17 replies to this topic

[Gregory Gross]

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!

[Optics Patent]

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).

[Gregory Gross]

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.

[davidmcgo]

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.

[Gregory Gross]

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?

[davidmcgo]

Original one is still on the scope.

 

Dave

[Alan French]

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

 

Clear skies, Alan

[Gregory Gross]

I thought I'd tie a bow on this old discussion thread by adding my current knowledge of the timeframe for Questar's changing focal length indications on the spec markings that appear on the bottom of Standard 3.5" Questar barrels.
 
The earliest indicated visual focal length, 42.4 inches at f/12.1, appeared from the beginning of production in 1954 to around April 1960 if not a bit later.
 
At this point, a focal length marking of 45.5 inches at f/13 began appearing on scopes. Questar would continue to indicate this updated spec on production units until around early 1963 if not much later, as Dave's 1965 quartz-mirrored Questar (noted above in post #4) indicates.
 
In 1963 or so, Questar began indicating a visual focal length of 50.5 inches at f/14.4, which remains the modern focal length marking.
 
I discuss the changing focal length markings in more depth here. As always, if anyone has any evidence that would improve our understanding of this little bit of Questar history minutiae, please don't be shy about posting here. Since Questar would so often modify older scopes sent in for service, early examples with a newer characteristic aren't as dependable in terms of serving as evidence as later examples with an older characteristic are.

[JamesMStephens]

My Duplex was made in August 1975.  The lens cell looks long enough so that i always thought it was an R2 design, but according to your timeline maybe I need to measure carefully.

 

Jim

[Gregory Gross]

There has been some discussion of changes to Questar's focal lengths and ratios in our Questars on eBay and Craigslist discussion thread, so I thought I'd consolidate some of that discussion in this thread just for the record.

 

I noted that #0-713 appeared on eBay with a focal length and ratio of 42.4 inches at f/12.1. Roberto kindly noted that his #0-744 has a focal length and ratio of 45.5 inches at f/13, which to my knowledge appeared on Questars into early 1963.

 

Considering that Questars sent in for service were (and continue to be) often subject to updates, I think that the appearance of a later characteristic on an earlier scope isn't as good of an indicator of when a change happened as much as an earlier characteristic appearing on a later scope is. But I think in this case of Questar design change minutiae, it's safe to say that Questar transitioned from 42.4 inches at f/12.1 to 45.5 inches at f/13 at some stage between #713 and 744. It'd be interesting to learn of examples sitting out there between these two numbers.

[R Botero]

Here are a few pictures of my f/13 one:

 

 

 

 

In use at 4 degrees north:

 

 

Roberto

[R Botero]

And the mirror, pristine after decades and decades with a tube that holds perfect collimation:

 

[norcalryder]

Here is mine.  Y2K I believe.  SN 0 - 9913 - BB

 

 

[Spikey131]

#3-1901.  1963.  No changes in 37 years!

Attached Thumbnails

• 

Edited by Spikey131, 03 February 2023 - 12:56 PM.

[Bomber Bob]

Great info.  My theory is that Questar wanted to keep us confused...  (Doesn't take much with me!)  My '58 Standard (# R-8-259) has a manufactured date of 4/16/1958, and its booklet says its F/12.1 & 42.4 inches.  It went back for servicing 10 years later, at which point the barrel / Moon map was changed, and it has the updated specs of F/14.4 & 50.5 inches...  So, when I'm calculating magnification, I have to remember 1280mm -- rather than the original 1080mm...

 

BIF:  Before getting my Q, I thought both original eyepieces were Japan-made ERFLES; but, my booklet says the 40x is a 26mm KOENIG, and the 80x is a 13mm Erfle.

[Alan French]

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

 

Clear skies, Alan

Since correspondence says this has caused some confusion, I asked my wife what she remembers, and she said it was #148. Some old emails, one from 2010, say she is correct.

 

Clear skies, Alan

[bmwscopeguy]

Hi Greg,

 

You had asked if I'd send the focal length specs to you once I received my new (to me) 3.5 - s/n #3QTZ-1697

 

Attached is a pix of the spec nameplate on the barrel.

 

Cheers

Malcolm Bird

Attached Thumbnails

• 

[RMay]

Here’s one (not mine) recently sold at auction for $2700, and perhaps for database reference only: Q3.5 S/N #64. No markings on tube/dew shield that I could see, but note etched on base, “will focus from 5’8” to infinity.”

 

Ron

Attached Thumbnails

•