There are several filters that are useful for astrophotography with a telescope.
Light Pollution and Deep-Sky Filters
Light pollution and deep-sky filters work by filtering out wavelengths of light dominated by man-made light pollution and letting wavelengths of light from astronomical objects pass through.
These filters help especially with red emission nebulas. They can even help at true dark-sky sites by filtering out the natural sky glow caused by airglow. Airglow is caused by oxygen atoms glowing in the upper atmosphere which have been excited by solar ultraviolet radiation.
These filters are especially useful for recording red emission nebulas with unmodified DSLR Cameras.
Light pollution filters technically don't require much additional exposure for the wavelengths from deep-sky objects, as they typically pass as much as 90 percent of this light. What they do is allow longer exposures. This is a good thing. You need to expose longer with one of these filters to get your basic exposure for the sky background up out of the readout noise of the camera. The good thing is that when you expose longer for the sky background, you get much more light proportionally from the deep-sky object.
For example, at a reasonably dark-sky observing site where the Milky Way is visible, with a naked-eye limiting magnitude of about 6th mag, but still with some light pollution present, you might only be able to expose for 2 minutes at f/6 at ISO 1600. With a light pollution filter you would be able to expose for 2.5x to 3x times that long, or 5 to 6 minutes. So for the same level of sky background brightness, you would have 2.5x to 3x the amount of nebula brightness.
Light pollution filters won't work quite so well for reflection nebulas and galaxies because these are "continuum" sources. Their light is comprised of wavelengths across the entire continuum of the spectrum, so some of it is filtered out along with the light pollution. But these filters will still help even with nebulas and galaxies.
The only problem with light pollution filters allowing longer exposures is that tracking can become more of a problem at a dark sky site because exposures may run to 5 or 6 minutes at slower focal ratios, and you may get trailing in the image from periodic error. But light pollution filters are very useful under all kinds of skies.
Here are some light pollution filters you can get:
Baader also makes an interesting semi-apo filter that combines a minus-violet filter with a neodymium light-pollution filter.
Most light pollution filters are fairly expensive. Here are some lower cost alternatives that won't work quite as well, but will still help with light pollution. They are made from Didymium and block the orange sodium vapor wavelengths of light pollution.
Be sure to get one that is the correct size for the lens or camera-to-scope adapter that you are going to use it with.
Optical systems with lenses work because light is refracted, or bent, by the glass in them. This is why a telescope made with a lens is called a refractor.
The problem is that different wavelengths of light are refracted a different amount by a single piece of glass. Optical designers learned to use two different pieces of glass, each with a different refractive index, together as a set in a lens to partially correct for this problem. These are called achromats. They usually bring the red and green visible wavelengths to focus together, but blue was harder to control.
More complex optical designs, called apochromats, some made with exotic glass in three or four elements, can control all of the visual wavelengths very well, but they are very expensive.
Lenses and refractors that do not focus all wavelengths at the same place cause the artifact of chromatic aberration. It usually appears as out-of-focus blue / purple halos or fringes around stars.
Although many inexpensive doublet refractors made with extra-low dispersion (ED) glass claim to be apochromatic, this frequently means for visual use only. For photography, they may still have some color fringing because digital sensors are sensitive to some wavelengths that the eye can't see that well and that these scopes are not corrected for.
Reflectors do not suffer from chromatic aberration because mirrors focus all light at the same place.
Minus-violet filters can reduce much of this blue / purple color fringing in images taken with refractors that suffer from chromatic aberration and yield smaller stars.
In the mouse-over comparison above, we can see how the minus-violet filter helps to greatly reduce the blue / purple fringing around the bright stars of in M45, the Pleiades, a difficult test for a refractor. Star sizes are also reduced with the filter.
Here are some places you can get minus-violet filters:
The IDAS LPS filter also reduces chromatic aberration and blue / purple color fringing.
Solar filters are used to cut down on the tremendous amount of light that normally comes from the Sun. Proper, safe, solar filters can be used with a telescope to take pictures of the Sun and sunspots. But you MUST use a SAFE solar filter or you can severely damage your eyesight.
Baader probably makes the best safe solar filter. It is made from a flexible, specially-made Mylar material that makes solar viewing and photography completely safe. It can be purchased from:
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