Astronomical filter
Research astronomers, on the other hand, use various band-pass filters for photometry on telescopes, in order to obtain measurements which reveal objects' astrophysical properties, such as stellar classification and placement of a celestial body on its Wien curve.
Most astronomical filters work by blocking a specific part of the color spectrum above and below a bandpass, significantly increasing the signal-to-noise ratio of the interesting wavelengths, and so making the object gain detail and contrast.
The broad-band and narrow-band filters transmit the wavelengths that are emitted by the nebulae (by the Hydrogen and Oxygen atoms), and are frequently used for reducing the effects of light pollution.
Small solar filters threaded behind eyepieces do not block the radiation entering the scope body, causing the telescope to heat up greatly, and it is not unknown for them to shatter from thermal shock.
Therefore, most experts do not recommend such solar filters for eyepieces, and some stockists refuse to sell them or remove them from telescope packages.
These glass filters can crack unexpectedly from overheating when the telescope is pointed at the Sun, and retinal damage can occur faster than the observer can move the eye from the eyepiece.
Neutral density filters are mainly used in traditional photography, but are used in astronomy to enhance lunar and planetary observations.
Emission nebulae mainly radiate the doubly ionized oxygen in the visible spectrum, which emits near 500 nm wavelength.
Common filters are often used with the Hubble Space Telescope, forming the so-called HST-palette, with colors assigned as such: Red = S-II; Green = H-alpha; Blue = O-III.
These filters are commonly specified with a second figure in nm, which refers to how wide a band is passed, which may cause it to exclude or include other lines.
Broadband filters have a wider range because a narrow transmission range causes a fainter image of sky objects, and since the work of these filters is revealing the details of nebulae from light polluted skies, it has a wider transmission for more brightness.
