Why 12.5 Inches?

Hi,

Does anyone know why 12.5 inches has always been such a common size for
a mirror?

Starry Skies,
Rich

On Fri, 03 Oct 2003 01:17:06 GMT, Richard DeLuca
wrote:

Hi,

Does anyone know why 12.5 inches has always been such a common size for
a mirror?

Diameter of a wheel on a chariot?

rim shot

Paul Below
Battle Point Astronomical Association
Bainbridge Island, WA, USA
http://bainbridgeisland.org/ritchieobs/

If I had to take a SWAG at it, I'd say the extra 1/2 inch is to mask a TDE.
Orion

"Richard DeLuca" wrote in message
news
Hi,

Does anyone know why 12.5 inches has always been such a common size for
a mirror?

Starry Skies,
Rich

--
To reply, remove the "z" if one appears in my address
"Richard DeLuca" wrote in message
news
Hi,

Does anyone know why 12.5 inches has always been such a common size for
a mirror?

Starry Skies,
Rich

Actually, this is an interesting question that I had never actually
considered before, simply because, for as long as I've been at this (and
that is since rocks were young), the quintessential "serious" amateur
telescope has nearly always been the venerable 12.5" f/6 Newtonian. Less
so now, perhaps, but much more so in the past. If you go back through the
old books and magazines (and I have a lot of oldies), 12.5" has been with
us since the dim times. Long before anyone thought to use concrete form
tubes as structures worthy of holding telescope optics. Nearly all the
earlier tubes were made of fiberglass, and before that, from metal...

But as for the answer to your question, I'm afraid I'll have to defer that
to a higher authority... Fortunately, we've got lots of 'em here!

Maybe someone had a lot of surplus 12.5" porthole glass in the old days...

"Paul Below" wrote in message
...
On Fri, 03 Oct 2003 01:17:06 GMT, Richard DeLuca
wrote:

Hi,

Does anyone know why 12.5 inches has always been such a common size for
a mirror?

Diameter of a wheel on a chariot?

rim shot

Don't you mean hip-shot, as in shooting without aiming? ^_^

The reasoning for 12.5 inches was that in theory it gave you a full half
magnitude gain over a 10 inch mirror (14.0 vs. 14.5 using the old magnitude
parameters from about 1960, when 12.5 inch mirrors surplanted those of 12
inches aperture), just as going from an 8 inch mirror to a 10 produced a half
magnitude jump. I guess you might consider it as a selling point.

John Bortle

THANKS!!!

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"JBortle" wrote in message
...
The reasoning for 12.5 inches was that in theory it gave you a full half
magnitude gain over a 10 inch mirror (14.0 vs. 14.5 using the old
magnitude
parameters from about 1960, when 12.5 inch mirrors surplanted those of
12
inches aperture), just as going from an 8 inch mirror to a 10 produced a
half
magnitude jump. I guess you might consider it as a selling point.

John Bortle

--
To reply, remove the "z" if one appears in my address
"JBortle" wrote in message
...
The reasoning for 12.5 inches was that in theory it gave you a full half
magnitude gain over a 10 inch mirror (14.0 vs. 14.5 using the old
magnitude
parameters from about 1960, when 12.5 inch mirrors surplanted those of
12
inches aperture), just as going from an 8 inch mirror to a 10 produced a
half
magnitude jump. I guess you might consider it as a selling point.

John Bortle

Sounds like following that pattern, the 17.5" (as opposed to 18") probably
came about based on the NEXT half-magnitude mark...

Thanks, John!

Like I said, a higher authority!

Yikes! I guess that would put the next half magnitude at 16", then...
and NOT 17.5"

I guess the 17.5" size must have been created to target 15.0 Magnitude...,
rather than extend the previous convention...

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"Jan Owen" wrote in message
news:wH5fb.45321$vj2.22890@fed1read06...


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"JBortle" wrote in message
...
The reasoning for 12.5 inches was that in theory it gave you a full
half
magnitude gain over a 10 inch mirror (14.0 vs. 14.5 using the old
magnitude
parameters from about 1960, when 12.5 inch mirrors surplanted those of
12
inches aperture), just as going from an 8 inch mirror to a 10 produced
a
half
magnitude jump. I guess you might consider it as a selling point.

John Bortle

Sounds like following that pattern, the 17.5" (as opposed to 18")
probably
came about based on the NEXT half-magnitude mark...

Thanks, John!

Like I said, a higher authority!

Was that what Coulter(or was it Odyssey) was thinking? Makes sense.

Best regards,
Bill

"Jan Owen" wrote in message
news:lR5fb.45341$vj2.16502@fed1read06...
Yikes! I guess that would put the next half magnitude at 16", then...
and NOT 17.5"

I guess the 17.5" size must have been created to target 15.0 Magnitude...,
rather than extend the previous convention...

--
To reply, remove the "z" if one appears in my address
"Jan Owen" wrote in message
news:wH5fb.45321$vj2.22890@fed1read06...


--
To reply, remove the "z" if one appears in my address
"JBortle" wrote in message
...
The reasoning for 12.5 inches was that in theory it gave you a full
half
magnitude gain over a 10 inch mirror (14.0 vs. 14.5 using the old
magnitude
parameters from about 1960, when 12.5 inch mirrors surplanted those of
12
inches aperture), just as going from an 8 inch mirror to a 10 produced
a
half
magnitude jump. I guess you might consider it as a selling point.

John Bortle

Sounds like following that pattern, the 17.5" (as opposed to 18")
probably
came about based on the NEXT half-magnitude mark...

Thanks, John!

Like I said, a higher authority!