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While not critical, filters (in front of the camera, or, in the case of a RASA, in front of the scope itself) can help improve the contrast and visibility of the image. Some cameras benefit from an IR blocking filter to reduce the amount of infrared light getting to the camera sensor, resulting in a sharper image. For urban and suburban observers, a filter that reduces the effect of light pollution can help improve image contrast. I didn’t realize how much difference a filter would make until I tried this without one. My first time out, I had installed a Celestron Light Pollution Imaging (LPI) filter that is recommended for use with my RASA. But many of the images looked a bit odd-colored afterward. (Green?) So the next time, I tried observing without it. Wow – super-washed out, low-contrast images – and a ton more light pollution. Maybe a filter wouldn’t be needed at a true dark-sky site. But here on the outskirts of a city of a million people, I now think of it as critical. Oh – I’ve learned to compensate for the green tint using basic color sliders in Sharpcap.

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