damage. The filters can be purchased in many sizes as appropriate for various models of telescopes, costing between one hundred and several hundred dollars.
Image Projection One of the simplest methods of displaying a solar image is projection. A white screen placed a foot or two behind a telescope will produce a very nice image of the Sun that can be safely viewed by a number of people simultaneously.
Keep in mind that the beam of light out of the eyepiece is quite intense, potentially hot enough to burn a careless finger placed near the exit from the telescope.

The method works best with small optical systems, binoculars or the smallest of telescopes. You do not need big optics to project a very nice solar image.

You do not have any optics? Use the simplest optical arrangement of all, the pinhole camera!

If you do not have a safe means of viewing the transit and do want a look, you can go to one of the many public events being organized. Here on the Big Island there are quite a few options, one should be close to you.

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.

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