QUESTAR SOLAR FILTERS

The Questar astronomical telescopes are a family of 3-½-inch, and 7-inch aperture Maksutov-Cassegrain based optical systems that have been since 1954 produced in the U.S.A., each made to industrial degrees of perfection and durability proven to have been well engineered to provide more than a lifetime of rewarding service. Among the accessories made for their 3-½-inch telescopes, and later for the Questar 7, is a solar filter that attaches to the front of the telescope.

Right: three different Questar 3-½-inch metal coated glass solar filters, each in their cells (holder). Shown are two off-axis arrangements, where the element is not centered to bypass the telescope central obstruction, with the current model at left and a 1950's model at right. Above them is a full aperture 3-½-inch solar filter. The Questar 7 filters closely resemble the two current filters shown here (63,163 bytes).

Company Seven offers a broad range of good to superb filters made to study the Sun including: Hydrogen-Alpha, Calcium K line, Helium and others. But it is the while light filter that are the most common: affordable, and the more readily available class of sun filters. So this is the filter with which most people, who have been fortunate to observe the Sun, employed.

White light filters attenuate the intense energy of the Sun, simultaneously rejecting portions of the spectrum that could damage the eye of an observer, or even the optics of some telescopes and their accessories. By darkening the Sun to comfortable levels we are able to observe its photosphere, the visible surface of the Sun that amateur astronomers are most familiar with.

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 surrounding the solar disk they see light has taken a slanting path through this layer to reach us and we only see through the upper, cooler and dimmer regions. This is referred to as limb darkening, that appears as a shading of the solar disc near the limb. Still, many features can be observed in the photosphere with a small telescope equipped with a suitable white light solar filter - quality becomes more vital then quantity.

A high quality “white light” solar filter, such as that we offer by Questar, can reveal sunspots and other events too including: a transit of a planet or Space Station, an eclipse of the Sun. The Sun goes through cycles of activity where the low point and high points peak about eleven (11) years apart, so there can be periods of weeks were very little sunspot activity can be observed; the photo below in 1999 for example was on a day of modest activity. While in 2022 and into 2023, approaching predicted Solar Maximum of 2024, the Sun has been absolutely jumping with activity.

Company Seven has some customers who bought their new Questar here with no initial thought of observing the Sun, but who have spent more time looking up by day than by night! For quite a few clients the “white light” filter became their stepping stone to a portable DayStar Hydrogen-Alpha solar observatory system for their telescope.

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 were 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 was calculated to pass enough light so that sunspots could be observed. However, these first filters permitted the then unknown harmful portions of the Sun’s 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 of and hazards of sunlight were better understood, and suitable solar media developed to permit the 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 20th century glass and metal deposition technology had evolved so that more precisely made filter could be manufactured, but through the 1960s these were generally very costly and limited to universities and organizations or persons of means. The Questar 3-½ Standard telescope, introduced in 1954 and produced since with some refinements, were one of the first to have included a safe solar filter as part of their standard accessories set. Questar included the smaller diameter off-axis filter, or the customer could opt for the choice of a full-aperture white light filter as an upgrade. The Questar solar filter could be stowed in a pouch within the 3-½ astro telescope carry case, or you could keep store it in Questar’s handcrafted thin mahogany wood (shown at right). Later in the 1980s Questar introduced a dovetailed oak wood solar filter case. Even today there remains the option of a wood case for their solar filter.

Right: Questar 3-½ full aperture Solar Filter in its machined aluminum cell in handmade mahogany wood case. The original instruction sheet and cotton padding are still there (47,108 bytes).
This is a second generation mahogany wood case, slightly differing from that of the 1950s production.
This is exhibited at Company Seven's showroom museum collection.

Most consumers who bought an amateur telescope might find it came with some device marketed as being for solar observing. This could be a thread-on dark green tinted, or neutral density gray, glass filter (absolutely dangerous) that were often included among department store amateur telescopes sold from the 1950s into the mid 1980s. While since the late 1800s the Herschel Wedge could offer good high resolution performance and were available into the 1970s, these were not without some risks. As law schools graduated more lawyers, makers of these dropped out one by one.

The 1980s saw the introduction of economical glass filters, and also some flexible aluminized mylar sheet filters entered the market. The glass filters tended to be made of simple float glass, the common greenish tinted window-pane glass that were not particularly plane parallel or well-polished, but when coated who cared about spectra? As for definition, they were acceptable especially at modest magnifications called for to observe the entire disc of the Sun. The mylar filters were even less costly than glass panes to buy, but aluminized mylar tends to show a bluish-biased color of the Sun. As the mylar filters were usually made with two air-spaced layers of mylar, the detail recognition of sunspots was not spectacular - but in optics we get what we pay for. Company Seven evaluated a number of these filters* and found them to generally have such a poor optical performance that we could not recommend them for use with high resolution telescopes. Later came another generation of filters with metallic coatings applied to thin polymer based materials that show sunspots much better, some being diffraction limited. By the 21st century Polymer displaced mylar as the choice of lightweight, economical filter substrate. But none of these flexible filters has the durability of glass, and while they can show good detail on sunspots neither provides the pleasing deep orange red rendition of the Sun which most amateur astronomers prefer.

* among many trials, we photographed a 1980s solar eclipse with a Questar 3-½ telescope optical tube assembly piggybacked atop a Celestron 11 Schmidt-Cassegrain telescope. The telescopes were equipped with full aperture solar filters by Questar and the other by a third-party, and the telescopes were configured for similar magnifications to show the full disc of the Sun and some bordering. Exposures alternated between the telescopes with the same Nikon F2AS film camera. There was no need to refer to our log as those photos taken by the Questar 3-½ clearly showed much finer detail, with more of a 3-D like pop, than those taken by the C-11 with a third-party economical full aperture glass solar filter of the day.

For those seeking the ultimate resolution and definition possible with a white light filter then there are few well made glass solar filters which employ plane parallel, polished smooth surfaced flats made of optical grade glass in production. Such filters including those which Company Seven offered by Carl Zeiss into the 1990s, those we still offer by Questar Corporation, and custom flats by Company Seven. These afford excellent views, durability, reliability, and safety.

Questar Solar Filter Features

The heart of the Questar Solar Filter is its durable homogeneous optical glass element, also made in the U.S.A.. This glass is precision ground to be plano-parallel (both surfaces parallel, free of wedge, to a high standard), and then polished smooth to an uncommonly high standard by an experienced optical laboratory. Precision optical flats are not inexpensive to produce, ranging in price (when this article was first hosted in 1994) from about $400 for a 3.5 inch (90mm) diameter filter, up to about $1,400 for a 7 inch (178mm) diameter filter.

Left: full aperture Solar Filters for Questar 3-½ and for the Questar 7, each in their machined ventilated aluminum cell. Each is shown in their optional storage case.
Note how the reflective properties of the coating is mirror-like at showing the case lid and ceiling tile (29,902 bytes).

Next the element is sent to an optical coatings company where in a vacuum chamber microns-thin layers of metal are applied. A triple Chromium (element “Cr”) alloy 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. This very hard, corrosion resistant coating rejects harmful infrared portions of the spectrum, while this also attenuates the brightness of the Sun uniformly across the visible spectrum. All harmful infrared and ultraviolet rays are rejected, while light in the visible spectrum is uniformly attenuated. Because of their metallic content, white light filters tend to appear as a mirror that is impossible to look through unless one is observing very bright sources including the Sun of course.

With the Questar Solar Filter the Sun is presented as a very pleasing, Orange Red disc framed against a black background of space. The filter is practical for safe observing by eye, and for photography/imaging.

Questar Solar Filter Convenience: the filter element is installed a precision machined cell (holder) made of high grade aluminum, then black anodized. The early production Questar telescopes of the 1950s have no threading at their front cell so the filter holder was simpler and made to snap-on. Those for later to current production telescopes thread-on to the front cell of the Questar telescope. A thread-on retaining ring allows the the filter to float in the cell, with space to expand or contract freely in response to ambient temperature changes.

With the Questar solar filter attached it becomes reassuringly safe to observe with no risk of eye damage, and when threaded-on the filter cannot blow or fall off the telescope.

Right: Questar full aperture Solar Filter in its machined aluminum cell. These are similarly machined for 3-½ or 7 inch models with venting slots, and knurling to facilitate handling (34,908 bytes).

Aside from the impressively high optical standards of the Questar filter element, these have always been so lightweight and thin as an assembly that each can fit into the thin square pouch of every generation of Questar 3-½-inch astro telescope carry case, whether the original 1950s handmade carrying case made of saddle leather in England, or current carry case.

Particularly since the COVID-19 era inflation resulted in costs of many items including the Questar filters to dramatically increase, Martin has moderated his intolerance of mediocrity.

Questar Solar Filter Density: the early to mid production Questar solar filters were more oriented for photography thus made with a higher transmission coating, this allowed finer-grained films and faster camera shutter speeds to be selected. These early filters show so much brighter a disc of the Sun that they can be uncomfortable to observe through, so some people try to attenuate their brightness with an optional in-line filter often with mixed results. Company Seven can provide the brighter solar filter elements, but only as a service to our own customers who may be more focused on imaging. Our new Questar telescopes sold include the later production darker density solar filter element, standard off-axis or optional full-aperture, and these are the version provided by Company Seven for new filters sold separately.

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 3-½-inch, of for their 7-inch aperture telescopes. The full aperture Solar Filter cell features vent slots designed to reduce any possibility of heating the telescope corrector lens by heat being convected from the black anodized metal cell. There is also machined knurling around the perimeter of the full aperture filter cell to facilitate handling. This is such a superb filter arrangement that Company Seven has made similar cells for some of our own customers in need of custom solutions for Hydrogen-Alpha pre-filter, and other applications.

The features visible in a white light filter may include: 1. 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.

Left: Total Eclipse of the Sun of 11 August 1999, after first contact but before totality. Taken at the Black Sea north of Varna, Bulgaria, by William Chandler with Questar Full Aperture Solar Filter. Note the pleasing orange red tone of the Sun's disk, the relatively dark color tone is due to his camera exposure settings. Image used by permission of Mr. Chandler, and all rights reserved (21,328 bytes).

The Questar filter coatings are optimized to provide a pleasing deep orange-red image, where subtle features will contrast starkly against the background. The image at left 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. The image below and to the right by Mr. D. Illig more accurately depicts the color tone you can expect to observe through the modern Questar 3-½ full aperture solar filer.

Right: truer example of the Questar solar filter color tone in the image taken on 28 Jun 2017 by D. Illig with his Questar 3-½ and with the full aperture solar filter, in Maryland. The image also demonstrates prime focus area coverage with a Canon EOS 6D, a full frame 35mm format DSLR camera, and are covered by the Sun's disc. Image courtesy of D. Illig, used by permission and all rights reserved (13,965 bytes).
Click on image to see enlarged view (25,138 byes).

These optical glass element in our Questar solar 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 fine optics for Questar Corp., and for others too. Their offerings include optically flat and extremely parallel elements, them even having made reference flats for National Bureau of Standards (NIST). This high degree of perfection means the performance of the telescope and filter are more likely to be limited by seeing conditions of an average sunny day, than by the optics themselves.