NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Off Center Sextant Scope Observations
From: Greg R_
Date: 2007 Nov 22, 20:51 -0800
From: Greg R_
Date: 2007 Nov 22, 20:51 -0800
I think you just handed me my next "experiment"
for whenever I get out to do some sights next time... ;-)
Seems like I always move the sun (or whatever
celestial body) into the center of the scope's field of view before taking the
final measurement, but now that I think about it I can't really be sure... so I
think I'll try some "off-center" sights just to see what happens (Astra metal
and Davis plastic sextants, though if our theory is correct it shouldn't make
any difference for either one).
> It shouldn't make any difference where they
are in
> the field of view since both rays of light,
from the
> sun and from the horizon, will travel
through the
> same spot in the optics of the
telescope
Agreed - since they're both going to be
affected by the same error (refraction, or otherwise) when both index and
horizon light rays go through the scope, then they should both appear in the
same relative position of the scope. The only other variables would be the
mirrors and the shade(s), hopefully both are acceptably "flat" to where any
difference between the light paths where they enter the scope should be
minimal. At least the way I see
it...
--
GregR
----- Original Message -----From: Gary J. LaPookTo: NavList@fer3.comSent: Thursday, November 22, 2007 8:01 PMSubject: [NavList 4018] Re: Off Center Sextant Scope ObservationsGary wrote:
Or, put the lower limb on the horizon near the bottom of the sextant field, as you usually do, then quickly move the sextant so you are looking through the top of the field of view and the sun should still be on the horizon. It shouldn't make any difference where they are in the field of view since both rays of light, from the sun and from the horizon, will travel through the same spot in the optics of the telescope.
gl
Greg R. wrote:I haven't researched this in depth, but my instinct is would it really matter? Assuming you're talking about some sort of refraction error (through the sextant scope optics, not the atmosphere), since you're using the sextant's mirrors to make the lower limb tangent to the horizon wouldn't both be refracted by pretty much the same amount if viewed at that point (or really any point) of the scope?But you can probably answer you own question - how close do your observations come to a known GPS position for the same location? If you can get to within a mile or so consistently from a shore location (no matter what part of the scope you use for the sight) I'd say not to worry about it. :-)--GregR----- Original Message -----From: "Greg Rudzinski" <gregrudzinski@yahoo.com>To: "NavList" <NavList@fer3.com>Sent: Thursday, November 22, 2007 4:39 PMSubject: [NavList 4015] Off Center Sextant Scope Observations>
> What happens to an image of a celestial body when viewed off center in
> a sextant scope? I have a bad habit of using the lower half of the
> field of view when taking lower limb observations of the sun. I
> suspect that this increases the observed altitude slightly.
>
> Greg Rudzinski
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