NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Lunar Obs
From: Ken Gebhart
Date: 2006 Jul 29, 22:21 -0500
on 7/28/06 22:46, Frank Reed at FrankReedCT@aol.com wrote:
>
> Here's today's lunars from the Chicago lakefront:
> Time(CDT) LD
> 18:23:00 41d 13.0'
> 18:26:55 41d 14.5'
> 18:29:17 41d 15.5'
> 18:32:10 41d 16.1
>
> The sextant is an "International Nautical" which is a Tamaya-alike and
> nearly identical to an "M.A.C.". The IC is -0.7' and T=90F, P=29.95 in.
>
> I get an average error of -0.2' in the lunar distance (cleared using my
> online clearing calculator) which corresponds to about 6.5' of
> longitude. Conditions for this lunar were very bad. At 41 degrees
> elongation, the Moon should be visible in daylight but I could not see
> it at all except using the sextant's 7x telescope, and even then it was
> indistinct in hazy skies. Took me about five minutes of waving the
> sextant around to find it.
>
> I discovered something interesting, and annoying, about my sextant
> while taking these sights. The shade in front of the index mirror does
> not completely cover the mirror. There is a small spot left unshaded.
> If I move the Sun's image to the far "lower right", I see the Sun
> unshaded --which I do not want to do more than once. Fortunately, the
> shade has an obvious "foot" which I can file down a little to solve the
> problem. The horizon shade had the same problem, but I fixed that one
> by unscrewing the whole unit and turning it around.
>
> -FER
> www.HistoricalAtlas.com/lunars
>
>
> >
Frank,
I hate revealing these little secrets, but most sextants have the same
problem. Both Astra and Cassens & Plath will show a little sun at low
angles, say between 5 and 17 deg. I don't know why this is so, but maybe
it's because no one ever complains. On the other hand, it has been
thoroughly vetted on the Nav-L list that such glimpses, even with higher
power scopes do less eye damage than naked eye exposure. And even this is
not very significant unless exposure is prolonged to the point of pain
(solar eclipses excluded of course).
Ken
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To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
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From: Ken Gebhart
Date: 2006 Jul 29, 22:21 -0500
on 7/28/06 22:46, Frank Reed at FrankReedCT@aol.com wrote:
>
> Here's today's lunars from the Chicago lakefront:
> Time(CDT) LD
> 18:23:00 41d 13.0'
> 18:26:55 41d 14.5'
> 18:29:17 41d 15.5'
> 18:32:10 41d 16.1
>
> The sextant is an "International Nautical" which is a Tamaya-alike and
> nearly identical to an "M.A.C.". The IC is -0.7' and T=90F, P=29.95 in.
>
> I get an average error of -0.2' in the lunar distance (cleared using my
> online clearing calculator) which corresponds to about 6.5' of
> longitude. Conditions for this lunar were very bad. At 41 degrees
> elongation, the Moon should be visible in daylight but I could not see
> it at all except using the sextant's 7x telescope, and even then it was
> indistinct in hazy skies. Took me about five minutes of waving the
> sextant around to find it.
>
> I discovered something interesting, and annoying, about my sextant
> while taking these sights. The shade in front of the index mirror does
> not completely cover the mirror. There is a small spot left unshaded.
> If I move the Sun's image to the far "lower right", I see the Sun
> unshaded --which I do not want to do more than once. Fortunately, the
> shade has an obvious "foot" which I can file down a little to solve the
> problem. The horizon shade had the same problem, but I fixed that one
> by unscrewing the whole unit and turning it around.
>
> -FER
> www.HistoricalAtlas.com/lunars
>
>
> >
Frank,
I hate revealing these little secrets, but most sextants have the same
problem. Both Astra and Cassens & Plath will show a little sun at low
angles, say between 5 and 17 deg. I don't know why this is so, but maybe
it's because no one ever complains. On the other hand, it has been
thoroughly vetted on the Nav-L list that such glimpses, even with higher
power scopes do less eye damage than naked eye exposure. And even this is
not very significant unless exposure is prolonged to the point of pain
(solar eclipses excluded of course).
Ken
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
