Filters

Eyepiece filters and filters that cover the front of the entire telescope are important additions to an astronomer's inventory.  Hundreds of different kinds are available, but choosing a few quality filters will suffice for 99% of an amateur's observing sessions.

Eyepiece filters usually screw into the bottom of the eyepiece and come in sizes that fit both 1.25 inch and 2 inch eyepieces.  These filters are a great way to enhance the contrast of low surface brightness objects like diffuse nebula.  They can  be used to enhance the surface features of the planets and Moon, reduce glare of very bright objects, and even enhance the definition of images.  There are filters that reduce the effects of light pollution and those that allow the spectrum to pass through at only one frequency.  Several are summarized below.

You should ONLY use Solar filters that cover the entire front of the telescope to allow the safe viewing of the Sun.

Solar Filter:  This is the filter that will allow you to safely observe the closest star to the Earth, the Sun.  These filters fit snugly over the front of the telescope and are made out of either coated glass, or a mylar material.

WARNING: If you choose to make your own filter, it is very important to use material that is specially designed for use as a solar filter.  Before using any solar filter you must inspect it by holding it up to a bright light (not the sun - duh) to check it for any pinholes or scratches.  If you find any at all, do not use the filter.  Period.  Your eyes will be permanently and immediately damaged by the Sun after viewing the concentrated image of the Sun through your telescope and through these defects.

My recommendation for long term durability and long term safety is a well cared for glass filter.  If you are going to do photography through the filter though, I would direct you to a thorough review of Solar Filters in Sky and Telescope Magazine, July, 2000.  Brands reviewed included Thousand Oaks Type 2, Thousand Oaks Polymer Plus, Thousand Oaks Type 2+, Orion, and the Tuthill Solar Skreen.  A follow up review included the test report for Baader solar filter mylar material, in September, 2000.  Mylar filters, when properly cared for are just as safe as glass filters.

Safe Solar Filter Suppliers include:

Blue Filter:  A Wratten #44A, 47B or 80A can be 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.  It can also be used to cut down the glare of the bright Moon.  The 80A is the filter to have if you only buy one filter.

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.

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 will be found to be one of the most useful filters to have.  It is used to bring out details on Mars and enhance some of the zonal details on Jupiter.  An orange filter darkens the blue sky so daytime observations of Jupiter, Venus and the Moon are much improved.

Red Filter:  A Wratten #23A, 25, or 25A can be 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.  Flipping back and forth between red and blue filters can sometimes bring out subtle coloration on the surface of the Moon.

Polarizing Filter:  This filter can cut down the glare when observing a nearly full Moon, making it easier to see ray structure.  It will also cut down daytime glare.  I prefer to use a variable Moon filter that allows me to "dial in" the amount of the neutral density light reduction.  If you will be doing any terrestrial photography with your telescope, this filter will reduce sky glare early in the morning and the late afternoon.

O-III Filter:  A bandpass filter used to enhance the contrast of diffuse nebula by allowing the doubly ionized oxygen band through.  Great for the Ring Nebula, M57, crucial  for the Veil Nebula.

Ultra-High Contrast Filter: A bandpass filter used to enhance the contrast of diffuse nebula.  Allows the O-III light through and Hydrogen Beta.  Great for the Swan Nebula.

Hydrogen Alpha Filter:  These filters are VERY expensive, but if you want to observe and photograph solar prominences, this filter is required.  Viewing the chromosphere outside of an eclipse is a challenge. The easiest way is to use this special filter that 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 656.3 nanometers (6563 angstroms) in the red.  

Hydrogen Beta Filter:  The only way to see the Horsehead Nebula and the California Nebula.  This filter is so specialized, it is really only good for two objects; IC 434, the emission nebula surrounding the Horsehead Nebula in Orion, and the California Nebula.  Both of these objects emit strongly in the hydrogen beta portion of the spectrum.  While it is possible, just barely, to catch a glimpse of the Horsehead without the H Beta, the view with it is far more stunning.  The filter emphasizes the glow of both of these nebulae, turning them red. In the case of IC 434, the red outline around the actual Horsehead itself (B33) is what makes it visible.  The California Nebula is similar in that it is just barely visible without the H Beta.  With it, the California "shape" becomes clearly visible.  (From http://sciastro.net/portia/advice/filters.htm)

Light Pollution Filter:  A relatively new class of filters known as light pollution reduction, or LPR, filters. It is designed to darken the background sky by blocking mercury vapor light transmission and enhancing 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, with the background light-polluted sky.  This filter doesn't work particularly well on other types of objects, but does a fine job with emission and planetary nebulae, because they emit light in the hydrogen alpha, hydrogen beta, and doubly ionized oxygen wavelengths.  (From  Maxitrol eye ointment price)

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