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Early Questar Focal Length: Report Yours Here

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#1 Gregory Gross

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Posted 26 December 2019 - 01:03 PM

I've been doing some investigation on the history of what focal length Questar implemented in the 1950s and early 1960s. There are some holes in the information I have, and I was hoping that owners of Questars from this era could report the focal length (in inches) and ratio that is indicated on their scope's barrels. For the purpose of establishing a chronology, it's important also to have the serial number of the scope. Also important is knowing whether the scope went through a wide field conversion at some point in its lifetime.

A bit of background: Company 7 writes that early production models had a "nominal effective focal length of 1,070 mm [42.4 inches] at f/12." They note further that "by late 1961 Questar made several changes to the design of the main optics." This change resulted in Questar implementing "an intermediate production focal length of 1,156mm [45.5 inches at f/13] before finally settling in on 1,280mm [50.5 inches at f/14.4] that has been the standard for Questar 3-½ telescopes since."

Information from a variety of sources helped me narrow down the transition to more specific dates:

(1) Questar began production of their 3.5" scope in 1954 using a focal length of 42.4 inches at f/12.1. The company's advertisements that appeared in the February 1957 and November 1958 issues of Sky & Telescope also indicated these specs. The latest Questar that I’m aware of with this focal length is #9-532 (1959).

(2) Sometime around late 1959 or so, Questar switched to using a focal length of 45.5 inches at f/13. The Questar booklets from September 1959 and from 1960 all indicate this focal length and ratio. The earliest specific Questar that I know of with these specs is #0-859 (1960), and the latest is #2-1486 (1962).

(3) At some point in the early 1960s, Questar began implementing a focal length of 50.5 inches at f/14.4, which has indeed been the standard ever since. The earliest concrete indication of this changeover that I know of is the July 1964 Questar booklet, whose spec listing shows this. But I suspect the change could have happened earlier, maybe even by a few years.

I'm aware of a few interesting cases that raised questions in my mind about whether there is a connection between changes in focal length and the wide field construction that Questar announced in their magazine advertising of February 1964. Both #0-829 (1960) and #2-1543 (1962) are two older scopes that went through wide field conversions at some point in their lifetime, and each one has the newest focal length of 50.5 inches at f/14.4 shown on its barrel. I wonder if these conversions involve replacement of the corrector lens and primary mirror. Since Questar's implementation of the newer wide field construction involved a wider main tube and mirror thimble (see pp. 2-4 of this PDF posted on Questar's website for more info), it's not unreasonable to conclude that it wasn't possible for Questar to transplant the existing primary mirror that rode on a smaller thimble in a scope undergoing a wide field conversion onto a wider thimble. I would reason that Questar would have reached for a new optics set as part of a wide field conversion. This would explain the implementation of the newest focal length in the two older scopes I mention above.

Considering all of this, it would be most useful to learn of the earliest and latest instance of that intermediate focal length of 45.5 inches at f/13. In other words, how much earlier before Questar #0-859 (1960) and how much later after #2-1486 (1962) did the company build their scopes with a focal length of 45.5 inches at f/13?

If anyone owns an instance from this era, don't be shy to post a report about it here or PM me. Thanks much!


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#2 Optics Patent

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Posted 26 December 2019 - 03:43 PM

1957 (refurb 1961) - no markings on moon map, no red triangle stickers on control box.  5mm of cell visible forward of moon map.

 

1962 - moon map marked 45.5" f13 (visual).  No red stickers, same 5mm cell.

 

1964 - moon map marked 50.5" f14.4.  (5mm)

 

1966 - same as 1964

 

1973 duplex - Visual 1300mm, f14.4.  (5mm)

 

Didn't check 80s and newer, and would be good to know when that cell got longer (for Pelican case fitting, etc).



#3 Gregory Gross

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Posted 26 December 2019 - 07:06 PM

Do your 1964 and '66 Questars have the wide field construction? If not, that would answer another question I had about whether narrow field Questars (i.e., the so-called "standard" construction Questars with smaller rear axial port) had the older 45.5" visual focal length at f/13 even into the mid-1960s or whether they had, along with the wide field models, the newer 50.5" focal length at f/14.4.

 

Some more background: Between 1964 and 1972, Questar continued to offer new scopes built using the "standard" construction, and they only offered the wide field construction as a $100 upgrade option. The 1971 Price Catalog still differentiates the "Standard Questar, Pyrex Mirror" ($1140) from the "Standard Questar, Pyrex Mirror, Wide Field" ($1240). As late as 1972, as the "lnstruments and Accessories" catalog from that year indicates, "A variation of the Standard Questar with wide-field construction, is of interest to all who take photographs with the instrument. Twenty-one major internal barrel and control box changes permit coverage of about 20% more of the 35mm. double frame without loss of resolving power and with better illumination." But by the next year, the 1973 "Instruments and Accessories" catalog indicates that "all Questars now have the wide-field construction." It therefore seems that 1973 was the year that the wide-field construction became standard.

 

It strikes me as being unlikely that the company would have ordered optics sets from Cumberland for two different focal lengths for the 3.5" Questar during the period between 1964 (maybe earlier) and 1972. It seems much more likely that all optics sets that Questar received from Cumberland after adopting the 50.5-inch focal length at f/14.4 were all figured the same, the only variation being the size of the center hole through which the larger or smaller mirror thimble fit.

 

For what it's worth, the matter of the switch from the R1 to the R2 position for the secondary spot and the subsequent lengthening of the corrector lens cell is a whole other tangled question that I commented about in this posting.



#4 davidmcgo

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Posted 26 December 2019 - 08:29 PM

My 1965 quartz had the narrow field and was 45.5 inch focus as marked on moon map and confirmed with star drift timings with a Baader 12mm Micro guide eyepiece. No red sticker on the control box and short front cell barely protruding past the Moon map.  Secondary on R1 surface.

 

Since  converted to new optics and 50.5 inch, also confirmed with star drift timings using the reticle on the Baader 12mm micro guide.

 

Dave


Edited by davidmcgo, 26 December 2019 - 08:30 PM.


#5 Gregory Gross

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Posted 27 December 2019 - 12:34 AM

Thanks for posting that information about your 1965 Questar, Dave. I realize now that my earlier speculation about Questar using only one focal length in their scopes after around 1964 was wrong. I appreciate the clarification -- very helpful.

 

I'm curious: after you had Questar replace the optics, did they also update the moon map to a new one with updated specs, or is the original one still on the scope?



#6 davidmcgo

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Posted 27 December 2019 - 09:39 AM

Original one is still on the scope.

 

Dave



#7 Alan French

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Posted 05 January 2020 - 09:25 PM

We owned #48 for more than a decade and the focal length was 45-inches.

 

Clear skies, Alan




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