Re: [ATM] help upper ring dia

["Bob May" <rmay@nethere.com>]

Stuff your finger into your mouth to wet it and then stuff it up
into the air.  That is where the two inches comes from.  It is a
crude approximation of the diameter of what you can see with a
two inch eyepiece of appropiate focal length and FOV.  The
approximation is in the right direction to keep from getting into
trouble by getting things too close to the real limit.
Bob May

rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net

----- Original Message -----
From: Nils Olof Carlin <nilsolof.carlin@telia.com>
To: <tcohen@blakeglobal.com>; <atm@atmlist.net>
Sent: Friday, April 25, 2008 11:54 PM
Subject: Re: [ATM] help upper ring dia


> Ted,
>
> "Just a bit curious as to how you conclude to a 2" FOV - not
that I
> disagree,
> but it's just not intuitive to me. Also, I assume the 2" means
2
> inches and
> not 2 arc-seconds, although wouldn't the FOV be more typically
> measured as
> subtending a certain angle."
>
> A 2 inches (sorry about that odd quotation mark!) dia. focuser
can
> show a no more than 2 in. dia of the focal plane - in real life
hardly
> more than 45 mm or so. thus, light from a star 1 in. from
center will
> have come as a light cylinder tilted by 1 inch relative to that
of a
> central star. So, all light that is focused within the FOV will
pass
> within a circle of diameter = mirror aperture + maximum FOV
(IIRC the
> secondary and the tube opening will tend to vignette at
opposite
> sides). Thus, using the focuser dia. as upper limit will always
be
> safe. Then again, a bit of vignetting by the truss or outer
tube
> opening is never serious, even if best avoided.
>
> Whether to measure field in linear or angular tems is a matter
of
> taste - an eyepiece with a 25 mm field stop will admit a 25 mm
field in
> any telescope, but the corresponding angle depends on the focal
length.
> (field stop divided by focal length - in radians, so multiply
by 57.3
> to get degrees).
>
> Nils Olof
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
>

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