I use a modified webcam for imaging in a suburban location. The visual limiting magnitude is about 4.5 in the best nights, so visual deep sky observing is difficult. The IDAS Light Pollution Suppression filter is specifically intended for imaging, according to the advertising. The IDAS LPS is said to effectively block the narrow spectral lines from mercury, neon and sodium lamps. The filter is available in many sizes, ranging from a 1.25 eyepiece thread to a whopping 82 mm. An individually tested transmission curve is included with the filter.

I decided to find out how long exposures I can use at my backyard using the filter before the image background becomes too bright. The test setup was my 80 mm refractor reduced to f/3.8 on an equatorial mount. The testing night was a typical moonless winter night with a limiting mag of 4.

I took some test image without and with the LPS filter. In images taken without the filter, the sky glow saturates the image background in a less than a minute exposure with my camera and scope. With the filter I can expose over 90 seconds before the image became too bright. Clearly can be seen that the filter does its job quite well.

My little test shows that I can use over twice as long exposure times with the filter before the sky glow becomes a limiting factor. Of course a filter like this wont be a substitute for a dark imaging site. Also I understand that no filter can totally block all artificial sky glow because some part of it has a continuous spectrum. But when travelling to dark sky location is not possible, the filter does a decent job of reducing sky glow and increasing the contrast of my webcam images.

From: | Created on: 10/16/2009 1:48:46 PM