NavList:

   

Reply

It seems to me that there's more to Alan's index-error problems than just
his use of a too-close roofline.

He has confirmed that there's been no fiddling with the mirror-screws
during this history of on-land observations, and that results are always
much more consistent when he checks index error using a real sea horizon.
One question that might provide some clue is this; when he observes a true
sea-horizon, what index error does he usually arrive at?

Both Geoffrey and Gary have pointed to the use of a too-close object for
aligning on. And indeed, if that object is as close as a quarter-mile
(Alan's estimate), then it would cause an error of around half an
arc-minute in the result. But that should be a consistent error, compared
with his sea-horizon value; always about half a minute off-the-arc, if Alan
always uses the same roofline, or another roof about the same distance off.
But whatever the distance, it should always be in the same direction,
off-the-arc, compared with a truly distant horizon.

But the errors Alan reports are very different from that picture. The
variation is much greater, and sometimes on-the-arc, sometimes off it.
What's going on?

Is Alan perhaps mistakenly attributing some of his off-the-arc observations
as on-the-arc, or vice versa? I ask him to explain exactly how he
distinguishes between them. It should depend only on the reading of the
micrometer drum. If the reading is just above zero, in the region of the
drum between the zero mark and, say, 10', then it's on-the-arc. If it's
less than zero, so in the region between 50' and zero, then it's
off-the-arc.

What makes me a bit suspicious is Alan's explanation of his procedure,
where he says-

"Zero the micrometer, I'm using an Astra 111B sextant. Then zero the index
arm.
Re the AH shots listed, I focoused the sextant scope on the roof lines of
small buildings, private residences, within 1/4 mile of where I stood,
beyond that proceeding as sextant instructions I've seen say, adjusting the
micrometer till I got an "unbroken horizon line", that being the
aforementioned roof line. Read the error off the micrometer, subtracting
for "on the arc", adding for "off the arc"."

Just for the FOCUSSING operation on the telescope, using rooflines a
quartter-mile away is perfectly adequate; it's insensitive to the exact
distance. But then for the ALIGNMENT of the two images, that calls for
sighting on something more than a mile away, as has been explained.

But I don't follow Alan's two-step process, as explained- "Zero the
micrometer ... then zero the index arm". As long as the pointer on the
index arm reads close to zero on its coarse, whole-degrees, scale, that's
good enough, and no further attention needs to be given to it. Then align
the two images to form an unbroken line, and read the micrometer minutes
off the drum, in divisions (and decimals) away from zero, on or off the arc
as stated above. That's it; job done; no call to "zero the index arm",
whatever that means.

Perhaps I'm misunderstanding Alan's procedure. I am clutching at straws,
somewhat, in trying to understand what is going so wrong.

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

   

Reply