Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Why 0.965”

  • Please log in to reply
58 replies to this topic

#26 Kokatha man

Kokatha man

    Voyager 1

  • *****
  • Posts: 13,369
  • Joined: 13 Sep 2009
  • Loc: "cooker-ta man" downunda...

Posted 09 February 2020 - 08:00 PM

Hmmm - 0.45mm wall thickness would be a very thin-walled tube if it really was a common product & one wonders how much use such a product would have had in general...leaving aside the fact that it is a mix of Metric & Imperial, the Imperial equivalent of 0.45mm is 0.0177etc inches which I'm unaware of having existed as any sort of standard anytime...but who knows..?

 

I can see a 25mm product as likely in Germany & other parts of Europe from which the Metric system spread (France under Napolean in fact) but I would've thought it would've been at least 0.5mm wall thickness - & most likely thicker still...we're talking about a time when nothing was done in "flimsy" modes. ;)

 

Focusing on the Japanese tendencies, historically Zeiss' involvment with Japanese optics goes back way before WW2 & Japanese industry had no problems mixing Imperial & Metric: the "Treaty of the Metre" was signed in 1885 & the British Imperial standard was made legal in 1909...

 

In 1924 Japan legally adopted the metric system but others were allowed as "transitional" & my understanding of why Imperial had a hold is because of the British Navy's prowess & adoption of said standard in specific industries during the time of Japan's modernisation...

 

Regardless, we still see Japanese industries' "mix & match" approach way after WW2 & the extent of Americanisation of Occupied Japan: Imperial still popping its' head up & other unusual elements like the "JIS" (Japanese Industry Standard) coarse 0.9pitch machine screws commonly used up to 1967...

 

None of the preceding really addresses European origins of the 0.965" fittings which are actually 24.511mm.....meaning that if they pared down some European (Metric) standard tubing of 25mm (if it did exist)...then they did so to leave a wall thickness of 0.2445mm - to me that makes the entire scenario even more curious!  rofl2.gif


Edited by Kokatha man, 09 February 2020 - 08:02 PM.


#27 Michael Covington

Michael Covington

    Author

  • *****
  • Vendors
  • Posts: 5,639
  • Joined: 13 May 2014
  • Loc: Athens, Georgia, USA

Posted 09 February 2020 - 09:17 PM

I like the theory that it was 25-mm tubing turned down in a lathe for roundness and precise diameter, giving 24.5 mm.

 

No confirmation of it, but it's the most plausible speculation so far.



#28 Michael Covington

Michael Covington

    Author

  • *****
  • Vendors
  • Posts: 5,639
  • Joined: 13 May 2014
  • Loc: Athens, Georgia, USA

Posted 09 February 2020 - 09:18 PM

 One of the other reason why Japanese telescope followed the Zeiss standard is that after WWII, the US was helping to rebuild Japanese economy and brought Zeiss into the teach the Japanese  mass production techniques for optics  So it makes sense that they would adopt the Zeiss standards.

     I just had another examples of this. A friend purchased a small Takahashi mount that was made in the early 80's off on Ebay. I made a new hand paddle to  replace the missing one to the control the electronics in the RA motor housing. He went to put the mount on a tripod and discovered that the threads for the tripod mount is a Whitworth type !  Whitworth is a British standard on a Japanese mount ? It is most likely left over from 35 years earlier. 

 

                  - Dave

Is it 1/4"-20 Whitworth?  That is the international standard for camera tripod sockets.  And another 3/8" Whitworth for larger cameras (still used to hold the heads on Italian-made Manfrotto tripods).  Standard American hardware fits them, but the Whitworth thread has sharper peaks, as you know.



#29 Terra Nova

Terra Nova

    ISS

  • *****
  • topic starter
  • Posts: 21,211
  • Joined: 29 May 2012
  • Loc: Middle Earth

Posted 09 February 2020 - 10:39 PM

I am skeptical of the focuser tubing idea.  My thinking:

 

By the 1890s, Germany would have been standardized on metric system so 25.0mm tubing have been standard.  A 24.5 mm ID would mean a wall thickness of 0.25mm. That's 0.010". That's very thin, about the thickness of the pop open lid on a can of cat food.

 

Even if they started with 1 inch tubing the wall would be 0.45mm = 0.018". Still very thin. 

 

For comparison, a 1.25 inch eyepiece has wall thickness of about 1.9 mm = 0.074 ".

 

Writing this, what does make sense to me is this:

 

Modern tubing can be very round, DOM, drawn on a mandrel maybe +/- 0.001 ".  Even then it can be out of round. In the 1890's, it probably was not as good as what we have today. But

 

But even today, if you start with a 25mm OD tube and want to make a batch of eyepiece barrels, the first thing you do is put them in a lathe and clean them up so they're straight and round. Taking 0.5 mm off the OD would pretty much guarantee you that all the tubing would clean up, I'd spec something like that. 

 

From my perspective as an engineer, this fits very nicely, it fits the metric system, it explains why it's 24.5 mm and not 25.0 mm.

 

Jon

I really like this idea. Having done exactly this with a lathe and round metal stock. It make a great deal of sense to me.


  • Jon Isaacs and bbqediguana like this

#30 Kokatha man

Kokatha man

    Voyager 1

  • *****
  • Posts: 13,369
  • Joined: 13 Sep 2009
  • Loc: "cooker-ta man" downunda...

Posted 09 February 2020 - 11:05 PM

I like the theory that it was 25-mm tubing turned down in a lathe for roundness and precise diameter, giving 24.5 mm.

 

No confirmation of it, but it's the most plausible speculation so far.

From what I posted in Post #26 I don't think that makes a lot of sense Michael - in manufacturing it is the O.D. (outside diameter) that would be machined down to create any uniform diameter...& turning out the I.D. to 0.965" which is the same as 24.511mm actually means the I.D. would have to be greater than this figure (24.511mm) to allow for fitting tolerances: this actually means less than 0.2445 wall thickness (25.0 - 24.511* = 0.489 ÷ 2 = 0.2445#)

 

* signifying the figure is very marginally greater than 24.511mm internal diameter. 

# signifying this figure is very marginally less than 0.2445mm wall thickness.

 

Nothing very precise in that imho! ;)



#31 Kokatha man

Kokatha man

    Voyager 1

  • *****
  • Posts: 13,369
  • Joined: 13 Sep 2009
  • Loc: "cooker-ta man" downunda...

Posted 09 February 2020 - 11:15 PM

As usual I'm looking at things from the reverse, concentrating on the focuser tubing to fit 0.965" ep barrels...but they're still not 24.5mm, but 24.511mm - which doesn't fit into some "nice 24.5mm" scenario ;)...perhaps Huygens or Newton et al ought to own up to some involvement..! :lol:

 

My "reverse rationale" about fitting tolerances might be inconsequential for the ep barrels, but there would still be a question on what "standard" the focuser tubes came from! :lol: 



#32 grendel

grendel

    Surveyor 1

  • -----
  • Posts: 1,540
  • Joined: 12 Apr 2009
  • Loc: Canterbury, Kent, UK

Posted 10 February 2020 - 03:10 AM

consider this, the machine for drawing brass telescope tubes was an english invention in 1782, that gives near a century of brass tubing production, could it be that one of the standard mandrel sizes gave a 1" outside diameter, and an inner diameter that was 24.5" and this was then taken up as a standard, has anyone ever measured the eyepiece size on an old broadhurst clarkson telescope and compared that, of cours 100 years after the introduction of the brass tubing machine each company building telescopes would by now own their own brass tubing machines, so each company would have standardised the tube sizes to just a few they used regularly hence the standardisation of the eyepieces.

then think along another tack, the size of the lenses in the eyepieces may well have dictated the size of the eyepiece barrels, was this the driver for the standard.



#33 grendel

grendel

    Surveyor 1

  • -----
  • Posts: 1,540
  • Joined: 12 Apr 2009
  • Loc: Canterbury, Kent, UK

Posted 10 February 2020 - 03:32 AM

just reading further through the broadhurst clarkson history (https://www.bcfteles...and-fuller-ltd/) it stated that in the late 18,00s early 1900's they were supplying brass tube to other telescope manufacturers, could this brass tube be where the standard originated.


  • Terra Nova likes this

#34 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 81,789
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 10 February 2020 - 05:32 AM

As usual I'm looking at things from the reverse, concentrating on the focuser tubing to fit 0.965" ep barrels...but they're still not 24.5mm, but 24.511mm - which doesn't fit into some "nice 24.5mm" scenario wink.gif...perhaps Huygens or Newton et al ought to own up to some involvement..! lol.gif

 

My "reverse rationale" about fitting tolerances might be inconsequential for the ep barrels, but there would still be a question on what "standard" the focuser tubes came from! lol.gif

 

0.965" is the nominal diameter, not the design diameter including clearances. The difference between 0.965" and 24.5 mm is 0.0004" or 11 microns.  

 

I don't know if you read my previous post where I proposed the 24.5 mm standard came from making eyepiece barrels from 25 mm tubing and turning them so theyre true. That's standard matching practice.

 

I just measured the OD of the 0.965" eyepiece I own, a Japanese 20 mm Hygenian. Using a Starrett micrometer, I measured 0.962". That's a reasonable clearance for 24.5 mm/0.965". 

 

I'm not a machinist, I'm an engineer who works very closely with machinists. I've designed a lot of expensive stuff using tubing.. Clearances are critical so there's always a difference between nominal and actual.

 

0.965" doesn't make sense in the inch system, 24.5 mm makes good sense in the Metric context.

 

Jon


  • Corcaroli78, Terra Nova and bbqediguana like this

#35 luxo II

luxo II

    Apollo

  • -----
  • Posts: 1,478
  • Joined: 13 Jan 2017
  • Loc: Sydney, Australia

Posted 10 February 2020 - 05:41 AM

Simple - Somebody made a mistake and thought 1” was 24.5mm not 25.4mm. Easy mistake if you were a bit scrambled with both metric and imperial, as my country was for decades.

Whoever it was set a precedent that stuck, unfortunately.

It never made much sense in any “system” and frankly 1.25” is just as arbitrary.

Edited by luxo II, 10 February 2020 - 05:42 AM.


#36 Kokatha man

Kokatha man

    Voyager 1

  • *****
  • Posts: 13,369
  • Joined: 13 Sep 2009
  • Loc: "cooker-ta man" downunda...

Posted 10 February 2020 - 06:27 AM

Your rationale might well be correct Jon, but I'm inclined to disagree. flowerred.gif As a former practising jeweller I have a fair bit of experience in drawing both rod & tubing down to a required size...tube "sinking" that is, where the rod or tube is drawn through a (wait for it!) "drawplate" which is simply a hardened metal plate with tapered dies or draw-holes to provide the correct size. wink.gif

 

This tube can well vary in wall thickness as opposed to "rod drawing" where the metal is drawn over a mandrel which regulates the O.D, I.D. & wall thickness, but in fact the material pulled through a simple die or drawplate is quite consistent in O.D. 

 

Grendel's comments might well hold more of the truth: the earliest "semi-precision metal-working lathes" were not around until the 1770's even though lathes per se are ancient devices: I guess the bone of my contention is that regardless of when this so-called "standard" was arrived at & whatever it is precisely,* I find it difficult to believe that drawn tube needed to be lathed to create early ep's as well as (pardon the pun!) draw-tubes as in focuser tubes...

 

*Btw I've measured numerous 0.965" ep barrels & there is a wide variety of difference on both the plus & minus side of that figure, some are considerably over & a few under which to my mind makes some notions or "standards" even more problematical...

 

But whatever, I think Terra has raised one of the more interesting & arcane aspects of telescope fittings' manufacturing & I'd like to dig deeper or hear more from anyone interested - & as several folks have suggested, it may well have originally been an idiosyncratic happenstance! waytogo.gif


  • Terra Nova likes this

#37 grendel

grendel

    Surveyor 1

  • -----
  • Posts: 1,540
  • Joined: 12 Apr 2009
  • Loc: Canterbury, Kent, UK

Posted 10 February 2020 - 07:26 AM

what I find interesting is that one company perfected the drawn brass tube, and that today the tube that holds the eyepiece is called the draw tube, could this be an indication of the source for such tubing, larger diameter tubing can be rolled and seamed, but for a smooth focussing action a seamless tube would have been a necessity, as it would for the extending tubes of nautical telescopes.


  • Terra Nova and cookjaiii like this

#38 Senex Bibax

Senex Bibax

    Mariner 2

  • -----
  • Posts: 286
  • Joined: 01 Feb 2019
  • Loc: Ottawa, ON

Posted 10 February 2020 - 07:51 AM

Looking to the British for an explanation seems a bit of a stretch to me, knowing their historic love of "every manufacturer for themselves" standards. Just try working on old British bicycles. You can tell which were made by Raleigh, even if sold under other names, because nothing fits from any other manufacturer, and vice versa.



#39 Terra Nova

Terra Nova

    ISS

  • *****
  • topic starter
  • Posts: 21,211
  • Joined: 29 May 2012
  • Loc: Middle Earth

Posted 10 February 2020 - 07:55 AM

0.965" is the nominal diameter, not the design diameter including clearances. The difference between 0.965" and 24.5 mm is 0.0004" or 11 microns.  

 

I don't know if you read my previous post where I proposed the 24.5 mm standard came from making eyepiece barrels from 25 mm tubing and turning them so theyre true. That's standard matching practice.

 

I just measured the OD of the 0.965" eyepiece I own, a Japanese 20 mm Hygenian. Using a Starrett micrometer, I measured 0.962". That's a reasonable clearance for 24.5 mm/0.965". 

 

I'm not a machinist, I'm an engineer who works very closely with machinists. I've designed a lot of expensive stuff using tubing.. Clearances are critical so there's always a difference between nominal and actual.

 

0.965" doesn't make sense in the inch system, 24.5 mm makes good sense in the Metric context.

 

Jon

I agree, it makes sense that 0.965” followed 24.5mm.

 

To through one more monkey wrench into the works, the standard diameter for microscope eyepieces is 23mm or 0.917”. But maybe this isn’t a monkey wrench after all, but really a clue. I think we can acknowledge that 23mm pretty much a round number, perhaps not as round as 20 or 25 but round enough. Also Zeiss was making microscopes long before they were ever making telescopes and microscopes were the biggest part of their business. If Zeiss established the the 23mm microscope standard, it leads me to believe that they established the 24.5mm standard as well. Of course this doesn’t tell us why it was 24.5mm and not 25 or 30, but it might narrow the search in terms of where to look for the answer.


Edited by Terra Nova, 10 February 2020 - 09:10 AM.

  • Jon Isaacs likes this

#40 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 81,789
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 10 February 2020 - 09:32 AM

This tube can well vary in wall thickness as opposed to "rod drawing" where the metal is drawn over a mandrel which regulates the O.D, I.D. & wall thickness, but in fact the material pulled through a simple die or drawplate is quite consistent in O.D.

 

Nonetheless:

 

Standard machining practice today for precision parts is to start with an oversized piece and machine it so it is both round and straight. 

 

I've built a lot of stuff using CDS (Cold drawn seamless) and DOM (Drawn over mandrel.)  The specs are pretty tight but if i want to avoid turning a long tube to spec'd I will design my clearances for the other parts in reference to the measured diameter of the tubing.

 

Jon


  • Terra Nova likes this

#41 Terra Nova

Terra Nova

    ISS

  • *****
  • topic starter
  • Posts: 21,211
  • Joined: 29 May 2012
  • Loc: Middle Earth

Posted 10 February 2020 - 10:02 AM

Nonetheless:

 

Standard machining practice today for precision parts is to start with an oversized piece and machine it so it is both round and straight. 

 

I've built a lot of stuff using CDS (Cold drawn seamless) and DOM (Drawn over mandrel.)  The specs are pretty tight but if i want to avoid turning a long tube to spec'd I will design my clearances for the other parts in reference to the measured diameter of the tubing.

 

Jon

Always!

Attached Thumbnails

  • D726652A-D219-4162-A10B-9CBB9BDFF49D.jpeg


#42 DAVIDG

DAVIDG

    Cosmos

  • *****
  • Posts: 9,235
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 10 February 2020 - 10:56 AM

I agree, it makes sense that 0.965” followed 24.5mm.

 

To through one more monkey wrench into the works, the standard diameter for microscope eyepieces is 23mm or 0.917”. But maybe this isn’t a monkey wrench after all, but really a clue. I think we can acknowledge that 23mm pretty much a round number, perhaps not as round as 20 or 25 but round enough. Also Zeiss was making microscopes long before they were ever making telescopes and microscopes were the biggest part of their business. If Zeiss established the the 23mm microscope standard, it leads me to believe that they established the 24.5mm standard as well. Of course this doesn’t tell us why it was 24.5mm and not 25 or 30, but it might narrow the search in terms of where to look for the answer.

From both an optical stand point and an economic one I can understand why Zeiss would have two different sizes for microscope vs telescope eyepiece. The optical designs can be different so you would want the correct eyepiece design to used on the instrument they were design for. Second if you want to sell the most eyepieces you want a different standard  for both.  As I said before, economic plays just as an  important role as scientific reasons why a standard is adopted. So a simple reason why the  24.5mm  standard was adopted  was it saved money and/or make money for Zeiss or whoever started it.

    Here is a wild guess but what is the OD of the draw tube for Zeiss microscope  focuser  of that period when the 24.5mm  telescope eyepiece was started ?  Is it 24.5mm OD with a  23mm ID  ? Maybe all Zeiss did was use the tubing for the microscope focusers which needs to be bigger in diameter then the 23mm microscope  eyepiece as the barrels for the telescope eyepieces.

                      - Dave


  • Terra Nova likes this

#43 Astrojensen

Astrojensen

    Voyager 1

  • *****
  • Posts: 12,615
  • Joined: 05 Oct 2008
  • Loc: Bornholm, Denmark

Posted 10 February 2020 - 11:40 AM

    Here is a wild guess but what is the OD of the draw tube for Zeiss microscope  focuser  of that period when the 24.5mm  telescope eyepiece was started ?  Is it 24.5mm OD with a  23mm ID  ? Maybe all Zeiss did was use the tubing for the microscope focusers which needs to be bigger in diameter then the 23mm microscope  eyepiece as the barrels for the telescope eyepieces.

                      - Dave

First of all, we still don't know exactly when or why the barrel diameter of 24.5mm was chosen, second, the eyepiece adapters of microscopes, just like on telescopes, was turned down from larger stock on a lathe. 

 

Even very early Zeiss microscopes are fairly elaborately machined, even more so by their day and age. 

 

 

Clear skies!
Thomas, Denmark


  • Jon Isaacs and Corcaroli78 like this

#44 Steve C.

Steve C.

    Viking 1

  • -----
  • Posts: 677
  • Joined: 24 Jan 2008
  • Loc: Sugar Land, TX

Posted 10 February 2020 - 11:46 AM

Could binoculars have been the original source? Decades ago, I remember fashioning a wide-field eyepiece by taking it out of the binoculars and it fit perfectly in my Tasco reflector which was set up for .965.   


  • wfj likes this

#45 Chuck Hards

Chuck Hards

    You don't know Swift from Astrola

  • *****
  • Administrators
  • Posts: 26,626
  • Joined: 03 May 2010

Posted 10 February 2020 - 12:29 PM

The military usually drives things like this.  I would speculate that all optical size standards had their genesis in former Zeiss military contracts with the German Imperial Army.



#46 davidc135

davidc135

    Viking 1

  • -----
  • Posts: 770
  • Joined: 28 May 2014
  • Loc: Wales, UK

Posted 10 February 2020 - 01:30 PM

I agree, it makes sense that 0.965” followed 24.5mm.

 

To through one more monkey wrench into the works, the standard diameter for microscope eyepieces is 23mm or 0.917”. But maybe this isn’t a monkey wrench after all, but really a clue. I think we can acknowledge that 23mm pretty much a round number, perhaps not as round as 20 or 25 but round enough. Also Zeiss was making microscopes long before they were ever making telescopes and microscopes were the biggest part of their business. If Zeiss established the the 23mm microscope standard, it leads me to believe that they established the 24.5mm standard as well. Of course this doesn’t tell us why it was 24.5mm and not 25 or 30, but it might narrow the search in terms of where to look for the answer.

Just had a look at my grandmother's 1910 microscope- definitely a classic. It's a Leitz rather than a Zeiss but the viewing end consists of an eyepiece 23.16 mm OD which slides into a tube of 23.8 OD which either screws into or becomes 25.1 OD which has a friction fit into another short tube/ ring of 26.45 OD which finally screws into the main tube of 29.65mm OD. 

 

I expect the Zeiss equivalent instrument would have much the same arrangement.

She bought it second hand in 1922 for one pound and ten shillings! I expect for field work during her Botany course.

 

David


Edited by davidc135, 10 February 2020 - 01:49 PM.

  • Terra Nova likes this

#47 JuergenB

JuergenB

    Vostok 1

  • -----
  • Posts: 117
  • Joined: 15 Nov 2015
  • Loc: Lilienthal, Germany

Posted 10 February 2020 - 01:36 PM

I agree, it makes sense that 0.965” followed 24.5mm.

 

To through one more monkey wrench into the works, the standard diameter for microscope eyepieces is 23mm or 0.917”. But maybe this isn’t a monkey wrench after all, but really a clue. I think we can acknowledge that 23mm pretty much a round number, perhaps not as round as 20 or 25 but round enough. Also Zeiss was making microscopes long before they were ever making telescopes and microscopes were the biggest part of their business. If Zeiss established the the 23mm microscope standard, it leads me to believe that they established the 24.5mm standard as well. Of course this doesn’t tell us why it was 24.5mm and not 25 or 30, but it might narrow the search in terms of where to look for the answer.

I always find 23.2 mm, not 23.0 mm as the O.D. of old microscope eyepieces. This is even stranger than the 24.5 mm we were discussing about.

 

Juergen


  • Terra Nova likes this

#48 Terra Nova

Terra Nova

    ISS

  • *****
  • topic starter
  • Posts: 21,211
  • Joined: 29 May 2012
  • Loc: Middle Earth

Posted 10 February 2020 - 05:23 PM

I always find 23.2 mm, not 23.0 mm as the O.D. of old microscope eyepieces. This is even stranger than the 24.5 mm we were discussing about.

 

Juergen

 

As Yogi Berra would say, “it’s deja vu all over again!” :lol:

 

Once again, it’s a mystery, wrapped in an enigma, wrapped in a conundrum! ;)


  • JuergenB likes this

#49 Kokatha man

Kokatha man

    Voyager 1

  • *****
  • Posts: 13,369
  • Joined: 13 Sep 2009
  • Loc: "cooker-ta man" downunda...

Posted 10 February 2020 - 07:50 PM

Once again, it’s a mystery, wrapped in an enigma, wrapped in a conundrum! wink.gif

...& obviously one that gets trotted out every so often on CN Terra - I've had a look through several threads here over the years & learnt stuff like "France went metric in the 1600'sbigshock.gif bangbang.gif  rofl2.gif & even Jon (tongue firmly in cheek, I'm sure!) posited it was during a conversation between Huygens & Nagler... hmm.gif  

 

Somewhere in the mists of time (& for older folks like myself this can mean yesterday, or 5 minutes ago scratchhead2.gif ) I recall the notion that as everyone made almost everything associated with whatever the finished article was comprised of that at some stage some influential manufacturer started the whole shebang re 0.965" ep's: Grendel's suggested this up the page a bit btw...

 

I have just found this youtube video which promises to (possibly!) shed more light on some of the aspects discussed here - I have to go & pick up my Royal Astro fabrications from Dennis right now so I cannot watch the 44 minute video & don't want to speed-read/look at it.....hopefully it'll shed that light! (maybe someone will report on it by the time we get back home...) https://www.youtube....h?v=rf93q626KhQ

 

  



#50 Ben H

Ben H

    Viking 1

  • -----
  • Posts: 683
  • Joined: 12 Jun 2012

Posted 10 February 2020 - 10:06 PM

Another thing to add, it is possible that Zeiss chose 24.511 mm because it was exactly one inch. But wait, you say, an inch is 25.4 mm!

Not exactly. By the early 1800s there were over 100 unique definitions for common measurements in the German states, often varying from city to city. On entering a city, you might even find the standards posted at its entrance.

The German inch, the zoll, was usually defined as 1/12th of a foot, or Fuß, though in some places it was 1/11th of a foot, or even 1/10th. The Fuß itself varied between 250 mm and 435 mm. 

I haven't yet found a historical definition of the Fuß for Jena, but ~294 mm wouldn't have been outside of possibility. Nearby Weimar and Leipzig's Fuß was 282 mm, Erfurt's was 283mm, but Freiburg's was 293mm. 

Even if Zeiss introduced the 0.965" standard after 1872 when the metric system became compulsory, the Jena machinists may have still chosen to use the zoll as the basis, even if they officially denominated it in millimeters. 

Of course, that might all be a total red herring because, as I've said I don't know what the Jena Fuß or zoll was. 


Edited by Ben H, 10 February 2020 - 10:08 PM.

  • Astrojensen, Corcaroli78, Terra Nova and 1 other like this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics