Filters Used In Astronomy By: Jeff Thrush

ND = 0.3 means 50% transmission

Minor Planets

Electromagnetic Spectrum

Visible Light  The wavelengths our eyes can detect is only a small portion of the electromagnetic spectrum. We call this the visible light spectrum.  If the visible portion of the light spectrum is divided into thirds, the predominant colors are red, green and blue. These three colors are considered the primary colors of the visible light spectrum.  All the other colors we can see in nature are found somewhere along the spectrum between blue and red.

Primary / Secondary Colors Primary colors can be arranged in a circle, commonly referred to as a color wheel. Red, green and blue (RGB) form a triangle on the color wheel. In between the primary colors are the secondary colors, cyan, magenta and yellow (CMY), which form another triangle.

Color Reproduction There are only two basic ways used to reproduce color: Additive Color System (RGB) Subractive Color System (CMY)

Additive Color System The additive color system involves light emitted directly from a source, before an object reflects the light. The additive reproduction process mixes various amounts of red, green and blue light to produce other colors. Combining one of these additive primary colors with another produces the secondary primary colors cyan, magenta, yellow. Combining all three primary colors produces white. All image capture devices utilize the additive color system to gather the information needed to reproduce a color image. These devices include digital cameras, flatbed scanners, drum scanners, and video cameras.

Subtractive Color System The subtractive system creates color by subtracting or absorbing certain wavelengths of color while reflecting other wavelengths back to the viewer. When combined in equal amounts, pure subtractive primary colors produce the appearance of black. Two subtractive primary colors are required to produce one additive primary color.

A Better Understanding

Characteristics of a red filter White light is composed of three primary colors: red, green and blue. A filter of a primary color will transmit its own color and absorb the other two

Characteristics of a yellow filter. Secondary colors are mixtures of primary colors. Yellow, for example, is a combination of red and green. Because a filter passes its own color and absorbs others, a yellow filter passes red and green and absorbs blue

Glass Filters These filters are nothing more than colored glass or plastic. Have lower light transmission levels requiring longer exposure times. Cost less and are easier to manufacture than dichroic filters. They can be purchased in a wide range of sizes and shapes.

Filters Yellow Filter: A Wratten #8, 12, or 15 can improve the markings in the clouds of Venus and enhance dust storms on Mars. Orange Filter: A Wratten #21 is one of the most useful filters. It brings out details on Mars and enhances some zonal details on Jupiter, also darkens the blue sky so daytime observations of Jupiter, Venus and the Moon are much improved. Red Filter: A Wratten #23A, 25, or 25A are used to enhance contrast on Mars, Jupiter, and Saturn. A red filter, however, is fairly dark, so it works best on larger aperture telescopes that give brighter images. Switching between red and blue filters can sometimes bring out subtle coloration on the surface of the Moon. Green Filter: A Wratten #58 allows you to see more clearly the edges of the Martian polar caps and enhances the belts and the Great Red Spot in the clouds of Jupiter.

Filters Blue Filter: A Wratten #44A, 47B or 80A is used to detect high altitude clouds on Mars, white ovals and spots in the belts of Jupiter, and the zones of the clouds of Saturn, and to reduce the glare of the bright Moon. The 80A is the filter to have if you only buy one filter. Polarizing Filter: Cuts down the glare when observing a nearly full Moon, making it easier to see ray structure. It will also cut down daytime glare. A “variable” Moon filter will allow you to dial in a specific amount of neutral density light reduction. For terrestrial photography with your telescope, this filter will reduce sky glare early in the morning and the late afternoon.

Filters Solar Hydrogen Alpha Filter: These expensive filters are essential if you want to observe and photograph solar prominences. This special filter transmits a very narrow slice of the spectrum, typically 1/5000 to 1/3000 of the visible spectrum's full width. The transmission window of these ultra-narrowband filters is generally centered on the bright hydrogen-alpha (H-alpha) line at a wavelength of nanometers (6563 angstroms) in the red.

Wide Band H-Alpha Filters Sharp-cut-off deep-sky photography filter with zero transmission below 630nm, 90% at Hydrogen-Alpha 656.3nm, and 90-98% from 657nm to the near infrared. Filter Factor - Exposures will be increased 2-10 times, depending on the faintness of the nebula. Hypersensitized Kodak Technical Pan 2415 is the only recommended film for use with this H-Alpha Filter; do not use Tri-X or any other B&W film. Can be used with CCD Cameras Not suitable for visual use.

Dichroic Filters Dichroic filters are amazing in their simplicity and efficiency. These filters are made of thin-film coatings applied to a glass substrate. The coatings can range from as few as three or five layers to as many as 30 or 40 layers. Each film layer is approximately one one-thousandth of a millimeter thick. The process is carried out in a vacuum chamber and is similar to the process used to make semiconductor chips. Dichroic or interference filters pass certain precise wavelengths and reflect all others.

LPR Filter LPR filters are designed to darken the background sky by blocking light pollution. Enhances transmission in the hydrogen beta, doubly ionized oxygen (OIII) and hydrogen alpha regions of the spectrum. The filter increases the contrast of deep-sky objects, emission nebulae in particular.

Ultra-High Contrast Filter The UHC filter is a narrowband LPR filters. The filter is used to enhance the contrast of diffuse nebula. This filter performs equally well under light polluted or dark skies. Allows the transmission of O-III, Hydrogen Beta and Hydrogen Alpha light. Great for the Swan Nebula.

O-III Filter An O-III filter is used to enhance the contrast of diffuse nebula by allowing only the doubly ionized oxygen band through. Great for the Ring Nebula, Veil Nebula, and the North American Nebula.

H-Beta Filter This filter is so specialized, it is really only good for two objects; IC 434, the emission nebula surrounding the Horsehead Nebula, and the California Nebula. This filter can also be used for viewing some nebulae; unfortunately most of them are quite faint.

Astronomik Ha Filter Transmissions as high as 97% 13 nm bandwidth Total blocking of all unwanted light from light-pollution Ideal for planetary nebulas and supernova remnants Completely resistant against high humidity and scratches Can be used on every clear night regardless of the Moon phase.

10nm vs 3nm H-Alpha Filter

CMY Filters The CMY method to restore the true color image is more complex than using traditional RGB imagery. These equations are applied to each pixel in an image taken using CMY filters. R=Y+M-C G=Y+C-M B=C+M-Y

LRGB & Photometric Filters

Astronomik RGB Filters Manufactured in Germany and distributed by Adirondak Video Astronomy in the U.S. Highest transmission areas of any RBG filters. Very broad red filter. Offers both NIR blocking and non- blocking luminance filters. NIR blocking filter reasonably matches RGB but lets additional UV in. OIII, H-b and H-a emission filters are parfocal with RGB. Dichroic. Blue:Green crossover nm At OIII - B=42%; G=58% At OIII - B=44%; G=93% At OIII - B=44%; G=93%

Custom Scientific RGB Filters Manufactured by Custom Scientific Generally provided with SBIG filter wheels. Luminance filter is not UV or near-IR blocking. Lower blue transmission than Astronomik or Hutech. Narrow red filter with high transmission. "Gap" placed between green and red filters centered at 590 nm - the sodium line - as a light pollution measure. Dichroic. Blue:Green crossover nm At OIII - B=44%; G=93%

Meade Pictar RGB Filters These filters have exceptionally high transmissions, and a broad blue filter to compensate for lower blue CCD efficiency. The O[III] transmission is high in BOTH the blue and green filters making this filter set optimal for acquiring nebulae signals. The clear filter is not NIR-blocking, and does not match the low wavelength edge of the blue filter. These are included in the Meade Color Filter Wheel and cannot be purchased separately at this time. Blue:Green crossover = 499 nm At OIII - B = 85%; G = 89%

Optec RGB Filters Unusual RBG filter set. Red filter is similar to the Schuler. Optec has also placed a "gap" for sodium light pollution between the green and red filters. The blue filter is very wide and extends below 350 nm. Good match between the luminance filter and the RGBs. RGB filters do have some UV transmission below 400 nm that will be detected. Red filter transmits considerable NIR. Blue:Green crossover nm At OIII - B=90%; G=39%

True Technology RGB Filters A 5-piece filter set requiring the use of a separate NIR-blocking filter (shown in black) to block the red filter. High filter transmission. There is a "gap" between the red and green filters, but it is not centered on the sodium light pollution. Green filter has violet transmission. Clear filter has high transmission. Blue:Green crossover nm At OIII - B=83%; G=68%

SBIG STV & ST-237 These small filters are included in SBIG's CFW5C filter wheel used with their STV and ST-237 CCD cameras. They are similar to the 1.25" RGB filters used in filter wheels for their ST-7/8/9/10 CCD cameras. The clear filter is not NIR-blocking, whereas the RGBs are. Narrow red filter with high transmission. "Gap" placed between green and red filters centered at 590 nm - the sodium line - as a light pollution measure. Dichroic. Blue:Green crossover nm At OIII - B=72%; G=72%

Schuler RGB Filters Colored glass filters. Twice as thick as the dichroics. Lower transmission than dichroics but broader filters making them as efficient as many of the dichroics for galaxies and globulars. Will need about 20% more time than dichroics for H-alpha and OIII signals from nebulae. Good NIR match for luminance filter, but it lets in considerable UV. Red filter transmits considerable NIR. Designed for superior color balance. Blue:Green crossover nm At OIII - B=34%; G=56%

Schuler UBVRI Filters These filter are used to take photometric measurements using a CCD camera. To learn to do photometric work, you only need the V Filter. Once your proficiency increases, you can use the whole range of UV, Bu, V, R, I filters. The Schuler filters are in use by many AAVSO members. You do not need to spend per filter to achieve high quality repeatable scientific results

RGB Filter Calibration

Results of star HIP FilterADUCoefficient R G B

Creating a TriColor Image

Questions