NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: "Table top" index error measurement
From: Frank Reed CT
Date: 2006 Jul 12, 05:02 -0500
Alex, you wrote:
"If I understand Frank's proposal correctly,
the laser has to be fixed behind the scope
(so that it sends the beam to the eyepiece of the scope).
So I cannot see how it can be fixed to the sextant frame.
I imagined using a vice or a clamp behind the
sextant, to fix the laser rigidly. "
What's happened is we are talking about two different uses for the same
laser level:
1) measuring index error, as described at the top of this thread, and
for this purpose, as you note, the laser is positioned "behind" the
sextant.
2) adjusting sextant telescope collimation. In this case, the laser
rests on the frame of the sextant, or more likely, on two supports of
identical height (whatever they might be).
And you wrote:
"Then it is probably not hard to achieve parallelism
of the beam to the telescope axis.
This parallelism has to be assured in any case, no matter
how fix the laser. "
Strict parallelism is not necessary for measuring index error. You move
the laser around until you see two images, one direct, one reflected,
and that's all you have to do in terms of pointing. The critical step
is adjusting the focus of the telescope so that the "smudges" of laser
light that you will see when you first set this up become sharply
focused points.
"The light switch problem you mention seems easy to fix.
(Using skotch tape, or an assistant:-) "
Yes. Absolutely. A presentation laser pointer can be made to work with
various fixes like this, but it's a bit of a nuisance taping down the
switch every time --I've tried this and it really spoiled the fun.
Carpenters' laser levels are moderately priced, more versatile, and
designed for jobs that are closer to what I'm describing here. That's
all. They're the right tool for the job.
Of the laser level, you asked:
"You think it has a straight edge strictly parallel to the beam? "
They're pretty good on this score. It's what they're designed for
--communicating a level at one location to another location some dozens
of feet away. You can count on 5 minute of arc parallelism. The
packaging for various brands in this price range usually claims twice
that accuracy or better, but I think +/-5' is a reasonable expectation.
"In any case, I don't understand how are you going to
place it on the frame BEHIND the scope.
It has to send the light so that the beam enters
the scope through the eyepiece and leaves through the
objective lense. "
Again, to clarify, we're talking about two different uses for the same
toy.
er... tool.
-FER
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
From: Frank Reed CT
Date: 2006 Jul 12, 05:02 -0500
Alex, you wrote:
"If I understand Frank's proposal correctly,
the laser has to be fixed behind the scope
(so that it sends the beam to the eyepiece of the scope).
So I cannot see how it can be fixed to the sextant frame.
I imagined using a vice or a clamp behind the
sextant, to fix the laser rigidly. "
What's happened is we are talking about two different uses for the same
laser level:
1) measuring index error, as described at the top of this thread, and
for this purpose, as you note, the laser is positioned "behind" the
sextant.
2) adjusting sextant telescope collimation. In this case, the laser
rests on the frame of the sextant, or more likely, on two supports of
identical height (whatever they might be).
And you wrote:
"Then it is probably not hard to achieve parallelism
of the beam to the telescope axis.
This parallelism has to be assured in any case, no matter
how fix the laser. "
Strict parallelism is not necessary for measuring index error. You move
the laser around until you see two images, one direct, one reflected,
and that's all you have to do in terms of pointing. The critical step
is adjusting the focus of the telescope so that the "smudges" of laser
light that you will see when you first set this up become sharply
focused points.
"The light switch problem you mention seems easy to fix.
(Using skotch tape, or an assistant:-) "
Yes. Absolutely. A presentation laser pointer can be made to work with
various fixes like this, but it's a bit of a nuisance taping down the
switch every time --I've tried this and it really spoiled the fun.
Carpenters' laser levels are moderately priced, more versatile, and
designed for jobs that are closer to what I'm describing here. That's
all. They're the right tool for the job.
Of the laser level, you asked:
"You think it has a straight edge strictly parallel to the beam? "
They're pretty good on this score. It's what they're designed for
--communicating a level at one location to another location some dozens
of feet away. You can count on 5 minute of arc parallelism. The
packaging for various brands in this price range usually claims twice
that accuracy or better, but I think +/-5' is a reasonable expectation.
"In any case, I don't understand how are you going to
place it on the frame BEHIND the scope.
It has to send the light so that the beam enters
the scope through the eyepiece and leaves through the
objective lense. "
Again, to clarify, we're talking about two different uses for the same
toy.
er... tool.
-FER
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
