Many amateur photographers like to use a filter on their lens for protection, in case the lens is used in conditions that might harm the coating on the front element of the lens. For example, a filter might be used to protect against the salt spray or sand at the beach. It might also be used just to protect the lens from fingerprints. Some filters, such as a UV (Ultra-Violet) filter can be used to cut down on the blue haze in the distance in landscape photography. In general it's probably better not to use a protective filter for astrophotography. It may cause reflections on bright stars or planets, and you lose some light due to absorption in the glass. Auroras in particular are better shot without a filter because filters can cause rings or circles in the image from interference fringes at certain wavelengths. The only thing you really have to protect the lens' front element from is dew. However a filter won't stop the dew, it will only condense on the filter, and this will ruin your photos too. As we discussed in section 208, Dealing with Dew, if you shoot in an area where dew is a problem, you need to use an anti-dewer to stop the dew. A filter won't help. An anti-dewer is a 12-volt heating strap that is wrapped around the lens to gently warm it. As long as the lens is kept just a couple of degrees warmer than the dewpoint, dew won't form. There are other filters which can be helpful for astrophotography though.
Light Pollution filters, such as Hutech's IDAS LPS filter and Lumicon's Deep-Sky filter, do a good job of filtering out wavelengths dominated by man-made light pollution, and passing wavelengths emitted by astronomical objects. This darkens the sky considerably and increases the contrast between the astronomical object and sky background at the same exposure. It also allows a longer exposure to record even more signal from the object. You will usually need to expose 2x to 3x longer with a light pollution filter. Light pollution reduction and suppression filters work very well under light-polluted skies, but also help even at dark-sky observing sites because they also reduce the natural brightness of the night sky caused by airglow. This also helps increase the contrast between the sky and object. Note that some light pollution filters that are made with interference coatings, such as the Hutech IDAS LPS filter, will not work with wide-angle lenses. These types of interference filters require parallel light rays. They may work on lenses down to 35mm of focal length when stopped down to about f/4. Didymium filters are a low cost alternatives to expensive light pollution filters. They block orange man-made light pollution from sodium vapor lights.
Fog Filters - A fog filter is a diffusion filters used in front of the lens that puts a halo or glow around bright stars.
This can make the bright stars of a constellation stand out, making its shape easier to recognize in a photograph that may contain thousands of other stars. Tiffen's Double Fog 3 is such a filter, as well as Hoya's Fog A filter. Make sure to get the one with the correct size filter threads for your lens.
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