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QUESTAR SOLAR FILTERS

Three of the better metal coated glass solar filters in cells. Shown are two Off-Axis and one Full Aperture Filter in their machined cells. (63,163 bytes).

The Questar is a family of telescopes and accessories have been made for more than fifty years in the U.S.A. to industrial degrees of perfection and durability. Questar telescopes and accessories are proven to have been engineered to provide more than a lifetime of rewarding service.

Right: Three of the Questar metal coated glass solar filters in cells. Shown are two Off-Axis arrangements with the current model at left and a 1950's model at right, and one Full Aperture Filter. Each coated glass element is shown installed as furnished in their machined cells. The Questar 7 filters closely resemble the two current filters shown here (63,163 bytes).

The photosphere is the visible surface of the Sun that amateur astronomers are most familiar with. This is readily observed with high quality "white light" solar filters such as that we offer by Questar. The visible surface of the Sun is not a solid surface but is actually a layer of the gas ball that is about 100 km thick; this is relatively thin compared to the 700,000 km radius of the Sun. When observing the center of the disk of the Sun we look straight in and see somewhat hotter and brighter regions. When one observes the limb of the solar disk that light has taken a slanting path through this layer and we only see through the upper, cooler and dimmer regions. This explains the "limb darkening" that appears as a darkening of the solar disk near the limb. A number of features can be observed in the photosphere with a simple telescope equipped with a suitable "White Light" solar filter.

A Brief History

The first astronomers who to observe features of the Sun developed a process to deposit carbon black from candle flames onto small clear glass windows. These filters are placed over the front of the observers eye, or later over a telescope in order to attenuate the brightness of the image made by the telescope to a comfortable level. The density of the filter is calculated to pass enough light so that sunspots could be observed. However, these first filters permitted the then unknown harmful portions of the spectrum to also pass through the filter and cause injury to the retina of the eye. Since there are no nerves sensitive to pain in this photosensitive area of the eye, one could go blind gradually and yet feel no pain. As a result many of the famous early astronomers (including Galileo) died blind; this blindness was accepted by some people as proof that "God" was angry with people who would attempt to peer into the Heavens. It would be some centuries later when the components and hazards of sunlight were fully understood, and suitable solar media developed to permit safe observing of the Sun. However, even today it is possible to find some filters marketed for solar observing that are simply not safe.

By the Twentieth Century glass and metal deposition technology had evolved so that more precisely made filter could be manufactured, but these were generally very costly and limited to Universities and organizations or persons of means. By the 1970's more economical commercial glass solar filters came onto the market, these were usually made from common window plate glass with an aluminum or other metal allow coating. While these less costly filters became acceptable for persons who looked for a durable and economical way to safely observe the major solar features, we have tested a number of these filters and found them to generally have such a poor optical figure that we could not recommend them for use with high resolution telescopes. By the 1980's Aluminized mylar and polymer filters entered the market, but none of these has the durability of glass, and neither provides the pleasing deep orange red rendition of the Sun which most astronomers prefer.

Questar Solar Filters

For those wishing the ultimate resolution and definition there are few well made glass solar filters which employ polished, smooth surfaced, plane parallel flats made of optical grade glass in production. Such filters including those which Company Seven offers by Questar Corporation which do afford excellent views and supreme durability, reliability, and safety. However, these are not inexpensive ranging in price from about $400 for a 3.5 inch (90mm) diameter filter, up to about $1,400 for a 7 inch (178mm) diameter filter housed in a precisely machined cell.

Total Eclipse of the Sun after first contact but before totality, 11 August 1999 taken at the Black Sea north of Varna. By William Chandler with Questar Full Aperture Solar Filter (21,328  bytes). Left: Total Eclipse of the Sun of 11 August 1999, after first contact but before totality. Taken at the Black Sea north of Varna by William Chandler with Questar Full Aperture Solar Filter. Note the pleasing orange red tone of the Sun's disk. (21,328 bytes).

The features visible in a white light filter may include: 1. dark sunspots, 2. the bright faculae, and 3. granules. One can also measure the flow of material in the photosphere revealing additional features including large scale flows and a pattern of waves and oscillations. Depending on the nature of the coating or metal used to make the filter the Sun may appear white, blue, yellow, or orange red. The Questar filter coatings are optimised to provide a pleasing deep orange-red image, where subtle features will contrast starkly against the background. The image above provides some approximation of the color tone, and although when the image was taken there was some sunspot activity recorded in this film image it does not show well in this low magnification unprocessed reproduction.

These elements used in our Questar filters feature excellent freedom from wedge, and very smooth polished surfaces and so these are superb glass windows for high resolution applications. These filter elements are made in an optics facility nearby Company Seven. The factory is well regarded for their ability to produce optical flat, and extremely parallel flat elements.

Questar Full Aperture Solar Filter Cell. Note vents and knurling to facilitate handling. (34,908 bytes). Right: Questar Full Aperture Solar Filter in machined aluminum Cell. Note vents and knurling to facilitate handling. (34,908 bytes).

A triple Chromium evaporate is deposited onto the polished parallel flat element so that only 17 millionths of the light from the Sun will pass through the filter. All harmful infrared and ultraviolet rays are rejected. The Sun is presented as a very pleasing, Orange Red disk framed against a black background.

The element is housed in a precisely machined aluminum cell which is anodized black. The cells are threaded for easy installation onto the front cell of the Questar 3-1/2 or 7 telescopes. The Full Aperture Solar Filter Cell features vent slots designed to reduce any possibility of heating the telescope corrector lens. There is also machined knurling around the perimeter of the Full Aperture filter cell to facilitate handling.

Off Axis or Full Aperture?

The Off Axis solar filter arrangement is an economical alternative to the Full Aperture filter. This is suitable for casual or introductory sunspot or eclipse observing. However, the Full Aperture filter arrangement provides almost triple the resolution and about seven times the brightness of the Off Axis filters. The Full Aperture Filter will show much finer details in sunspot structure and faculae on steady observing sessions, and will provide much faster possible shutter speeds to help "freeze" atmospheric turbulence. Therefore, Full Aperture filters are the most attractive for serious study or photography of the Sun.

Company Seven routinely stocks the Questar Solar Filters at our Laurel, Maryland showroom. For further information about pricing and availability of these items please contact Company Seven. For those interested in observing the Sun we highly recommend the book written by Beck, Hilbrecht, Reinsch, and Volker Solar Astronomy Handbook.

MAINTENANCE AND CLEANING INSTRUCTIONS:

  1. The metallic coating on the glass is applied to the inner surface of the element, the surface that faces the telescope objective lens. It is best to protect the filter from sharp edges or any item that could cause a scratch or abrade the film.

  2. While the glass optical element is installed in a metal cell, to provide for expansion due to temperature changes the filter "floats" in the cell and so it is not insensitive to shock, or vibration.

  3. When not in use, store the filter in a rigid container (such as a "TupperwareTM" or other plastic air tight container), or a fitted carrying case. Ideally the container should hold the filter in place without contacting the filter element, a soft velour like pouch or lining may accomplish this goal. Do not store the filter into an airtight containers when it is wet (from dew, etc.) for long periods of time; whenever possible let the filter "dry out" in order to reduce the potential for problems associated with Fungus, etc.

  4. Draw any large, loose bits of foreign matter from the surface of the prisms with an air bulb or small vacuum. Stubborn particles can be removed with a soft camel's hair brush.

  5. Place a few drops of an approved lens cleaning solution (by Kodak, etc.) or mild soap solution of clear dish washing liquid diluted with distilled water onto a cotton swab. Then gently wipe in a circular motion with little or no pressure. The filter can be damaged if there are tiny abrasive particles on the surface which are pressed in by rubbing hard.

For more information about how to work your way through the decision making process of choosing and specifying a Questar 3-1/2" telescope, the common accessories, and desirable options please feel free to call us or send an E-mail inquiry, or visit our showroom. You may find more help by referring to the articles in our Questar Library section on line.


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