Inches vs. millimeters

[deanm]

Why is it that folk typically refer to the aperture of their reflector in inches, but the refractor gang use millimeters?

I know that's not always the case, but why?!

Dean

[JA]

Good question - Maybe there was a time where a lot of the reflectors were made in the USA (or possibly some in UK) and the refractors more so from the rest of Europe and that has somehow morphed into some sort of defacto naming convention.

Best
JA

[Kunama]

It's Aperture Envy..... 102mm sounds so much more than 4"....

[julianh72]

I've always found it confusing that people use a 25 mm focal-length eyepiece with a 1 1/4" filter and diagonal, attached to their 8" SCT, which has a focal length of 2000 mm. The visual back uses a 2" x 24 TPI thread. Their astro-camera might have a 1/3" sensor with 2.2 micron pixels.

:shrug:

[N1]

They tend to operate in different aperture classes. Small newtonian sizes which are fairly common such as 76mm and 114mm are given in mm also, most of the time it would seem.

@Matt - and here we have it. It's not aperture that rules, it's aperture envy :D

[Renato1]

I was quite surprised that some of my astro-buddies in Italy referred to their telescopes as being 8 or 10 "pollici" (one pollice= one inch).
Cheers,
Renato

[skysurfer]

Indeed, I am also surpised.

Telescope apertures and screen (TV, laptop, tablet, cellphone) diagonals are expressed in inches.
Despite 95% of the world population (incl Australia and EU) uses metric.
The only exception is aviation altitude in feet and speed in nautical miles (=1.852km other than US miles which are 1.609km each).
It is probably too much American influence, despite screen manufacturers are usually Asian.
Telecope manufacturers are in many cases American.

Another issue is that much software uses the confusing M/d/y date and AM/PM time notation and this confusion is not enough, as 11pm is followed by 12am, which is 23:00 resp 0:00.

[Stardrifter_WA]

Some interesting observations here.

I am a machinist by trade and when I did my time in the 70's it was all manual and all imperial and it wasn't until the mid 80's that I saw my first metric machines and NC machines. NC is not to be confused with CNC. Numerical Controlled machines in the 80's required manual programming using a punch tape. It was so high tech in those days.

I think metric is so much easier to deal with than imperial measurements. So, in my view you still have a legacy of imperial through oldies like me. It doesn't help that the big market of the US still uses imperial and isn't likely to change.

Why you can get a mix of both metric and imperial in the same instruments is also probably a legacy of the imperial size standards were just too hard for manufacturers to change to metric. Just think if they suddenly changed these sizes to metric, your accessories would no longer fit. Do you fancy having to buy a whole new set of accessories?

I still use metric and imperial and constantly swap between using the two, and am comfortable in using either, and sometimes at the same time.

So, I think it will be a long time before it changes, if ever.

[PeterM]

I think your age definitely has something to do with how accepting your are of inches and mm.
At nearly 60 I was schooled in inches and then the metric switch came. I always use metric when measuring distance, height, weight but when discussing my telescope mirrors its always inches.
Historical value/significance and tying our hobby to its own glorious past. For instance the 200inch Hale Telescope on Mount Palomar. Sure its mirror is 5.1m but will always be known as the 200inch Telescope. Maybe as amateur astronomers that gives meaning/
comparison to our own telescopes.
I always refer to my scopes in inches, even at hundreds of school/public nights I have done over the years and nobody has ever questioned this.

[JA]

"Pollice" is Italian for thumb, also about an inch wide and somewhere back in time the reasoning for its adoption to its use in measurement. Just like the foot - "piede".

At a pinch, when your are out shopping and caught without a tape measure who has used their feet (circa 300mm) to measure the length of a couch, piece of timber, distance etc... ?


Best
JA

PS: dont forget lowly A4 paper ruler EVERYWHERE- it's your circa 300 (297)mm x 210mm reference

[csb]

Now that is just a really helpful tip.
Thank you!

[firstlight]

I remember asking a similar question of my late friend Nick Williams years ago. Actually I referred to my 10" dob as 250mm, and he said that this was the effect of British and European influence and the correct way was to describe it in inches... that was until i asked him what was the focal length of his 'scope :lol:

[rat156]

I have worked on my friend's ex Garry Wilmington XD Falcon.

https://www.tradeuniquecars.com.au/feature-cars/1505/ex-garry-willmington-xd-group-c-falcon

Strangely the XD uses imperial for the mechanicals, but metric for the body. I grew up working on metric cars, so these make much more sense to me, 10 mm is smaller than 11 mm. I'm forever asking the guys in the pits what's the next size up/down from whatever imperial tool I have in my hands. Sometimes I can work it out, but once you get in 32nds of an inch I'm lost.

Then again, tyres are still measured in imperial for diameter, but metric for width and profile (which is a ratio, so is probably dimensionless). Unless you have one of those eccentric French cars which tried to metricate tyre sizes.

Cheers
Stuart

[Stardrifter_WA]

Hi Stuart,

I can understand there being a difference as it probably comes down to where the parts are sourced.

I grew up learning imperial at school and then having to learn both imperial AND metric going through my trade. I went through the stage of transition, so I am comfortable with both, otherwise it would have damn confusing. So, I guess I am lucky in that regard, I understand both systems.

As you point out Stuart, it must be hard for those that have only learned the metric system. Metric is certainly easier to understand.

Most of my measuring tools were imperial dial gauges and verniers, until I recently upgraded to a digital Mitutoyo Verniers. And that was an expensive exercise, but it sure makes it easier having a digital gauge as you can swap between measurements. I also bought a small pocket digital vernier from the US that has metric, imperial and 'fractions', which is unusual, but sure is handy.

Cheers Peter

[AstralTraveller]

I'm also in that two-system generation. I try to work in metric but at home I still have spanners, drill bits etc in imperial. We also have instruments at work which have imperial nuts and bolts. I've had to train students from Europe and China, which are fully metric, to work on the gear and so need to introduce them to the imperial system. Talk about wonder and amazement!! If having to work in 1/16th" units isn't enough to do their head in, talk of SAE, Whitworth and NPT certainly is.

[Wavytone]

Ok.. reality check.

In most advanced countries there is an official standard for weights and measures used for commercial purposes - i.e. the buying and selling of goods.

SI (metric) units are used internationally in the scientific community as a matter of convenience and as a defacto standard because these days the primary standards for weight, length, frequency and so-on are all defined as SI units.

1. But... officially the USA uses US imperial units which means inches feet and miles, US gallons (not UK gallons) and so-on and did not adopt SI units. But not SI metric units. And you can blame the republicans for this mess.

2. Officially the UK remains defiantly non-metric and uses imperial units (inches, feet, yards, chains, miles, ounces, pounds, pints, gallons etc) and because of the cultural connotations (they'll never admit they agree with those froggy *******s across the channel) the UK will probably stay that way.

3. Although Australia has been officially metric since the 1970;s for weights and measures, the snag is that our major trading partners are a mixed bag, Japan, China and Europe are metric, but the US and UK aren't. C'est la vie, pussycat.

But for scientific purposes SI rules.

4. A hell of a lot of machine tools - starting with the lathes used to cut screw threads - were based on the whitworth screw thread standard. This is a UK imperial standard that goes back to the industrial revolution in 19th century and was so widespread as a de-facto standard - to ensure compatibility between nuts and bolts from different manufacturers - that even though Australia has been officially metric since the 1970's the use of whitworth threads machinery remains common.

Little things like the camera industry adopting the 1/4" 20 TPI thread as the interface for camera tripods ensure this will remain in use for a very long time to come, despite the mechanical advantages of modern 6mm x 1mm metric machine screws.

4. Relative light grasp of telescopes (the square of aperture) vs mental arithmetic. Knowing that 4 squared is 16 vs 6 squared is 36 and so-on is a heck of a lot easier than 102mm squared = 10404 sq.mm, or 152.4mm squared is 23,226 sq mm.

So yes there is a good reason to express aperture in inches.

As for focal length - the pre-eminent early lens manufacturers were german (Zeiss, Leica) followed by the Japanese (Canon, Nikon, Pentax) who all expressed focal length in SI units (mm). So it became fairly natural to find telescope focal lengths following the lens tradition, i.e. express in mm. It also turns out to be quite convenient when doing astrometry.

But f/ratio has to be calculated as a length/length, and this has to be in the same units (metric or imperial).

[raymo]

We still have fathoms and knots of course. With imperial threads we have British Standard Whitworth, [BSW], British Standard Fine [BSF],
Unified Coarse [UNC], Unified Fine {UNF], and British Association, 2BA, 4BA, 6BA etc.
raymo

[Renato1]

Thanks, but my Italian dictionaries says "Pollice" has two meanings, one is a thumb and the other is an inch.

I bought a TV set over there which had a 32 pollice screen.

On another matter, when one goes into a Subway for a roll, does one ask for a foot long roll or a 30.5 cm roll?
And what do they ask for in Europe, I wonder?
Regards,
Renato

[PCH]

And if I'm not mistaken, don't the poms still have the 10,000 guineas horse race event at some point during the year?

[JA]

Yes that's right ... and what I was alluding to (although I didn't say both) .... The thumb and its use as a unit of measure somewhere back historically as it was an inch or so wide, just like the foot (piede)

Best
JA

[julianh72]

In Europe, they sell 15 cm and 30 cm subs.

[raymo]

We seem to have an inbuilt desire to emulate America in every way we
can, so maybe that's why we use foot long subs here, rather than the 30cm
that our use of metric measurement would seem to dictate.
raymo

[JA]

15cm/30cm or 6inch/12inch rolls reminded me of these foot-longs OR NOT, a while back ......

Best
JA

https://youtu.be/-EftSWAMQHg