NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
"Table top" index error measurement
From: Frank Reed CT
Date: 2006 Jul 12, 00:24 -0500
There is a "table top" method for measuring IC which is independent of the
observer's eye. You'll need an open space or long corridor with 500 feet or so
that you can access securely, and it will help to have an assistant.
Get a laser from the local hardware store. Set your sextant on its side on a
table and aim the laser through the telescope. Place a target, like an easel
with a sheet of graph paper on it, about fifty feet away. You'll get two
spots side-by-side a few inches apart. Focus the sextant's telescope until the
laser dots are as small as possible --they will be very small and intense
points of light. Carefully mark off the distance between them on the graph paper.
Now walk the easel down to the far end of your test area. Let's make it 3438
inches (286.5 feet) away from the first observation point (one inch at a
distance of 3438 inches subtends one minute of arc). Ask your assistant to
re-focus the sextant's telescope until the laser dots are sharp again and
re-measure the distance between the dots. If the sextant's index error is exactly
zero, the beams of laser light emerging from the sextant will remain parallel
over this whole distance and the distance between the dots will not change. But
if they are separated by some distance, let's say 3/4 of an inch, then that
means the sextant has a non-zero index correction. The amount in minutes of
arc, if I haven't made a mistake along the way, is simply equal to the change
in separation distance in inches.
There should not be any problem measuring a change in the separation
distance to the nearest tenth of an inch so you can get a very accurate index
correction this way. It's repeatable and accurate. I've tried it with excellent
results --but only twice. It's worth a try at least. If you're worried about
measuring that small separation distance, get yourself a longer corridor. Poor
telescope optics might present a problem. Make sure no one can walk through
your set up and look into the focused laser beam.
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
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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: Frank Reed CT
Date: 2006 Jul 12, 00:24 -0500
There is a "table top" method for measuring IC which is independent of the
observer's eye. You'll need an open space or long corridor with 500 feet or so
that you can access securely, and it will help to have an assistant.
Get a laser from the local hardware store. Set your sextant on its side on a
table and aim the laser through the telescope. Place a target, like an easel
with a sheet of graph paper on it, about fifty feet away. You'll get two
spots side-by-side a few inches apart. Focus the sextant's telescope until the
laser dots are as small as possible --they will be very small and intense
points of light. Carefully mark off the distance between them on the graph paper.
Now walk the easel down to the far end of your test area. Let's make it 3438
inches (286.5 feet) away from the first observation point (one inch at a
distance of 3438 inches subtends one minute of arc). Ask your assistant to
re-focus the sextant's telescope until the laser dots are sharp again and
re-measure the distance between the dots. If the sextant's index error is exactly
zero, the beams of laser light emerging from the sextant will remain parallel
over this whole distance and the distance between the dots will not change. But
if they are separated by some distance, let's say 3/4 of an inch, then that
means the sextant has a non-zero index correction. The amount in minutes of
arc, if I haven't made a mistake along the way, is simply equal to the change
in separation distance in inches.
There should not be any problem measuring a change in the separation
distance to the nearest tenth of an inch so you can get a very accurate index
correction this way. It's repeatable and accurate. I've tried it with excellent
results --but only twice. It's worth a try at least. If you're worried about
measuring that small separation distance, get yourself a longer corridor. Poor
telescope optics might present a problem. Make sure no one can walk through
your set up and look into the focused laser beam.
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
