Great idea Gianni!
Will have to try it.
Thanks
John C.
On Fri, 5 Jul 2002 18:39:57 +0200 Gianni Ferrari [EMAIL PROTECTED] wrote:
I have read with a lot of interest the numerous and very interesting
messages that concern the Shadow Sharpener and I would like to make a little
observation.
Gianni and John,
A good front face mirror for the sun can be made from a scrap of
ordinary
window glass. Mark it with a glass cutter (or a carbide scribe) and make
controlled breaks along the pre-scribed lines to obtain the size you
want.
Soften the sharp edges and corners with wet or dry
Hi Dialling colleagues,
Patrick Powers asked if anyone had practical experience of the bead-in-a-hole
(pinspeck) shadow sharpener.
I used one experimentally o my Isaac Newton mean-time equatorial dial (see
www.flowton-dials.co.uk). It consisted of a 3mm dia phosphor bronze bead
suspended in
-
From: Patrick Powers [EMAIL PROTECTED]
To: sundial@rrz.uni-koeln.de
Sent: Thursday, June 06, 2002 3:14 PM
Subject: Re: Shadow Sharpener Again
Message text written by INTERNET:sundial@rrz.uni-koeln.de
John said: ...but do you think your formula could help determine the
optimum
size
Carmichael [mailto:[EMAIL PROTECTED]
Sent: 07 June 2002 16:50
To: sundial@rrz.uni-koeln.de
Subject: Re: Shadow Sharpener Again
It seems as though the only practical use for a bead-in-hole is on the
alidade of an equatorial heliochronometer Since for it to work properly, as
John Davis pointed out, it must
Patrick Powers wrote ...
The basic formula is actually f=(s^2)/(L), where f is
the focal length, s is the radius of the (infinitely thin!)
hole and L is the wavelength of the light.
I would express this a bit differently, since a pinhole does not form an
image in the sense that a lens
PROTECTED]
To: sundial sundial@rrz.uni-koeln.de
Sent: Wednesday, June 05, 2002 4:30 PM
Subject: Re: Shadow Sharpener Again
John, I clicked 'send' on my last message without adding the detail of the
formula!
The basic formula is actually f=(s^2)/(L), where
f is the focal length, s is the radius
Message text written by INTERNET:sundial@rrz.uni-koeln.de
John said: ...but do you think your formula could help determine the
optimum
size of the gap between the bead and hole of a bead-in-hole sharpener?
Yes it can help but it's not the same! Strictly the process is different
but the
[EMAIL PROTECTED]
To: sundial@rrz.uni-koeln.de
Sent: Wednesday, June 05, 2002 3:23 AM
Subject: Re: Shadow Sharpener Again
John wrote:
I just tested a bead-in-hole shadow sharpener and five pinhole sharpeners
ranging in diameter from 2mm to 6mm. I used the shadow edge of my house's
roof (which
Message text written by INTERNET:sundial@rrz.uni-koeln.de
That's a very useful formula! Not that it would be of practical value,
because I'm just using visible light, but does that mean that the pinhole
size would change with different wavelenght of light?
Yes, the pinhole effect is driven by
John, I clicked 'send' on my last message without adding the detail of the
formula!
The basic formula is actually f=(s^2)/(L), where
f is the focal length, s is the radius of the (infinitely thin!) hole and L
is the wavelength of the light.
Sunshine has a representative wavelength of 550
-
From:
[EMAIL PROTECTED]
To: sundial@rrz.uni-koeln.de
Sent: Thursday, May 30, 2002 6:04
PM
Subject: Re: Shadow Sharpener Again
In a message dated
05/30/2002 9:25:21 AM Eastern Daylight Time, [EMAIL PROTECTED]
writes:
I apologize for my original explanation
To determine offset of the perceived shadow edge, I built a simple device consisting of a cardboard test pattern, a white cardboard screen, and a stick to separate them about 36". The test pattern was two parallel strips each exactly 1" wide with a 1" gap between them. Over a three month period
I think my best bet will be to use the 2 mm pinhole. But I will take all of them to the mountain because I want to test all the sizes using the telescope's shadow edge. Maybe the larger holes will work better if the distance to the style is increased? Whadya think?
John,
I agree, the larger
oeln.de
Sent: Thursday, May 30, 2002 7:04 PM
Subject: Re: Shadow Sharpener Again
Pete, Thanks
for clearing this up. Amazing how our fingers do not type what we think we
have said! I think your results are in good agreement with mine. Your
difference in degrees between the center of the Sun's ima
a double-edged
gnomon such as a rod, cable, pinhole, slit, etc.
Thanks. Pete S.
- Original Message -
From: Pete
Swanstrom
To: sundial@rrz.uni-koeln.de
Sent: Tuesday, May 28, 2002 10:14 AM
Subject: Re: Shadow Sharpener Again
On a brightday with clear
skies, the perceived edge
- Original Message -
From:
Pete Swanstrom
To: sundial@rrz.uni-koeln.de
Sent: Thursday, May 30, 2002 5:12
AM
Subject: Re: Shadow Sharpener Again
I apologize for my original
explanationregardingthe perceived edge of a shadow, it was
wrong. The corrected first
I apologize for my original explanation regarding the perceived edge of a shadow, it was wrong. The corrected first paragraph should read as follows:
On a bright day with clear skies, the perceived edge of a shadow appears near the inner edge of the penumbra (near the umbra). With increasing
Title: Re: Shadow Sharpener Again
blockquote, dl, ul, ol, li { margin-top: 0 ; margin-bottom: 0 }
-->
On a
brightday with clear skies, the perceived edge of a shadow
appearsnear the outer edge of the penumbra. With
increasing haze or whenever the edge of a cloud passes by, the
perceived e
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