FILTER

Filters are glass plates inserted into the optical train to limit the light, in some way, that falls on the camera detectors. The filters, due to the coatings on them, pass light in a specific range of frequencies and block all the rest.

The following standardized codes are defined by the AAVSO and, although somewhat cryptic, are necessary due to some software processing packages needing the codes instead of a more readable name.

The filters and associated codes that are available for my images are:

• TR – (Tri-color red) Red channel in a DSLR or color CCD camera.
• TG – (Tri-color green) Green channel in a DSLR or color CCD camera.
• TB – (Tri-color blue) Blue channel in a DSLR or color CCD camera.
• CV – (Clear) No filter used.
• L – (Luminance) This filter passes all the above color range but blocks near-infrared and ultraviolet.

• B – (Johnson blue – 445 nm) Broadband photometric filter.
• V – (Johnson visual – 551 nm) Broadband photometric filter.
• R – (Cousins red – 658 nm) Broadband photometric filter.
• SG – (Sloan green – 475 nm) Broadband photometric filter.
• SR – (Sloan red – 620 nm) Broadband photometric filter.
• HA – (Hydrogen Alpha – 656.28 nm) Narrowband filter. This frequency is emitted by ionized hydrogen which is common in nebulae or on the surface of the Sun.
• UV*- (UVenus – 325-381 nm) Broadband filter. This filter is used primarily to image detail in the cloud tops of Venus.
• D1*- (≈325-800 nm) 100 lpi diffraction grating. This device produces a low-resolution spectrum.
• D2*- (≈325-800 nm) 200 lpi diffraction grating. This device produces a low-resolution spectrum.

* – not a standard AAVSO filter code.