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Frank,

Here are the latest series of lunars.  The position is N36*46.8',
W81*50.7'.  The "Clrd D" column is the cleared lunar distance observed
with the sextant, and the "Calc D" is the expected value.  These are in
decimal degrees.  The "Diff'" column is their difference, in minutes of
arc.  Note that for observation #39, the two Moon-Antares shots, I was
only off by a few tenths of minutes of arc.  In the others, I was off
by a minute or two of arc, and very consistently off!  I also checked
the sextant with some careful altitude shots and was out by less than
0.05+/-0.23' & 0.14+/-0.30' of arc (mean and std deviation of three
reps each) at sextant angles of 38 and 100 degrees, respectively.  I
don't think it's the sextant; it's given pretty accurate altitude shots
with one telescope that is properly aligned.  Operator error of some
sort, unless it's the shades.

In observation #37, I reset the index arm between the second and third
shots and the fourth and fifth.

These were corrected for non-standard pressure and temperature.  At
2100' elevation, our standard pressure is about 28" rather than 30", so
the correction can be up to a 0.3 or 0.4' of arc at high temps and low
sextant altitudes.

Fred


#       Objects Date & GMT                      Clrd D  Calc D  Diff'
36      Moon, Sun       09/20/2004 22:04:49     80.25643        80.24506        0.68
36      Moon, Sun       09/20/2004 22:05:42     80.26952        80.25311        0.98
36      Moon, Sun       09/20/2004 22:06:26     80.27711        80.25979        1.04
36      Moon, Sun       09/20/2004 22:07:32     80.29761        80.26982        1.67
36      Moon, Sun       09/20/2004 22:08:17     80.30030        80.27666        1.42
36      Moon, Sun       09/20/2004 22:09:04     80.30809        80.28380        1.46
37      Moon, Sun       09/21/2004 21:08:43     92.91720        92.88494        1.94
37      Moon, Sun       09/21/2004 21:09:48     92.92667        92.89487        1.91
37      Moon, Sun       09/21/2004 21:10:53     92.91957        92.90480        0.89
37      Moon, Sun       09/21/2004 21:11:56     92.92899        92.91442        0.87
37      Moon, Sun       09/21/2004 21:13:09     92.94219        92.92557        1.00
37      Moon, Sun       09/21/2004 21:14:11     92.95657        92.93504        1.29
38      Moon, Sun       09/22/2004 21:56:00     106.54126       106.51259       1.72
38      Moon, Sun       09/22/2004 21:59:15     106.55013       106.54236       0.47
39      Moon, Antares   09/23/2004 00:38:40     38.44910        38.44678        0.14
39      Moon, Antares   09/23/2004 00:41:07     38.46465        38.47096        -0.38

On Sep 22, 2004, at 11:22 PM, Frank Reed wrote:

> Fred H wrote:
>  "My understanding is that observed lunar distances will be too large
> if
>  the sextant is not properly oriented.  If anybody could speak to this,
>  it would help me solve a current problem I am having with lunar
>  observations; my telescope does not have cross hairs."
>
>  The main thing that you need to know about this issue, Fred, is that
> you can SEE it if it's a real problem for your sextant. Next time you
> shoot a lunar, bring the two objects into "perfect" contact as nearly
> as you can. Then "walk" the two objects around the field of view
> (you've got 12 seconds before the Moon moves a tenth of a minute of
> arc). If they remain in "perfect" contact at various points across the
> field of view of your sextant's telescope, you don't need to worry
> about this issue. Also, consider trying this with star-star sights
> since the apparent distance changes much more slowly (due to changing
> refraction as the stars move across the sky). IF your sextant's
> telescope is parallel to the sextant frame, you should find no change
> in contact unless the telescope has a very low magnification in which
> case the images will separate very slightly on the left and right
> sides of the field of view. If the telescope is NOT parallel to the
> frame, you may see a substantial increasing monotonically from one
> side of the field of view to the other. If you see that, you will want
> to correct the telescope's alignment.
>
>  I think you mentioned a couple of days ago that you were getting
> errors in observed distances larger than a minute of arc. Can you post
> the actual sight data? Have you tried any star-star sights in this
> distance range? Myself, I'm skeptical of the arc error certificates
> which accompany modern sextants. I wouldn't be surprised if a
> significant fraction of them do not accurately represent the errors of
> the instruments.
>
>  Frank R
>  [ ] Mystic, Connecticut
>  [X] Chicago, Illinois

   

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