§ 5.1. Product Refinements
Gregory Gross

§ 5.1. Product Refinements

On This Page

  1. Secondary Mirror Relocation
  2. Zerodur Primary Mirrors
  3. Other Changes

By the middle of the 1970s, the Standard Questar telescope had been subject only to a handful of alterations. In prior years, the finish of its star map dew shield lost its etched and enamel-filled markings in favor of screen-printed ones, the design of its case changed from the original English saddle leather to an updated one that was made in the United States, and the eyepieces that came with it changed from a design that used Japanese-made lenses to oculars made by Vernonscope. Other changes included the addition of a solar filter for its finder assembly and a dust cap for the corrector lens.

Still, a Questar built in 1975 was nearly undistinguishable from one built in 1965. Only a savvy aficionado could spot any differences.

This process of slow and incremental change continued into the late 1970s and the 1980s. Questar’s carrying case underwent another slight alteration, details on its fork mount arms and base changed, and a new primary mirror substrate option appeared. But perhaps most significant was a design change that represented the conclusion of a story that began during the earliest days of Questar’s design and development.

Secondary Mirror Relocation

Optical radii of a Maksutov-Cassegrain telescope
In a typical modern Maksutov-Cassegrain telescope as illustrated here, the secondary mirror spot—the R4 surface—is applied directly onto the R2 (inside) surface of the corrector lens. Gregory Gross

Dmitri Maksutov’s original catadioptric design called for spherically-figured optical elements that were relatively easy to make. The four main surfaces—the outside and inside surfaces of the corrector lens, the primary mirror, and the secondary mirror spot—were to have curvatures with their own specific radii. With “R” standing for “radius of curvature,” the outside surface of the corrector lens was referred to as the telescope’s R1 surface, the inside of the corrector lens represented its R2 surface, the primary mirror was the R3 surface, and the secondary mirror spot comprised its R4 surface.

In the late 1940s, when Lawrence Braymer was busy creating the design for what became the Questar telescope, he originally intended to place the secondary mirror spot in its most logical place: on the inside or R2 surface of the meniscus corrector lens. Indeed, most Maksutov-Cassegrain telescopes built today have a secondary mirror that is simply an aluminized spot applied directly to the R2 surface. But due to an intellectual property claim that Albert Bouwers and others had held since 1950,[1] Braymer was forced to place the secondary spot on the outside or R1 surface of the meniscus instead. This less efficient design required light to travel through the corrector lens three times instead of only once. But this compromise did not adversely affect the performance of Braymer’s system, and it enabled him to move forward with production of the Questar telescope.

Considering that U.S. patents typically protect the intellectual property claims of their holders for seventeen years, Bouwers’s 1950 patent expired in 1967. With the disappearance of that constraint, Questar was free to implement the optical design that Lawrence Braymer had originally intended: placement of the secondary mirror spot on the inside or R2 surface of the corrector lens.[2]

The Nature and Significance of the Design Change

After Bouwers’s patent expired, Questar proceeded deliberately and cautiously with altering the design of a telescope that had proven to be a commercial success since it had first appeared on the market in 1954. The company spent a number of years creating designs for an instrument with a secondary mirror spot that was positioned for better performance. In 1972, the company sent optical drawings for the 3.5- and seven-inch Questar telescopes to its design consultant, Edward Kaprelian, for analysis.[3]

After Kaprelian completed his optical analysis of the relocated secondary spot, Questar moved forward with implementing the new design. In November 1977, as company records show, Questar performed its first testing for a 3.5-inch telescope with a secondary spot on the R2 surface.[4] The design passed muster. It then turned to implementing all of the necessary changes that had to be made to accommodate the new arrangement.

Dimension Changes

Relocating the secondary mirror spot from the inside to the outside surface of the corrector lens triggered a cascade of changes.

Since the distance between the R3 and R4 surfaces—that is, the primary mirror and secondary spot surfaces—needed to remain the same, Questar shifted the position of the corrector lens itself forward by a length that was approximately equal to the thickness of the lens. Doing so maintained the position of the secondary mirror spot relative to the primary mirror.

With this move, the length of the black anodized aluminum cell that housed the corrector lens also had to be extended forward. Between Questar telescopes with R1 and R2 secondary spot positioning, the length of corrector lens cell as measured from the front edge of the lens cell to its back edge, where it met the telescope barrel’s moon map skin, became longer. It changed from 0.218 to 0.420 inches, a difference of a bit less than a quarter-inch.[5] The length of the corrector lens cell is perhaps the most obvious visual way to distinguish whether a Questar has its secondary spot on the R1 or R2 optical surface.

Altogether, the entire length of the telescope tube increased about twice the thickness of the meniscus corrector lens.[6]

Longtime Questar manager Jim Perkins notes that, as of 2020, 3.5-inch systems with R2 optics have a computer-modeled focal length of 1269.7mm (50.0 inches) and an effective focal ratio of f/14.2. Slight variations may exist due to machining tolerances and optical curve variation.[7]

Improvements for Light Transmission

Each time that light encounters an optical element—that is, when light bounces off an optical surface or goes through it—a certain percentage of light becomes absorbed or is scattered by those elements. As an optical system becomes more complex, light transmission decreases. Simplifying that system results in more efficient light transmission and better performance overall.

Upon each point of contact with a Questar corrector lens surface with standard magnesium fluoride coatings, approximately 1.4% of light is lost. With broadband coatings, about 0.5% of light is lost. Reflections off primary and secondary mirror surfaces also result in 6% of light being lost with magnesium fluoride coatings and 2% with broadband coatings.[8]

The following chart assembled by Jim Perkins summarizes the percentage of light loss at each point of contact in Questar telescopes with R1 and R2 secondary mirror spots and with magnesium fluoride and broadband coatings:

Corrector Primary Mirror Corrector Secondary Mirror Corrector Total Light Loss
Front Back Back Front Front Back
Magnesium Fluoride Coatings R1 1.4 1.4 6 1.4 1.4 6 1.4 1.4 20.4
R2 1.4 1.4 6 - - 6 - - 14.8
Broadband Coatings R1 0.5 0.5 2 0.5 0.5 2 0.5 0.5 7
R2 0.5 0.5 2 - - 2 - - 5

In Questars with magnesium fluoride coatings, relocating the secondary mirror spot from the R1 to the R2 surface reduced light loss by 73%. Instruments with broadband coatings saw a 71% reduction in light loss.[9]

Images off the secondary mirror spot and the inside unmirrored surface of the corrector lens
Using a Questar with a secondary mirror spot on the outside (R1) surface of the corrector lens, one can bring the reflection of the Moon off the unmirrored inside surface to focus. On the left is the image of a crescent Moon as it would normally appear off the secondary mirror. The image on the right shows the focused reflection off the unmirrored inside surface of the corrector lens and the surrounding glare of unfocused light off the secondary mirror. Gregory Gross

Moreover, the redesign significantly decreased the amount of glare that an observer could see around bright objects like the Moon. Because of the unforgiving amount of light these luminous objects emit, scattering was inevitable if light had to pass through the corrector lens twice on its way to and from a secondary mirror on the outside surface of the lens. In a Questar with an R1 secondary spot, bright objects often appear with a noticeable amount of glare around them when their images are brought to normal focus. That glare represents unfocused reflected light off the unmirrored inside surface of the corrector lens. By working the focuser knob counterclockwise, an observer can move the primary mirror back to the point where the reflection off the unmirrored inside surface of the corrector lens is brought to a ghostly focus.

Production

Before Questar switched production to the updated optics, it first stayed true to the approach that Lawrence Braymer had always followed: use up all existing stock, and never waste anything. The company ran through all of the older R1 optics sets before ordering new ones with the updated design. Jim Perkins remembers that, in the late 1970s, the company typically had around two hundred optics sets and fifty completed telescopes on hand. While it consumed its existing stock, Questar and its optical suppliers developed new coating fixtures, updated grinding plates, changed test plates, and re-machined corrector lens cells.[10] Finally, as company documents show, units with R2 optics entered production in April 1978.[11]

Zerodur Primary Mirrors

Not long after it began production of telescopes with secondary mirror spots on the R2 surface of the corrector lens, Questar ended its use of Cer-Vit and switched to Zerodur for its primary mirror substrate upgrade option.

Introduced in 1968 by Schott AG, a Carl Zeiss subsidiary, Zerodur appeared on the market around the same time as Cer-Vit.[12] But Schott’s alternative consisted of a glass ceramic “semi-alloy” that combined rigidity and thermal stability. The manufacturer claimed that Zerodur’s amorphous crystalline structure made it highly resistant to the kind of long-term “flow” that more common types of glass supposedly exhibit. Moreover, its linear thermal expansion coefficient was practically zero at useful temperatures. It offered a level of stability far greater than Pyrex or fused silica mirrors could.[13]

In spite of these advantages, Questar opted for Cer-Vit mirrors because of the company’s commitment to finding as many domestic suppliers as it could for the parts that went into its products.[14] But by 1978, Corning Glass Works discontinued its production of Cer-Vit. For the next two years, Questar worked through its remaining stock of primary mirrors made out of that material before adopting ones made of Zerodur.[15]

In Questar promotional literature, the option for Zerodur mirrors first appeared in company’s 1980 Instruments and Accessories price list. A Standard Questar equipped with a Zerodur primary mirror and magnesium fluoride coatings cost $125 more than the base model with Pyrex mirror.[16]

In the 1987 Instruments and Accessories price list, the option to order a 3.5-inch Questar with a Zerodur primary mirror upgrade but with standard magnesium fluoride coatings disappeared.[17] Because most customers who opted for the mirror upgrade also chose broadband coatings, Questar simplified its offerings to match predominant demand by coupling Zerodur primary mirror and broadband coating options.[18]

Other Changes

During the late 1970s and the 1980s, Questar implemented other changes that were perhaps far less obvious.

The tabletop tripod legs that the company included with Standard and Duplex Questars slowly evolved during this period. Keen observers will notice that the rubber tips became larger between 1974 and 1977. Later, push-in tripod legs changed to threaded screw-in ones in 1981.[19]

The plugs that covered the two tripod leg holes on the side of the fork mount base also changed. Since the beginning of production in 1954, Questar included a clever way to hang one’s telescope off the window of an automobile using leg hole plugs that doubled as hangers. But sometime in the mid-1980s, this feature disappeared in favor of far simpler flat plugs.[20]

Questar side arm logo badges
Side arm logo badges with and without the "R" symbol. Stewart Squires via groups.io

Other changes were more cosmetic in nature. In 1981, the company made a slight alteration to the logo badges it applied to the side arm of the Questar’s fork mount. It added a small “®” symbol on the lower right-hand side of the badge.[21]

More subtle alterations to the side arms appeared. In late 1986, Questar transitioned from side arm edges with a polished finish to ones with a brushed, satin finish.[22] The change was the company’s way of adapting to the decline in the cosmetic quality of castings for these aluminum parts.[23]

Questar’s vinyl carrying case of the mid-1980s
Questar’s vinyl carrying case of the mid-1980s. Questar Corporation

The mid-1980s also saw the introduction of an updated carrying case, one that featured a vinyl exterior finish. It replaced the leather-clad model that the company had included with Standard and Duplex Questars since the early 1970s.[24]

And in the last few years of the 1980s, the company transitioned from handwritten to stenciled serial number markings on the base plates of Standard and Duplex Questars.[25]

The type and pace of updates and refinements that Questar introduced in the late 1970s and the 1980s were consistent with the slow and incremental approach the company had always takes with its products. For some, that approach was too slow and incremental.

Next: § 5.2. Competition Intensifies

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Notes

1 Albert Bouwers, Johannes Becker, and Adriaan Hendrik van Gorum, 1950, Reflecting Type Telescope Having a Spherical Mirror, U.S. Patent 2,504,383, filed December 18, 1945, and issued April 18, 1950, https://patents.google.com/patent/US2504383, accessed December 29, 2019.

2 Ben Langlotz, online forum posting, Cloudy Nights, March 12, 2019, https://www.cloudynights.com/topic/653162-an-observing-quirk-with-early-questar-scopes/?p=9209136, accessed December 29, 2019.

3 Jim Perkins, “Questar Serial Number Systems” (unpublished manuscript, August 20, 2020), typescript.

4 Jim Perkins, “Questar Serial Number Systems” (unpublished manuscript, August 20, 2020), typescript.

5 Jim Perkins, email message to author, October 2, 2020.

6 Alt-Telescopes-Questar Majordomo list message, September 8, 1999, digest 409, https://groups.yahoo.com/neo/groups/Questar/files/Alt-Telescopes-Questar%20Digests/, accessed October 14, 2019.

7 Jim Perkins, email message to author, October 6, 2020.

8 Jim Perkins, Alt-Telescopes-Questar Majordomo list message, November 11, 1997, digest 85, “Jim Perkins Questar data.doc,” https://groups.yahoo.com/neo/groups/Questar/files/FAQ/, accessed October 15, 2019.

9 Jim Perkins, Alt-Telescopes-Questar Majordomo list message, November 11, 1997, digest 85, “Jim Perkins Questar data.doc,” https://groups.yahoo.com/neo/groups/Questar/files/FAQ/, accessed October 15, 2019.

10 Jim Perkins, email message to author, August 24, 2020.

11 Jim Perkins, “Questar Serial Number Systems” (unpublished manuscript, August 20, 2020), typescript.

12 “Mid Production Questar Standard 3-½ Telescopes: Featuring SN 1043 from 1961,” Company Seven, n.d., http://www.company7.com/library/questar/que61.html, accessed July 11, 2019.

13 Jim Perkins, Alt-Telescopes-Questar Majordomo list message, November 11, 1997, digest 85, “Jim Perkins Questar data.doc,” https://groups.yahoo.com/neo/groups/Questar/files/FAQ/, accessed October 15, 2019.

14 “Mid Production Questar Standard 3-½ Telescopes: Featuring SN 1043 from 1961,” Company Seven, n.d., http://www.company7.com/library/questar/que61.html, accessed July 11, 2019.

15 “Mid Production Questar Standard 3-½ Telescopes: Featuring SN 1043 from 1961,” Company Seven, n.d., http://www.company7.com/library/questar/que61.html, accessed July 11, 2019.

16 Questar Corporation, Instruments and Accessories catalog, 1980.

17 Questar Corporation, Instruments and Accessories catalog, 1987.

18 “Mid Production Questar Standard 3-½ Telescopes: Featuring SN 1043 from 1961,” Company Seven, n.d., http://www.company7.com/library/questar/que61.html, accessed July 11, 2019.

19 Ben Langlotz, online forum posting, Cloudy Nights, June 12, 2017, https://www.cloudynights.com/topic/580599-questar-design-change-history/?p=7935420, accessed July 11, 2019.

20 Ben Langlotz, online forum posting, Cloudy Nights, June 12, 2017, https://www.cloudynights.com/topic/580599-questar-design-change-history/?p=7935420, accessed July 11, 2019.

21 Ben Langlotz, online forum posting, Cloudy Nights, June 12, 2017, https://www.cloudynights.com/topic/580599-questar-design-change-history/?p=7935420, accessed February 16, 2021. As of February 16, 2021, the last Questar known to the author to have a side arm logo badge without an “®” symbol is #1-Z-8370-BB, built in 1981 (“Questar 3.5 Zerodor with Broad Band coatings,” Astromart, March 31, 2016, https://astromart.com/classifieds/astromart-classifieds/telescope-catadioptric/show/questar-35-zerodor-with-broad-band-coatings, accessed February 16, 2021).

22 Ben Langlotz, online forum posting, Cloudy Nights, October 7, 2019, https://www.cloudynights.com/topic/580599-questar-design-change-history/?p=9689859, accessed October 7, 2019; architel, online forum posting, Cloudy Nights, October 7, 2019, https://www.cloudynights.com/topic/580599-questar-design-change-history/?p=9690518, accessed October 7, 2019.

23 “Early Production Questar 3-½ Telescopes: 1954 and 1955,” Company Seven, n.d., http://www.company7.com/library/questar/que54-55.html, accessed July 11, 2019.

24 Ben Langlotz, online forum posting, Cloudy Nights, June 12, 2017, https://www.cloudynights.com/topic/580599-questar-design-change-history/?p=7935420, accessed July 11, 2019. The latest example known to the author to include the redesigned case that appeared in the early 1970s is #2-DP-Z-8498-BB, built in 1982 (“Questar Duplex, Zerodur, Broadband,” Astromart, February 2, 2010, https://astromart.com/classifieds/astromart-classifieds/telescope-catadioptric/show/questar-duplex-zerodur-broadband-pending-david, accessed February 4, 2021).

25 As of December 19, 2021, the latest Questar known to the author to have its serial number hand-etched into its base plate is #9-9754-BB, built in 1989 (“Questar 3.5" Mak (Standard -?) - SN: 9-9754-BB,” Cloudy Nights, December 18, 2021, https://www.cloudynights.com/classifieds/item/280010-questar-35-mak-standard-sn-9-9754-bb/, accessed December 19, 2021).