Thousand Oaks full spectrum (white light) solar filters are made of high quality glass, with Thousand Oaks’ exclusive Solar II Plus multicoatings, the most durable on the market. They consist of thin layers of vacuum-deposited nickel, chrome, and stainless steel on the inside surface of the glass, where they are protected against damage during use. The coatings reject 99.999% of the Sun’s entire spectrum of energy – from ultraviolet to infrared – protecting both your telescope optics and your eyesight.
A lower cost alternative is Thousand Oaks R-G Solar Film filters. These filters use a flexible film, vacuum-coated with metal to reject 99.999% of the Sun’s energy. The opposing faces of glass substrates have to be polished precisely plane and parallel to avoid internal reflections that can affect image quality. Unlike glass, R-G film is thin in relation to the wavelengths of the white light spectrum, so that the film does not affect the homogeneity of the wavefront of the light hitting it and slight wrinkles in the film have no effect on the image. The RG film actually offers the highest resolution at high magnifications, with the glass substrates a very close second. Under low to moderate magnifications, however, it is difficult see a performance difference between R-G film and glass.
Each filter (whether glass or film) is mounted in a polished friction-fit aluminum cell with a lip that slips over the objective end of the telescope. This prevents the Sun’s heat from building up inside the telescope and possibly damaging the components inside. The filter cell is held snugly in place by 1/32” thick felt strips glued to the inside of the cell lip.
The filters below list the inside diameter (I.D.) of their cell’s lip. They are designed to fit telescopes with an outer diameter no larger than 1/16” smaller than the cell size. (The 1/16” difference is the total thickness of the felt holding the filter cell in place.) If you do not see your brand of telescope and model listed, simply choose the closest filter cell I.D. size that is larger than the outside diameter measurement of your telescope. For example, a 4.25” I.D. filter would fit telescope diameters from 4” to 4-3/16”. Do not choose an I.D. size the same as your telescope outer diameter, since it will be too tight and will not allow room for the felt lining. Ample extra self-adhesive felt is included so you can double or triple the layers of felt for a custom fit.
The glass filters have a 15-year manufacturer’s warranty against flaws in materials and workmanship. The R-G film filters have a five-year warranty against flaws in materials and workmanship. If handled with care and common sense, both types can last 20 years to life.
The Sun appears yellow-orange through a Thousand Oaks filter, standing out briskly against a black sky. The filter can be used for both visual and photographic observation, using a camera ISO setting of 100 or higher. Thousand Oaks white light solar filters show only sunspots and convection-cell granulation of the Sun’s surface. A much higher cost hydrogen-alpha line band filter is needed to see flares, spicules, and solar prominences.
Full aperture solar filters use a filter element with a diameter close to or equal to that of the aperture of your telescope itself. The resolution of the filtered image is thereby equal to that of your telescope, showing the most amount of solar detail. Off-axis solar filters have a small diameter filter element mounted off to one side of the filter cell, rather than in the center of the cell. This keeps the secondary mirror of a reflector or catadioptric telescope from blocking a portion of the light path and reducing contrast. However, the resolution of a telescope using an off-axis filter is reduced to that of a scope with an aperture equal to that of the filter element diameter. In other words, an 8” scope using a 3” off-axis filter will only have the resolution of a 3” scope. So why choose an off-axis filter?
Seeing conditions are generally poorer in the turbulent warm air of the day than in the cooler and calmer air of the evening. Perfect daytime “seeing” only occurs about 1% of the time. A large scope with a full aperture filter is therefore more affected by typical poor daytime seeing than the same scope with a smaller off-axis filter. An off-axis filter reduces the aperture, thus reducing the turbulent effects. An off-axis filter often yields less detail, but sharper and steadier images than a full aperture filter.
For telescopes up to 5” in aperture, an off-axis filter offers no advantage. For average seeing conditions, an off-axis filter is generally used for 10” and larger telescopes. For scopes between 5” and 10”, if your daytime observing location is in the middle of a large expanse of asphalt or concrete (a parking lot or school yard, for example), or looks over the roof of a nearby building, an off-axis filter that is less affected by poor seeing conditions may provide more satisfactory viewing and photography. If full aperture resolution is still preferred, a full aperture filter can be stopped down using a simple off-axis mask if seeing conditions dictate.
Always remember to install the solar filter before pointing your telescope at the Sun to avoid instant and irreversible damage to your eyesight. Remove or cover the finderscope of your telescope as well, to avoid burning up its crosshair reticle, as well as to prevent accidental eyesight damage.
Pinholes and minor surface scratches are common in vacuum-deposited metallic solar filter coatings. They do not let in enough extra light to cause a visual danger or scope-heating problem, and are no cause for alarm. All filters are tested by the manufacturer for safety before shipment. A “bright” pinhole may cause minor ghosting due to scattered light. If this is seen while observing, the pinhole(s) should be covered on the inside of the filter (the coated side) with a fine point black felt tip marker, a small drop of paint, or a small dab of typewriter white-out. There is no optical danger if small pinholes or scratches are present, and the touch up procedure will not lessen the optical performance.
The illustrations shown below are typical of Thousand Oaks filters and do not necessarily show the specific filters described.